Download 01.01.2001 – 31.03.2003 - Archiv Physik

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Graduiertenkolleg 695
Nichtlinearitäten optischer Materialien
Arbeits- und Ergebnisbericht
01.01.2001 – 31.03.2003
Teil 1
(Anhänge sind im Teil 2 zu finden)
Gefördert von
der Deutschen Forschungsgemeinschaft
und dem Land Niedersachsen
1
Inhalt
Seite
Vorwort
3
1. Umsetzung der Zielsetzung und Konzeption des Kollegs
1.1 Kurzberichte zu den Forschungsbeiträgen der beteiligten Hochschullehrer
1.2 Einzelberichte der in der vergangenen Periode geförderten Kollegiat(inn)en
4
4
29
2. Auflistung aller Kollegiat(inn)en
116
3. Auswahl der Kollegiat(inn)en
117
4. Durchführung des Studienprogramms
117
5. Angaben zur Vergabe der Koordinationsmittel
118
6. Interne Erfolgskontrolle des Kollegs
118
7. Gastwissenschaftlerprogramm
119
8. Zwischenbilanz des Kollegs
122
Anhänge sind im Teil 2 dieses Berichts zu finden:
Anhang 1: Vorlesungsskript P. Hertel: ‚Linear Response Theory’
Anhang 2: Vorlesungsskript E. Krätzig, K. H. Ringhofer: ‚The Photorefractive Nonlinearity’
Anhang 3: Vorlesungsskript H.-J. Schmidt: ‚Nonlinear Wave Equations’
Anhang 4: Seminarprogramme SS 01, WS 01/02, SS 02, WS 02/03
Anhang 5: Durchgeführte Workshops
2
Vorwort
Das Graduiertenkolleg 695 ‚Nichtlinearitäten optischer Materialien’ startete Anfang
2001. Die Betreuer (Punkt 1.1) vergaben im Laufe des dieses Jahres alle 13
Stipendien, und zusätzlich wurden 3 weitere Kollegiat(inn)en aufgenommen. Die
bearbeiteten Projekte sind unter Punkt 1.2 beschrieben, weitere Informationen zu
den Kollegiat(inn)en finden sich unter den Punkten 2 und 3. Im Jahr 2001 begann
ebenfalls das Studienprogramm des Kollegs, das in Punkt 4 und den Anhängen im
Teil 2 dieses Berichts dargelegt ist. Zur internen Erfolgskotrolle (Punkt 6) diente auch
das Seminar des Kollegs, in dem alle Kollegiat(inn)en mindestens einmal im Jahr die
Fortschritte ihrer Arbeiten vorstellten. Regelmäßige Workshops und ein
Gastwissenschaftlerprogramm (Punkt 7) waren ebenfalls Schwerpunkte der
Ausbildung. Die Zwischenbilanz fällt aus unserer Sicht positiv aus (Punkt 8). Das
Graduiertenkolleg 695 hat sich zu einem wichtigen Schwerpunkt der
Graduiertenausbildung in den Naturwissenschaften an der Universität Osnabrück
entwickelt.
Die Leitung des Graduiertenkollegs bestand in der ersten Periode aus K. Betzler, E.
Krätzig, E. Rühl und F. Rahe (Vertreter der Kollegiat(inn)en), als Sprecher fungierte
E. Krätzig, sein Vertreter war K. Betzler. Für die zweite Periode kandidierte E. Krätzig
aus Altersgründen nicht mehr, und K. Betzler wurde zum Sprecher gewählt. Da E.
Rühl im Herbst 2002 einem Ruf an die Universität Würzburg folgte, wurde H.-J.
Steinhoff als Nachfolger in die Leitung aufgenommen.
Osnabrück, den 28.02.03
K. Betzler
E. Krätzig
3
1. Umsetzung der Zielsetzung und Konzeption des Kollegs
1.1 Kurzberichte zu den Forschungsbeiträgen der beteiligten
Hochschullehrer
Prof. Dr. Klaus Bärwinkel,
Apl. Prof. Dr. Heinz-Jürgen Schmidt,
Priv.-Doz. Dr. Jürgen Schnack
Forschungsübersicht
Neben den bisherigen Arbeitsgebieten (Transporttheorie, Quantenthermostaten,
Thermodynamik kleiner Quantensysteme, Fermionische Molekulardynamik,
Relativitätstheorie) ist seit 1999 die theoretische Behandlung kleiner Spinsysteme
und magnetischer Moleküle ein neuer Schwerpunkt der Arbeitsgruppe. Hier werden
mit exakten und approximativen Methoden die Eigenschaften des Energiespektrums
und
thermodynamische
Eigenschaften
einschließlich
Spin-SpinKorrelationsfunktionen untersucht. Wichtige Teilergebnisse sind z. B. Regeln für die
k-Quantenzahlen von relativen Grundzuständen in Spinringen, die Entdeckung
spezieller exakter Grundzustände (independent magnon states), die zu
makroskopischen Magnetisierungssprüngen führen sowie die Erklärung von
Rotationsbändern im Spektrum vieler Spinsysteme.
Forschung im Kolleg
Seit 2001 werden im Zusammenhang mit dem Graduiertenkolleg 695 zusätzlich
optische und magnetische Solitonen untersucht, wobei durchaus inhaltliche und
methodische Zusammenhänge mit den oben genannten Themen bestehen. Für die
Untersuchung von magnetischen Solitonen (Stipendiat Pavlo Shchelokovskyy) ist
dies offensichtlich: Hier sollen quantenmechanische Analoga zu den bekannten
klassischen Solitonen in Spinringen
gefunden und deren experimentelle
Realisierung diskutiert werden.
Ein zweites Projekt (Stipendiat Felix Homann) ist der Entwicklung von
Näherungsmethoden für die Beschreibung von Solitonen gewidmet, die auch in
solchen Fällen Ergebnisse liefert, in denen die exakten Methoden der inversen
Streutheorie versagen. Hier werden Ideen aus der Fermionischen Molekulardynamik
und Methoden der analytischen Mechanik Hamiltonscher Systeme verwendet.
Kooperationen im Kolleg
Neben der engen Zusammenarbeit innerhalb der Arbeitsgruppe gab es
Berührungspunkte und Diskussionen mit den von Prof. Schürmann betreuten
Projekten und Kollegiat(inn)en sowie mit der Arbeitsgruppe von Prof. Ringhofer zum
Thema „Metamaterialien“.
Publikationen im Zusammenhang mit dem Graduiertenkolleg (ab 2000)
K. Bärwinkel, H.-J. Schmidt, J. Schnack, Energy bounds for n-partite spin systems,
Eur. Phys. J. B (2003) submitted
4
H.-J. Schmidt, J. Schnack, Symmetric polynomials in physics,
Plenary talk at the G24 conference, Paris, 2002, contribution to the proceedings 2003
H.-J. Schmidt, M. Luban, Classical ground states of symmetrical Heisenberg spin
systems, J. Phys. A: Math. Gen. (2003) submitted
M. Exler, J. Schnack, Evaluation of the low-lying energy spectrum of magnetic Keplerate molecules with DMRG, Phys. Rev. B (2003), submitted
H.-J. Schmidt, Linear energy bounds for Heisenberg spin systems, J. Phys. A: Math.
Gen. 35 (2002) 6545-6555
J. Schulenburg, A. Honecker, J. Schnack, J. Richter, H.-J. Schmidt, Macroscopic
magnetization jumps due to independent magnons in frustrated quantum spin lattices, Phys. Rev. Lett. 88 (2002) 167207
H.-J. Schmidt, J. Schnack, Partition functions and symmetric polynomials,
Am. J. Phys. 70 (2002) 53-57
J. Schnack, H.-J. Schmidt, J. Richter, J. Schulenburg, Independent magnon states on
magnetic polytopes, Eur. Phys. J. B 24 (2001) 475
J. Schnack, M. Luban, R. Modler, Quantum rotational band model for the Heisenberg
molecular magnet Mo_72Fe_30, Europhysics Letters 56 (2001) 863
D. Mentrup, J. Schnack, Isothermal quantum dynamics: Nosé-Hoover method for
coherent states, in Advances in Quantum Many-Body Theory, Proceedings of "The
11th International Conferences on Recent Progress in Many-Body Theories",
Manchester, edited by Raymond F. Bishop, Tobias Brandes, Klaus A. Gernoth, Niels
R. Walet, and Yang Xian (UMIST, Manchester, UK, 2001), World Scientific
J. Schnack, M. Luban, R. Modler, Rotational band structure of low-lying excitations in
small Heisenberg systems, in Advances in Quantum Many-Body Theory, Proceedings of "The 11th International Conferences on Recent Progress in Many-Body Theories", Manchester, edited by Raymond F. Bishop, Tobias Brandes, Klaus A.
Gernoth, Niels R. Walet, and Yang Xian (UMIST, Manchester, UK, 2001), World Scientific
H.-J. Schmidt, J. Schnack, M. Luban, Heisenberg exchange parameters of molecular
magnets from the high-temperature susceptibility expansion, Phys. Rev. B 64 (2001)
224415
A. Müller, M. Luban, C. Schröder, R. Modler, P. Kögerler, M. Axenovich, J. Schnack,
P.C. Canfield, S. Bud'ko, and Neil Harrison, Classical and Quantum Magnetism in
Giant Keplerate Magnetic Molecules, Chem. Phys. Chem. 2 (2001) 517
D. Mentrup, J. Schnack, Nose-Hoover dynamics for coherent states, Physica A 297
(2001) 337-347
H.-J. Schmidt, J. Schnack, M. Luban, Bounding and approximating parabolas for the
spectrum of Heisenberg spin systems, Europhysics Letters 55 (2001) 105 –111
5
H.-J. Schmidt, M. Luban, Continuous families of isospectral Heisenberg spin systems
and the limits of inference from measurements, J. Phys. A: Math. Gen. 34 (2001)
2839-2858
J. Schnack, M. Luban, Rotational modes in molecular magnets with antiferromagnetic
Heisenberg exchange, Phys. Rev. B 63 (2001) 014418
J. Schnack, Properties of the first excited state of nonbipartite Heisenberg spin rings,
Phys. Rev. B 62 (2000) 14855-14859
H.-J. Schmidt, F. Homann, Photon Stars, General Relativity and Gravitation (GRG)
Vol. 32, No. 5 (May 2000) 919 – 931
H. Feldmeier, J. Schnack, Molecular Dynamics for Fermions, Rev. Mod. Phys. 72
(2000) 655-688
K. Bärwinkel, H.-J. Schmidt, J. Schnack, Ground state properties of antiferromagnetic
Heisenberg spin rings, Journal of Magnetism and Magnetic Materials 220 (2000) 227
D. Mentrup, H.-J. Schmidt, J. Schnack, M. Luban, Transition from quantum to classical Heisenberg trimers: Thermodynamics and time correlation functions,
Physica A 278 (2000) 214-221
Y. Furukawa, M. Luban, F. Borsa, D.C. Johnston, A.V. Mahajan, L.L. Miller, D.
Mentrup, J. Schnack, A. Bino, Spin dynamics of the magnetic cluster
[Cr_4S(O_2CCH_3)_8(H_2O)_4](NO_3)_2H_2O, Phys. Rev. B 61 (2000) 8635
K. Bärwinkel, H.-J. Schmidt, J. Schnack, Structure and relevant dimension of the
Heisenberg model and applications to spin rings, Journal of Magnetism and Magnetic
Materials 212 (2000) 240-250
6
Apl. Prof. Dr. Klaus Betzler
Forschungsübersicht
Unsere Forschungsgruppe ‚Nichtlineare Optik’ bearbeitet Problemstellungen, die mit
elektrooptischen und nichtlinear optischen Anwendungen zusammenhängen. Unter
anderem werden tensorielle optische Eigenschaften von neuen Materialien für diesen
Anwendungsbereich experimentell untersucht und theoretisch oder numerisch
modelliert. Beispiele solcher Tensoreigenschaften sind die lineare und nichtlineare
Suszeptibilität (Tensoren 2. bzw. 3. Stufe). Materialien mit günstigen linearen und
nichtlinearen optischen Eigenschaften werden benötigt, um effiziente Lichtquellen für
neue Wellenlängenbereiche zu realisieren (Frequenzkonversion). Eine weitere
Zielsetzung ist die Entwicklung und Anwendung neuer Messverfahren – vornehmlich
zerstörungsfreier optischer Verfahren – zur Kristallcharakterisierung. Im Zeitraum der
vergangenen drei Jahre wurden die folgenden Schwerpunkte bearbeitet:
 Untersuchung von neuen nichtlinear optischen Kristallen,
 Numerische Modellierung der optischen Eigenschaften verschiedener Kristalle,
 Kristallcharakterisierung durch nichtkollineare Frequenzverdopplung, Entwicklung
von automatisierten Auswerteverfahren,
 Optische und dielektrische Eigenschaften von undotiertem Strontium-BariumNiobat (SBN) in Abhängigkeit von der Zusammensetzung,
 Optische Frequenzverdopplung an SBN in der Nähe des Phasenübergangs,
 Optische Eigenschaften von reinen und dotierten Gläsern und Keramiken.
Forschung im Kolleg
Im Kolleg betreuen wir die Projekte ‚Growth and characterization of nonlinear SBN
crystals’ (zusammen mit Dr. R. Pankrath) und ‚Optical nonlinearities near the phase
transition of SBN’. Im ersten Projekt (Stipendiat M. Ulex) werden undotierte SBNKristalle mit unterschiedlicher Zusammensetzung hergestellt. Es geht unter anderem
darum, den Existenzbereich ‚x’ des Mischkristallsystems SrXBa1-XNb2O6 genauer zu
definieren und das Phasendiagramm auszumessen. Der Schwerpunkt des zweiten
Projekts (Stipendiat A. Tunyagi) liegt auf den nichtlinearen Eigenschaften dieser
Kristalle. SBN ist bei Zimmertemperatur im gesamten Zusammensetzungsbereich
azentrisch (Raumgruppe P4bm) und zeigt hohe nichtlineare Suszeptibilitäten. Bei
höheren Temperaturen erfolgt ein Phasenübergang in eine zentrosymmetrische
Phase. Dieser Übergang lässt sich sehr gut durch die Messung der optischen
Frequenzverdopplung (SHG) charakterisieren. Bei Temperaturen unterhalb des
Phasenübergangs zeigen sich neuartige, bisher nicht bekannte SHG-Effekte, deren
Ursache (SHG-Interferenzen aus antiparallelen ferroelektrischen Domänen) geklärt
werden konnte. Nähere Informationen zu den beiden Projekten enthalten die
Berichte der Stipendiaten.
Kooperationen im Kolleg
Die beiden Projekte werden in enger Zusammenarbeit mit den Forschungsgruppen
von E. Krätzig und M. Wöhlecke sowie mit der Abteilung Kristallzüchtung (H. Hesse,
R. Pankrath) durchgeführt. Eine Kooperation zur Berechnung linearer und
nichtlinearer Eigenschaften optischer Materialien besteht mit Dr. Dongfeng Xue
(zurzeit: National Institute for Materials Science, Tsukuba, Japan).
7
Publikationen im Zusammenhang mit dem Graduiertenkolleg
K. Betzler, H. Hesse, R. Jaquet, D. Lammers: Optical second-harmonic generation in
lead formate. J. Appl. Physics 87, 22 (2000).
D. Xue, K. Betzler, H. Hesse, D. Lammers, S. Zhang: Theoretical studies of nonlinear
optical properties of compounds K4Ln2(CO3)3F4 (Ln=Pr, Nd, Sm, Eu, Gd). J. Appl.
Physics 87, 2849 (2000).
D. Xue, K. Betzler, H. Hesse: Structural characteristics and second order nonlinear
optical properties of borate crystals. Trends in Optics and Photonics Series 34, 542
(2000).
D. Xue, K. Betzler, H. Hesse: Dielectric constants of binary rare-earth compounds. J.
Phys.: Condens. Matter 12, 3113 (2000).
D. Xue, K. Betzler, H. Hesse, D. Lammers: Nonlinear optical properties of borate
crystals. Solid State Communications 114, 21 (2000).
D. Xue, K. Betzler, H. Hesse: Chemical bond analysis of the second order nonlinear
optical behavior of Mg-doped lithium niobate. J. Phys.: Condens. Matter, 12, 6245
(2000).
D. Xue, K. Betzler, H. Hesse: Dielectric properties of lithium niobate-tantalate crystals. Solid State Communications, 115, 581 (2000).
D. Xue, K. Betzler, H. Hesse: Chemical bond analysis of the second order nonlinear
optical behavior of Zn-doped lithium niobate. Optics Communications, 182, 167
(2000).
D. Lammers, K. Betzler, D. Xue, J. Zhao: Optical Second-Harmonic Generation in
Benzophenone. physica status solidi (a) 180/2, R5 (2000).
D. Xue, K. Betzler, H. Hesse: Dielectric properties of I-III-VI2 type chalcopyrite semiconductors. Physical Review B15, 62, 13546 (2000).
D. Xue, K. Betzler, H. Hesse, D. Lammers: Temperature dependence of the dielectric
response of lithium niobate. J. Phys. Chem. Solids 62, 973 (2001).
D. Xue, K. Betzler, H. Hesse: Second order nonlinear optical properties of In-doped
lithium niobate. J. Appl. Phys. 89, 849 (2001).
D. Xue, K. Betzler, H. Hesse: Induced Li-site vacancies and nonlinear optical behavior of doped lithium niobate crystals. Optical Materials 16, 381 (2001).
D. Xue, K. Betzler: Influence of optical-damage resistant dopants on the nonlinear
optical properties of lithium niobate. Applied Physics B 72, 641 (2001).
D. Xue, K. Betzler, H. Hesse, H. Ratajczak: Theoretical analysis of the chemical
bonding behavior and the dielectric properties of different phases of ice. Bulletin of
the Polish Academy of Sciences Chemistry 49, 289 (2001).
D. Xue, K. Betzler, H. Hesse, D. Lammers: Linear and nonlinear optical susceptibilities of orthorhombic rare earth molybdates RE2(MoO4)3. J. Phys. Chem. Solids 63,
359 (2002).
D. Xue, K. Betzler, H. Hesse, H. Ratajczak: Chemical bond analysis on second order
nonlinear optical properties of Na2SeO4·H2SeO4·H2O. Bulletin of the Polish Academy
of Sciences Chemistry 50, 289 (2002).
D. Xue, K. Betzler, H. Hesse: Chemical bond analysis of the nonlinear optical properties of the borate crystals LiB3O5, CsLiB6O10, and CsB3O5. Applied Physics A 74,
779 (2002).
K.-U. Kasemir, K. Betzler: Detecting ellipses of limited eccentricity in images with
high noise levels. Image and Vision Computing, 21, 223 (2002).
Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke:
Composition dependence of the ultraviolet absorption edge in lithium tantalate.
J. Appl. Physics, in print (2003).
8
Prof. Dr. Karsten Buse
Forschungsübersicht
Karsten Buse hat sich an der Universität Osnabrück in der Gruppe von Herrn Prof.
Krätzig habilitiert und 2000 zwei Rufe auf C4-Professuren erhalten. Er nahm dann
einen Ruf an die Universität Bonn auf die Heinrich-Hertz-Stiftungsprofessur der
Deutschen Telekom AG an. Seine Arbeitsgruppe ist Zug um Zug bis Mitte 2002 nach
Bonn umgezogen. Das Arbeitsgebiet gliedert sich in drei Teile: Holographie,
Nichtlineare Optik und Nahfeldoptik.
Forschung im Kolleg
Periodisch-gepolte Lithiumniobat- und Lithiumtantalat-Kristalle (LiNbO3 und LiTaO3)
sind von sehr großem Interesse für die nichtlineare Optik, da in diesen Kristallen für
fast jede Wellenlängenkombination die Phasenanpassungsbedingung erfüllt werden
kann. Zusammen mit den großen nichtlinearen Koeffizienten entsteht daraus die
Möglichkeit, Frequenzverdoppler und optische parametrische Oszillatoren zu
realisieren. Dieses geschieht zur Zeit weltweit in zahlreichen Universitäts- und
Industrie-Laboren. Zwei Materialprobleme erschweren jedoch den Einsatz der
Kristalle: Zum einen tritt bei Beleuchtung mit intensivem sichtbaren Licht "optical
damage"
auf.
Darunter
werden
unerwünschte
Brechungsindexund
Absorptionsänderungen verstanden. Zum anderen bereitet die Erzeugung von
Domänenmustern mit kleinen Periodenlängen (unter 3 μm) große technische
Probleme. Das Ziel des im Graduiertenkolleg verfolgten Projekts ist, Lösungen für
diese Probleme aufzuzeigen und zu erproben, damit bessere optische parametrische
Oszillatoren realisiert werden können.
Das "optical damage" beruht auf der lichtinduzierten Umverteilung von Ladungen, die
dann von Störstellen eingefangen werden. Ein Lösungsansatz ist die starke
Protonierung des Materials. Das erzeugt eine ionische Dunkelleitfähigkeit, die alle
lichtinduzierten Raumladungsfelder kurzschließt. Ein weiterer Ansatz ist die sehr
hohe Dotierung mit Übergangsmetallen wie z. B. Eisen. In geringen Konzentrationen
erhöht diese Dotierung das "optical damage", in großen Konzentrationen bildet die
Dotierung aber ein Störstellen-Band mit sehr hoher Leitfähigkeit, wodurch die
Raumladungsfelder ebenfalls kurzgeschlossen werden.
Zur Strukturierung der Domänen wollen wir homogene elektrische Felder und
räumlich modulierte Lichtmuster einsetzen. Daher haben wir zunächst sehr genau
das ferroelektrische Verhalten insbesondere von LiNbO3-Kristallen studiert. Ein
Ergebnis ist, dass die Koerzitivfeldstärke nach einem Umpolungsvorgang zunächst
reduziert ist und nach ca. 30 s praktisch den alten Wert erreicht. D. h. innerhalb
dieser Zeit lassen sich die Domänen etwas leichter in die ursprüngliche Richtung
zurückpolen. Eine Beeinflussung der Geschwindigkeit, mit der die Koerzitivfeldstärke
auf den alten Wert zurückgeht, durch Beleuchtung konnte noch nicht nachgewiesen
werden. Hier versprechen Lichtpulse mit besonders hohen Intensitäten Fortschritte.
Der Effekt ist so wichtig, weil sich damit die Koerzitivfeldstärke entsprechend dem
Lichtmuster räumlich modulieren lassen sollte. Dann können homogene elektrische
Felder zu dem gewünschten Domänenmuster führen.
Durch Beleuchtung mit einem ultravioletten Lichtmuster, das während mehrerer
Polungszyklen kontinuierlich vorhanden war, wurde jedoch bereits eine semipermanente räumliche Modulation der Koerzitivfeldstärke erreicht. Wird ein
Polungszyklus dann im richtigen Moment abgebrochen, so stellt das Domänenmuster
eine Replik des Lichtmusters dar. Die erzielten Strukturlängen liegen noch im Bereich
9
30 μm, und es ist zu prüfen, ob das Verfahren auch zu feineren Strukturen führen
kann.
Bei der Untersuchung des ferroelektrischen Verhaltens ist ein neuer Effekt
aufgefallen: Die Beleuchtung der Kristalle während des Polungsvorgangs führt hinter
der Probe zur Auffächerung des Strahls, so dass ein 6-zahniger Stern entsteht.
Dieser rührt von einer Lichtablenkung an den Domänenwänden her und erlaubt es,
viel Information über den Polungsvorgang und die Beeinflussung der Polung mit Licht
in-situ zu erhalten.
Kooperationen im Kolleg
Obwohl Herr Dipl.-Phys. Manfred Müller, der mit einem Stipendium des
Graduiertenkollegs an dem Forschungsprojekt arbeitet, seit 2001 in Bonn tätig ist, hat
seine Arbeit sehr von den Kooperationen im Graduiertenkolleg profitiert. Einige
Beispiele: Für die Realisierung der Wasserstoff-Eindiffusion wird ein feldunterstütztes
Verfahren eingesetzt. Hier haben Herr Prof. Kapphan und seine Mitarbeiter mit ihrer
Erfahrung sehr geholfen, dass eine entsprechende Anlage gebaut wurde. Bei einigen
Dotierungen spielen Kristallfeldübergänge eine wesentliche Rolle. Dabei haben wir
sehr von Diskussionen mit Herrn apl. Prof. Wöhlecke profitiert. Das Projekt und seine
Ergebnisse wurden mehrfach im Rahmen des regelmäßigen Seminars des
Graduiertenkollegs vorgestellt. Die sich daran anschließenden Diskussionen haben
viele wertvolle Impulse erbracht. Insbesondere Diskussionen mit Herrn Prof. Krätzig,
die auch außerhalb des Seminars bei mehreren Gelegenheiten stattgefunden haben,
waren sehr hilfreich. Und die Kristallzuchtabteilung, Herr Dr. Pankrath und Herr Dr.
Hesse, haben mit präparativen Tipps und Hilfeleistungen ebenfalls viel zum Erfolg
des Projekts beigetragen.
Publikationen im Zusammenhang mit dem Gradiertenkolleg (ab 2000)
1. I. Nee, M. Müller, K. Buse, E. Krätzig, "Role of iron in lithium-niobate crystals for
the dark-storage time of holograms", J. Appl. Phys. 88, 4282-4286 (2000)
2. I. Nee, M. Müller, and K. Buse, "Development of thermally fixed photorefractive
holograms without light", Appl. Phys. B 72, 195-200 (2001)
3. M. Wengler, M. Müller, E. Soergel, and K. Buse, "Dynamics of ferroelectric domain reversal in lithium niobate crystals", Appl. Phys. B, accepted
4. M. Müller, E. Soergel, M. Wengler, and K. Buse, "Star-patterns - a new diffraction
phenomenon from domain structures in lithium niobate crystals", submitted
5. M. Müller, E. Soergel, M. Falk, J. Hukriede, and K. Buse, "Reduction of optical
damage in lithium niobate crystals by hydrogen loading", in preparation
10
Prof. Dr. Peter Hertel
Wegen der Übernahme des Amtes des Vizepräsidenten der Universität Osnabrück
im Herbst 2000 konnte das Graduiertenkolleg lediglich durch die Vorlesung
„Nonlinear response theory“ im WS 2001/02 sowie durch Diskussionen und
Beratungen unterstützt werden.
11
Prof. Dr. Siegmar Kapphan
Forschungsübersicht
Die Arbeitsgruppe "Laseroptik" beschäftigt sich mit störstellenrelevanten Problemen,
insbesondere in oxidischen Kristallen die für photorefraktive und laseroptische
Anwendungen von Interesse sind. Mit Fourier-IR Spektroskopie UV-VIS
Absorptionsmessungen, Lumineszenz und optischer Frequenzverdopplung werden
in einem großen Temperatur- und Spektralbereich die physikalischen Eigenschaften
und das Zusammenspiel von Störstellen und Wirtsgitter untersucht. Vor allem die
Fourier IR-Spektroskopie hat sich in den letzten Jahren für die Untersuchung von
lichtinduzierten polaronischen Zuständen als sehr nützlich erwiesen, da diese
Polaronen charakteristische optische Übergänge in Nah-Infrarot Bereich besitzen.
Forschung im Kolleg
Das Forschungsprojekt im Kolleg konzentriert sich auf lichtinduzierte
Absorptionseffekte in SrxBa1-xNb2O6 (SBN) und in Ba1-yCayTiO3 (BCT) Kristallen.
Diese Kristalle können in der Kristallzucht (Dr. Pankrath) in einer kongruenten
Zusammensetzung
(x = 0,61 für SBN und y = 0,23 für BCT) gezogen werden. In dieser
Zusammensetzung sind große homogene Kristalle hoher Qualität möglich, die für
Anwendungen gute Voraussetzungen bietet. Die photorefraktiven Eigenschaften
können durch passende Dotierungen (z. B. Cer und Chrom) erhöht und optimiert
werden. Ein lichtinduzierter Ladungstransport von den Dotierungsstörstellen zu
polaronischen Zentren, der vereinfacht als Cr3+ + Nb5+ ↔ Cr4+ + Nb4+ (z.B. für
SBN:Cr) skizziert werden kann, ist als erster Schritt in der Kette identifiziert worden
die zum Aufbau von Raumladungsfeldern führt, die den Brechungsindex
modifizieren. Die Nb4+-Elektronenpolaronen (Ti3+-Polaronen in BCT) besitzen
charakteristische breite Absorptionen im NIR deren Eigenschaften und nichtlineare
Abhängigkeiten von Lichtintensität, Dotierung und Temperaturen es zu beschreiben
gilt. Neben den NIR-Absorptionen treten weitere breite Absorptionen in VIS-Bereich
auf, deren Natur sich noch in der Diskussion befindet. Beide Zentrenarten zeigen bei
Raumtemperatur eine hohe thermisch-induzierte Beweglichkeit, die zu einer raschen
Abnahme der Zentrenkonzentration in den beleuchteten Zonen führt. Dieses
thermisch-induzierte "Hopping" ist bei tiefen Temperaturen unterdrückt, so dass im
Tieftemperaturbereich hinreichend große polaronische Zentrenkonzentration
spektroskopisch untersucht werden können. Mit Frau Inna Kislova konnten wir
kürzlich (2002) qualitative Untersuchungen zu einer erstmals beobachteten Kr+-Laser
induzierten Dissoziation von VIS-Zentren durchführen, die als Dissoziationsprodukt
freie Nb4+ (bzw. Ti3+) Polaronen-Absorptionen ergab und damit erste experimentelle
Hinweise auf die Natur dieser VIS-Zentren. Frau Kislova beendete ihren Aufenthalt in
Deutschland aus persönlichen, familiären Gründen und das Promotionsvorhaben
musste daher vorzeitig abgebrochen werden (Okt. 2002).
Die Untersuchungen zu diesem Zentren und insbesondere zu der Dissoziation der
VIS-Zentren sollen mit Herrn A. Goubaev fortgesetzt (Beginn 12/02) und mit
Methoden der Photoleitfähigkeit und des Photo-Hall-Effektes ergänzt werden, um
eine Klärung der lichtinduzierten Vorgänge und ihres Einflusses auf Aufbau
(Einschreiben) und Speicherung von photorefraktiven Prozessen in diesen
Materialien zu erzielen.
Kooperationen im Kolleg
Eine enge Zusammenarbeit besteht insbesondere mit der Abteilung Kristallzüchtung
(H. Hesse, R. Pankrath), die die benötigten dotierten Kristalle in hoher Qualität
12
herzustellen vermögen. Darüber hinaus bestehen enge Kontakte mit den
Forschungsgruppen von E. Krätzig, K. Betzler und M. Wöhlecke für die zum Teil
auch Messungen im IR Bereich mit dem Fourierspektrometer durchgeführt werden.
Eine Kooperation zur theoretischen Modellbetrachtung von polaronischer Zentren
und zu den nichtlinearen Eigenschaften optischer Materialien besteht mit Prof. Dr. V.
Vikhnin und Prof. V. Trepakov, Ioffe Inst., RAS, St. Petersburg, Russland
Publikationen im Zusammenhang mit dem Graduiertenkolleg
M. Gao, S. Kapphan, R. Pankrath, J. Zhao, ‘NIR-Absorption of Nb4+-Polarons in Reduced SBN-Crystals’, phys. stat. sol. (b), Vol. 217, 999 (2000)
V. Vikhnin, S. Kapphan, H. Liu, W. Jia, V. Trepakov, L. Jastrabik, ‘Polaron and
Charge Transfer Vibronic Exciton Phenomena in Ferroelectrics’, Ferroelectrics, 237,
81-88 (2000)
I. I. Tupitsyn, A. Deineka, V. Trepakov, L. Jastrabik and S. Kapphan, ‘Li-Doping Effect on the Energy Structure of KTaO3’, Ferroelectrics, Vol. 237, 9-16 (2000)
V. Trepakov, A. Skvortsov, S. Kapphan, L. Jastrabik and V. Vorlíček, ‘Comparative
Studies of Luminescence in Congruent and Stoichiometric VTE-Treated LiNbO3:Cr’,
Ferroelectrics, Vol. 239, 297-304, 1167 - 1174 (2000)
Ming Gao, S. Kapphan, R. Pankrath, Xiqi Feng, Yuanfen Tang, V. Vikhnin, ‘Lightinduced VIS-absorption and light-induced charge transfer in pure and doped SBN
crystals’, J. of Phys. Chem. Sol., 61, 1775-1787 (2000)
M. Gao, S. Kapphan, R. Pankrath, ‘Photoluminescence and thermoluminescence in
SBN:Cr crystals’, J. of Phys. Chem. Sol., 61, 1959-1971 (2000)
M. Gao, S.Kapphan, R. Pankrath, J. Zhao, ‘NIR Absorption of Nb4+ Polarons in reduced SBN crystals’, Ferroelectrics, 239, 251-256, 1121 - 1126 (2000)
S. A. Basun, A. A. Kaplyanskii, A. B. Kutsenko, V. Dierolf, T. Tröster, S. E. Kapphan,
and K. Polgar, ‚Dominant Cr3+ Centers in LiNbO3 under Hydrostatic Pressure’ Phys.
of Sol. State, Vol. 43, No. 6, 1043-1051 (2001)
V. S. Vikhnin, R. I. Eglitis, E. A. Kotomin, S. Kapphan, G. Borstel, ‚New PolaronicType Excitons in Ferroelectric Oxides: INDO-Calculations and Experimental Manifestation’, Mat. Res. Soc. Symp. Proc., Vol. 677, AA 4.15.1 (2001)
Zhao Jian-Lin, Wang Bin, Wu Jian-Jun, Yang De-Xing, S. Kapphan, R. Pankrath, ‚Investigation of photorefractive Two-Wave Coupling in Cr-doped Strontium Barium Niobate Crystal’, Chin. Phys. Soc., Vol. 10, 739 – 742 (2001)
V. S. Vikhnin, R. I. Eglitis, S. E. Kapphan, E. A. Kotomin, G. Borstel, ‚A new phase in
ferroelectric oxides: The phase of charge transfer vibronic excitons’, Europhys. Lett.,
56, 702 – 708 (2001)
S. A. Basun, A. A. Kaplyanskii, A. B. Kutsenko, V. Dierolf, T. Troester, S. E. Kapphan, K. Polgar, ‚Optical characterization of Cr3+ centers in LiNbO3’, Appl. Phys. B,
73, 453 – 461 (2001)
Zhao Jianlin, Wu Jianjun, Wang Bin, Yang Dexing, S. Kapphan, R. Pankrath, ‚Image
Edge-Enhancement using photorefractive Two-Wave Coupling in Cr:SBNCrystal’,
Acta Optica Sinica, Vol. 21, No. 11, 1343 (2001)
M. Wierschem, T. Lindemann, R. Pankrath, S. E. Kapphan, ‚NIR Absorption and
light-induced charge transfer in photorefractive Ba0.77Ca0.23TiO3 crystals doped with
iron’, Ferroelectrics, Vol. 264, pp. 315-324 (2001)
13
V. S. Vikhnin, S. E. Kapphan, ‚Local Configuration instability as an origin of RelaxorType Properties of Ferroelectric solid solutions SBN, SCT and KLTN, Ferroelectrics
Letters’, Vol. 28(5-6), pp. 123-133 (2001)
V. S. Vikhnin, A. A. Kaplyanskii, A. B. Kutsenko, G. K. Liu, J. V. Beitz, S. E. Kapphan,
,"Charge transfer-lattice" clusters induced by charged impurities’, Journal of Luminescence 94-95, 775-779 (2001)
V. S. Vikhnin, I. Kislova, A. B. Kutsenko, S. E. Kapphan, ‚Charge transfer vibronic
excitons and excitonic-type polaron states: photoluminescence in SBN’, Solid State
Comm., 121, 83 – 88 (2002)
V. S. Vikhnin, R. I. Eglitis, S. E. Kapphan, G. Borstel, E. A. Kotomin, ‚Polaronic-type
excitons in ferroelectric oxides: Microscopic calculations and experimental manifestation’, Physical Review B. Vol. 65, 104304 (2002)
V. S. Vikhnin, R. I. Eglitis, S. E. Kapphan, ,Charge Transfer Vibronic Excitons in Incipient Ferroelectrics and Related Problems’, Ferroelectrics, Vol. 265, pp. 177 –178
(2002)
R. I. Eglitis, V. S. Vikhnin, E. A. Kotomin, S. E. Kapphan, G. Borstel, ,Theoretical
Prediction and Experimental Confirmation of Charge Transfer Vibronic Excitons and
Their phase in ABO3 Perovskite Crystals’, Mat. Res. Soc. Symp. Proc., Vol. 718
(2002)
I. L. Kislova, M. Gao, S. E. Kapphan, R. Pankrath, A. B. Kutsenko, V. S. Vikhnin,
,Photo- and Thermoluminescence in congruent SBN Crystals doped with Ce and Cr’,
Ferroelectrics, Vol. 273, pp. 187-192 (2002)
V. S. Vikhnin, S. Avanesyan, H. Liu, S. E. Kapphan, ,An origin of light induced centers in the visible range in ferroelectric oxides: possible role of the states of charge
transfer vibronic excitons’, Journal of Physics and Chemistry of Solids, Vol. 63, 16771683 (2002)
Z. Bryknar, V. Trepakov, Z. Potucek, L. Jastrabik, S. Kapphan, ‘Photoluminescence
Spectroscopy of Chromium doped Cd2Nb2O7’, Ferroelectrics, Vol. 272, 363 - 368
(2002)
V. S. Vikhnin, R. Blinc, R. Pirc, S. E. Kapphan, I. L. Kislova, P. A. Markovin, ‘A Model
of Polar Clusters in Ferroelectric Relaxors of PMN-Type: Polaronic and Charge
Transfer Effects’, Ferroelectrics, Vol. 268, 257 – 262 (2002)
R. I. Eglitis, E. A. Kotomin, V. A. Trepakov, S. E. Kapphan and G. Borstel, ‚Quantum
chemical modelling of electron polarons and ‚green’ luminescence in PbTiO3 perovskite crystals’, J. Phys.: Condens. Matter, 14, L 647 – L 653 (2002)
D. Millers, L. Grigorjeva, V. Pankratov, V. A. Trepakov, S. E. Kapphan, ‘Pulsed electron beam excited transient absorption in SrTiO3’, NIM B, 194, 469 – 473 (2002)
A. G. Badalyan, P. G. Baranov, V. A. Trepakov, C. B. Azioni, P. Gabinetto, M. C.
Mozzati, L. Jastrabik, J. Rosa, M. Hrabovský, ‘Recent researches of the Copper Centres in Potassium Tantalate Single Crystals’, Ferroelectrics, Vol. 272, 205 – 210
(2002)
14
Prof. Dr. Detlef Kip
Forschungsübersicht
Nach der Habilitation in der Arbeitsgruppe Elektrooptik von E. Krätzig im Jahre 1999
hat Detlef Kip im Sommer 2002 einen Ruf auf eine Professur für Optische
Technologien an die TU Clausthal angenommen. Die Forschung der noch in
Osnabrück durchgeführten Arbeiten sowie der in Clausthal im Aufbau befindlichen
Abteilung Optische Technologien beschäftigt sich mit verschiedenen Bereichen der
Photonik. In einem ersten Schwerpunkt geht es um die Entwicklung und Optimierung
oxidischer Kristalle. Für Anwendungen im Bereich der Integrierten Optik entwickeln
wir Wellenleiter, z.B. in den Substratmaterialien Lithiumniobat, Lithiumtantalat und
Strontium-Barium-Niobat. Unser besonderes Interesse gilt hier der Entwicklung von
schmalbandigen integriert-optischen Wellenlängenfiltern für Anwendungen in der
optischen Nachrichtentechnik und der effizienten Frequenzverdopplung in optischen
Wellenleitern. In einem weiteren Schwerpunkt geht es um die Erzeugung und
Untersuchung von optischen räumlichen Solitonen. Konkret untersuchen wir hierbei
die Existenzbereiche und Wechselwirkungseigenschaften solcher räumlicher
Solitonen in photorefraktiven Kristallen, die in diesem Zusammenhang ein
ausgezeichnetes, experimentell vergleichsweise leicht zugängliches Modellsystem
für die universellen Eigenschaften von Solitonen darstellen.
Forschung im Kolleg
Im Graduiertenkolleg wird von uns das Themengebiet der optischen
Frequenzverdopplung (SHG) in oxidischen Wellenleitern bearbeitet. Hierbei
konzentrieren wir uns auf die Kristalle LiNbO3 und LiTaO3, die kommerziell als Wafer
in hervorragender optischer Qualität erhältlich sind. In beiden Materialien ist für eine
effiziente Frequenzverdopplung und der Nutzung des größten SHG-Koeffizienten d33
eine Quasi-Phasenanpassung durch räumlich periodische Modulation der
ferroelektrischen Domänen notwendig. Das Schalten der Domänen geschieht mit
Hilfe von Hochspannungspulsen und lithographisch strukturierten Fingerelektroden
mit Gitterperioden von einigen Mikrometern. Die Wellenleiterherstellung erfolgt durch
Eindiffusion von Titanstreifen in das Substratmaterial. Besonderes Augenmerk gilt
der Vermeidung von photorefraktiven Effekten im Wellenleiter. Hierzu werden
Substratmaterialien mit veränderter, nahezu stöchiometrischer Zusammensetzung
durch VTE-Behandlung (Vapor Transport Equilibration) untersucht. Nähere
Informationen zu den laufenden und geplanten Arbeiten enthält der entsprechende
Bericht.
Kooperationen im Kolleg
Die Untersuchungen in diesem Projekt werden in enger Zusammenarbeit mit den
Forschungsgruppen von E. Krätzig, M. Imlau und K. Buse (jetzt Universität Bonn)
sowie mit der Abteilung Kristallzüchtung (H. Hesse, R. Pankrath) durchgeführt.
Publikationen in Zusammenhang mit dem Graduiertenkolleg
V. Shandarov, M. Wesner, J. Hukriede und D. Kip: „Observation of dark spatial
photovoltaic solitons in planar waveguides in lithium niobate“. J. Opt. A: Pure Appl.
Opt. 2, 500 (2000)
15
J. Hukriede, D. Kip und E. Krätzig: “Investigation of titanium- and copper-indiffused
channel waveguides in lithium niobate and their application as holographic filters for
infrared light”. J. Opt. A: Pure Appl. Opt. 2, 481 (2000)
M. Wesner, C. Herden, D. Kip und P. Moretti: “Photorefractive steady-state solitons
up to telecommunication wavelengths in planar SBN waveguides”. Opt. Commun.
188, 69 (2001)
M. Wesner, C. Herden und D. Kip: “Electrical fixing of waveguide channels in srontium-barium niobate crystals”. Appl. Phys. B 72, 733 (2001)
M. Wesner, C. Herden, R. Pankrath, D. Kip und P. Moretti: “Temporal development of
photorefractive solitons up to telecommunication wavelengths in SBN”. Phys. Rev. E
64, 36613 (2001)
J. Hukriede, D. Kip und E. Krätzig: "Permanent narrow-band reflection holograms for
infrared light recorded in LiNbO3:Ti:Cu channel waveguides". Appl. Phys. B 72, 749
(2001)
D. Kip, C. Herden und M. Wesner: “All-optical signal routing using interaction of mutually incoherent spatial solitons”. Ferroelectrics 274, 135 (2002)
J. Xu, V. Shandarov, M. Wesner und D. Kip: “Observation of two-dimensional spatial
solitons in iron-doped barium-calcium titanate crystals”. phys. stat. sol. (a) 189, R4
(2002)
J. Imbrock, A. Wirp, D. Kip und E. Krätzig: “Photorefractive properties of lithium and
copper in-diffused lithium niobate crystals”. J. Opt. Soc. Am. B 19, 1822 (2002)
16
Prof. Dr. Eckhard Krätzig
Forschungsübersicht
Unserer Gruppe „Angewandte Physik: Elektrooptik“ beschäftigt sich mit den
Schwerpunkten „Photorefraktive Effekte“ und „Integrierte Optik“. Photorefraktive
Effekte braucht man zur Aufzeichnung von Volumenphasenhologrammen, die etwa
zur Speicherung von Information, zur optischen Phasenkonjugation, zur
Lichtverstärkung und Oszillation, zur Bild- und Signalverarbeitung, zur
holographischen Interferometrie oder zur Filterung herangezogen werden können.
Die Integrierte Optik zielt darauf ab, möglichst viele optische Komponenten auf einem
gemeinsamen Substrat zu vereinen. In elektrooptischen Materialien kann man mit
verschiedenen Methoden - der Eindiffusion, der Ionenimplantation oder dem
Ionenaustausch - Wellenleiter mit relativ geringer Dämpfung erzeugen. Im
Berichtszeitraum haben wir folgende Themen bearbeitet (http://www.physik.uniosnabrueck.de/elektrooptik):
 Der lichtinduzierte Ladungstransport in elektrooptischen Kristallen
 Holographische Streuung, Lichtverstärkung und Oszillation
 Photorefraktives Schreiben mit IR-Licht und Hologrammstabilisierung
 Reduktion lichtinduzierter Brechungsindexänderungen
 Raumladungswellen in photorefraktiven Kristallen
 Photorefraktive Wellenleiter
 Solitonen in photorefraktiven Kristallen
Forschung im Kolleg
Im Kolleg behandeln wir die Projekte „Raumladungswellen in photorefraktiven
Kristallen“ und „Nichtlineare optische Eigenschaften photorefraktiver SBN-Kristalle“.
Beim ersten Projekt (Stipendiat F. Rahe) liegt der Schwerpunkt auf der Erforschung
nichtlinearer Wechselwirkungen der Raumladungswellen in photorefraktiven
Kristallen, die in Analogie zur „Nichtlinearen Optik“ zur Frequenzverdopplung und zur
optischen Gleichrichtung führen. Daneben treten aber auch Effekte auf, die in der
„Nichtlinearen Optik“ nicht bekannt sind, nämlich räumliche Gleichrichtung ohne
zeitliche Gleichrichtung und räumliche Verdopplung ohne zeitliche Verdopplung. Im
zweiten Projekt (Kollegiatin M. Wesner) geht es um die Untersuchung des attraktiven
photorefraktiven Materials Strontium-Barium-Niobat in Bezug auf aktuelle
Fragestellungen der nichtlinearen Optik und deren Anwendungen. Interessant sind
insbesondere die Erzeugung optischer Solitonen bis zu Wellenlängen von 1.5 µm
und eine neue Methode des elektrischen Fixierens. - Weitere Informationen sind in
den Berichten der Kollegiat(inn)en zu finden.
Kooperationen im Kolleg
Obige Projekte werden in Zusammenarbeit mit den Gruppen von K. H. Ringhofer
sowie K. Betzler, S. Kapphan und M. Wöhlecke durchgeführt. Weiter arbeiten wir in
enger Kooperation mit den früheren Gruppenmitgliedern K. Buse (jetzt Universität
Bonn) und D. Kip (jetzt Universität Clausthal). Die Kristalle erhalten wir von der
Gruppe von H. Hesse und R. Pankrath, die vielfach Proben auf unseren Wunsch hin
modifiziert haben. Weitere Kooperationen gibt es mit M. P. Petrov (Ioffe Institute,
St.Petersburg, Russland), P. Moretti (Universität Lyon, Frankreich), S. Odoulov
(Academy of Sciences, Kiev, Ukraine), V. Shandarov (Universität Tomsk, Russland)
und Jinjun Xu (Universität, Tianjin, VR China).
17
Publikationen in Zusammenhang mit dem Graduiertenkolleg
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S, Odoulov, B. Sturman, E. Krätzig, Seeded and Spontaneous Light Hexagons in
LiNbO3:Fe, Appl. Phys. B 70, 645 (2000)
M. P. Petrov, A. P. Paugurt, V. V. Bryksin, S. Wevering, E. Krätzig, Spatial Rectification of the Electric Field of Space Charge Waves, Phys. Rev. Lett. 84, 5114
(2000)
M. P. Petrov, V. V. Bryksin, V. M. Petrov, S. Wevering, E. Krätzig, Spectra of
Space Charge Waves in Photorefractive Crystals, Technical Digest CLEO Europe, 121 (2000)
J. Hukriede, D. Kip und E. Krätzig: “Investigation of titanium- and copperindiffused channel waveguides in lithium niobate and their application as holographic filters for infrared light”. J. Opt. A: Pure Appl. Opt. 2, 481 (2000)
M. P. Petrov, A. P. Paugurt, V. V. Bryksin, S. Wevering, E. Krätzig, Spatial
Rectification of the Electric Field of Space Charge Waves in Sillenites, Technical
Digest CLEO Europe, 377 (2000)
M. Goul'kov, S. Odoulov, O. Shinkarenko, E. Krätzig, R. Pankrath, Threshold of
Oscillation in a Ring-Loop Phase Conjugator as a Second Order Optical Phase
Transition, Appl. Phys. B 72, 187 (2001)
M. Wesner, C. Herden, D. Kip, E. Krätzig, P. Moretti, Photorefractive Steady State
Solitons up to Telecommunication Wavelengths in Planar SBN Waveguides, Optics Commun. 188, 69 (2001)
M. P. Petrov, A. P. Paugurt, V. V. Bryksin, S. Wevering, E. Krätzig, Dynamic Electrooptic Effect Induced by Space Charge Waves in Sillenites, Optical Materials 18,
99 (2001)
J. Hukriede, D. Kip und E. Krätzig: "Permanent narrow-band reflection holograms
for infrared light recorded in LiNbO3:Ti:Cu channel waveguides". Appl. Phys. B 72,
749 (2001)
M. P. Petrov, V. V. Bryksin, S. Wevering, E. Krätzig, Nonlinear Interactions and
Scattering of Space Charge Waves in Sillenites, Appl. Phys. B 73, 669 (2001)
M. Goulkov, S. Odoulov, Th. Woike, J. Imbrock, M. Imlau, E. Krätzig, C. Bäumer,
H. Hesse, Holographic Light Scattering in Photorefractive Crystals with Local Response, Phys. Rev. B 65, 195111 (2002)
M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Optically Induced Nonlinear Wave Processes in Photorefractive Crystals, Technical Digest IQEC 2002,
375 (2002)
S. Schwalenberg, F. Rahe, E. Krätzig, Recording Mechanisms of Anisotropic Holographic Scattering Cones in Photorefractive Crystals, Optics Commun. 209, 467
(2002)
M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Overall Rectification and
Second Harmonic Generation of Space Charge Waves, Phys. Rev. B 66, 085107
(2002)
J. Imbrock, A. Wirp, D. Kip, E. Krätzig, Photorefractive Properties of Lithium and
Copper In-diffused Lithium Niobate Crystals, J. Opt. Soc. Am. B 19, 1822 (2002)
Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke,
Composition Dependence of the Ultraviolet Absorption Edge in Lithium Tantalate
J. Appl. Phys., in print (2003)
M. P. Petrov, V. V. Bryksin, F. Rahe, C. E. Rüter, E. Krätzig, Space Charge Rectification Effects in Photorefractive Bi12TiO20 Crystals, Optics Commun., submitted
18
Dr. Rainer Pankrath
Forschungsübersicht
Die Arbeitsgruppe Kristallzucht beschäftigt sich mit der Herstellung oxidischer
Kristalle aus Schmelzen oder Schmelzlösungen für photorefraktive Anwendungen
sowie für die optische Frequenzverdopplung (SHG). Dabei lag der Schwerpunkt im
Berichtszeitraum auf:
 Züchtung von BaxCa1-xTiO3 mit verschiedenen Dotierungen für
Grundlagenuntersuchungen und photorefraktive Anwendungen.
 Züchtung von LiTaO3 mit verschiedenen Dotierungen für photorefraktive
Anwendungen sowie von reinen Kristallen für die SHG.
 Vapour transport equilibration-Behandlungen dieser Kristalle mit dem Ziel, das
„optical damage“ zu minimieren.
 Herstellung von Gläsern im System Bi2O3-B2O3 mit verschiedenen
Dotierungen (z.B.: Er, Nd).
 Optimierung
von
Sr2FeMoO6-Keramiken
für
Untersuchungen
der
magnetischen Eigenschaften.
 Züchtung von SrxBa1-xNb2O6 (SBN).
Forschung im Kolleg
Im Rahmen des Projektes „Growth and characterization of SrxBa1-xNb2O6 crystals
with x ranging from 0.2 to 0.8“ (Stipendiat M. Ulex) werden undotierte Sr xBa1-xNb2O6Mischkristalle über den gesamten Bereich der Mischkristallbildung hergestellt. Dabei
geht es unter anderem darum, den Existenzbereich der Mischkristalle abzugrenzen
und das Phasendiagramm des Systems zu bestimmen. Zu diesem Zweck wurde ein
bereits vorhandener Ofen modifiziert, in dem Liquidustemperaturen bestimmt werden
können. Die Zusammensetzung der gezüchteten Kristalle wird mit der
Röntgenfluoreszenz bestimmt.
Während SrxBa1-xNb2O6 mit kongruent schmelzender Zusammensetzung (x=0.61) in
unseren Anlagen mit hoher optischer Qualität gezüchtet werden kann, ist die
optische Qualität von Kristallen mit höherer und niedrigerer Sr-Konzentration in der
Regel deutlich vermindert. Ursache ist die An- oder Abreicherung bestimmter
Komponenten an der Phasengrenze zwischen Kristall und Schmelze. Durch Variation
der Rotationsgeschwindigkeit und der vertikalen Temperaturgradienten in der
Schmelze wird versucht, die Inhomogenitäten zu verringern.
Die Charakterisierung der gezüchteten Kristalle erfolgt in anderen Arbeitsgruppen im
Hause und in Kooperation mit Arbeitsgruppen an anderen Universitäten (siehe
Kooperationen im Kolleg).
Kooperationen im Kolleg
Das Projekt wird in enger Zusammenarbeit mit den Gruppen von K. Betzler und M.
Wöhlecke durchgeführt. Eine Kooperation zur Bestimmung der Gitterkonstanten und
der Sr,Ba-Verteilung zwischen den entsprechenden kristallographischen Positionen
besteht mit Prof. Dr. Schmahl (Ruhr-Universität Bochum). Die spezifische Dichte der
Kristalle wird in Zusammenarbeit mit Prof. Dr. Bohaty (Universität zu Köln) bestimmt.
19
Publikationen in Zusammenhang mit dem Graduiertenkolleg
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J. Dec, W. Kleemann, Th. Woike, R. Pankrath: Phase transitions in
Sr0.61Ba0.39Nb2O6:Ce3+: I. Susceptibility of clusters and domains. Eur. Phys. J.
B14, 627-632 (2000)
J. Dec, W. Kleemann, Th. Woike, R. Pankrath: Phase transitions in
Sr0.61Ba0.39Nb2O6:Ce3+: II. Linear birefringence studies of spontaneous and
precursor polarization. Eur. Phys. J. B14, 633-637 (2000)
Th. Woike, T. Granzow, U. Dörfler, Ch. Poetsch, M. Wöhlecke, R. Pankrath:
Refractive Indices of congruently melting Sr0.61Ba0.39Nb2O6. phys. stat. sol. (a)
186, R13 (2001)
T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.
Kleemann: Influence of pinning effects on the ferroelectric hysteresis in cerium-doped Sr0.61Ba0.39Nb2O6. Phys. Rev. B 6317, art. no. 174101 (2001).
T. Woike, U. Dörfler, L. Tsankov, G. Weckwerth, D. Wolf, M. Wöhlecke,T.
Granzow, R. Pankrath, M. Imlau, W. Kleemann: Photorefractive properties of
Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration.
Appl. Phys. B-Lasers Opt. 72, 661-666 (2001).
W. Kleemann, P. Licinio, T. Woike, R. Pankrath: Dynamic light scattering at
domains and nanoclusters in a relaxor ferroelectric. Phys. Rev. Lett. 86, 60146017 (2001).
T. Woike, T. Granzow, U. Dörfler, C. Pötsch, M. Wöhlecke, R. Pankrath: Refractive indices of congruently melting Sr0.61Ba0.39Nb2O6. Phys. Status Solidi
(a) 186, R13-R15 (2001).
J.L. Zhao, B. Wang, J.J. Wu, D.X. Yang, S. Kapphan, R. Pankrath: Investigation of photorefractive two-wave coupling in Cr-doped strontium barium niobate crystal. Chin. Phys. 10, 739-742 (2001).
J. Dec, W. Kleemann, V. Bobnar, Z. Kutnjak, A. Levstik, R. Pirc, R. Pankrath:
Random-field Ising-type transition of pure and doped SBN from the relaxor into
the ferroelectric state. Europhys. Lett. 55, 781-787 (2001).
M. Wesner, C. Herden, R. Pankrath, D. Kip, P. Moretti: Temporal development
of photorefractive solitons up to telecommunication wavelengths in strontiumbarium niobate waveguides. Phys. Rev. E 6403, art. no. 036613 (2001).
T. Volk, L.Ivleva, P. Lykov, N. Polozkov, V. Salobutin, R. Pankrath, M. Wöhlecke: Effects of rare-earth impurity doping on the ferroelectric and photorefractive properties of strontium-barium niobate crystals. Opt. Mater. 18, 179182 (2001).
R. Blinc, A. Gregorovic, B. Zalar, R. Pirc, J. Seliger, W. Kleemann, S.G. Lushnikov, R. Pankrath: Nb-93 NMR of the random-field-dominated relaxor transition in pure and doped SBN. Phys. Rev. B 6413, art. no. 134109 (2001).
V.V. Gladkii, V.A. Kirikov, E.V. Pronina, T.R. Volk, R. Pankrath, M. Wöhlecke:
Anomalies in the slow polarization kinetics of a ferroelectric relaxor in the temperature region of a diffuse phase transition. Phys. Solid State 43, 2140-2145
(2001).
P. Lehnen, E. Beckers, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric
domains in the uniaxial relaxor system SBN:Ce, Cr and Co. Ferroelectrics 253,
567-575 (2001).
W. Kleemann, V. Bobnar, J. Dec, P. Lehnen, R. Pankrath, S.A. Prosandeev:
Relaxor properties of dilute and concentrated polar solid solutions. Ferroelectrics 261, 707-716 (2001).
20
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P. Lehnen, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric nanodomains
in the uniaxial relaxor system Sr0.61-xBa0.39Nb2O6:Ce-x(3+). Phys. Rev. B 6422,
art. no. 224109 (2001).
T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.
Kleemann: Local electric-field-driven repoling reflected in the ferroelectric polarization of Ce-doped Sr0.61Ba0.39Nb2O6. Appl. Phys Lett. 80, 470-472 (2002).
W. Kleemann, J. Dec, P. Lehnen, R. Blinc, B. Zalar, R. Pankrath: Uniaxial relaxor ferroelectrics: The ferroic random-field Ising model materialized at last.
Europhys. Lett. 57, 14-19 (2002).
T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.
Kleemann: Evidence of random electric fields in the relaxor-ferroelectric
Sr0.61Ba0.39Nb2O6. Europhys. Lett. 57, 597-603 (2002).
P. Lehnen, J. Dec, W. Kleemann, T. Woike, R. Pankrath: Domain response
features of SBN:Ce. Ferroelectrics 268, 533-538 (2002).
W. Kleemann, J. Dec, R. Blinc, B. Zalar, R. Pankrath: Random fields at transitions from relaxor to glassy and ferroelectric states. Ferroelectrics 267, 157164 (2002).
W. Kleemann, J. Dec, S. Miga, T. Woike, R. Pankrath: Non-Debye domainwall-induced dielectric response in Sr0.61-xCexBa0.39Nb2O6. Phys. Rev. B 65,
art. no. 220101 (2002).
I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A.B. Kutsenko, V.S. Vikhnin:
Photo- and thermoluminescence in congruent SBN crystals doped with Ce and
Cr. Ferroelectrics 273, 2565-2570 (2002).
H.L. Zhao, Q.T. Xu, W.M. Zhou, D.S. Yang, S. Kapphan, R. Pankrath: Photorefractive edge-enhancement joint transform correlator. Opt. Commun. 212,
287-292 (2002).
S. Kapphan, B. Pedko, V. Trepakov, M. Savinov, R. Pankrath, I. Kislova: Variation of doping-dependent properties in photorefractive SrxBa1-xNb2O6:Ce, Cr,
Ce+Cr. Radiat. Eff. Defects Solids 157, 1033-1037 (2002).
I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A.B. Kutsenko, V.S. Vikhnin:
Congruent Sr0.61Ba0.39Nb2O6 doubly doped with Ce and Cr: Photo- and thermoluminescence investigations. Radiat. Eff. Defects Solids 157, 1015-1020
(2002).
21
Prof. Dr. Klaus Ringhofer, Dr. Maxim Gorkounov
Forschungsübersicht
Die Arbeitsgruppe Theoretische Optik beschäftigt sich mit der Beschreibung
nichtlinearer optischer Effekte in photorefraktiven Kristallen. In solchen Kristallen
verursacht eine Intensitätsmodulation, die von zwei interferierenden Lichtstrahlen
erzeugt wird, ein moduliertes elektrisches Raumladungsfeld, das eine entsprechende
Modulation des Brechungsindex hervorruft. Wir modellieren derartige nichtlineare
Wechselwirkungen optischer Strahlen in Kristallen mit unterschiedlichen Symmetrien
unter verschieden externen Bedingungen. Außerdem beteiligen wir uns an der
Forschung elektromagnetischer Eigenschaften der neuen Metamaterialen, die als
Analoga optischer Kristalle im Mikrowellenfrequenzbereich gelten. Die
mikroskopischen Eigenschaften solcher künstlich hergestellten Materialen sind relativ
leicht kontrollierbar, während die makroskopischen Eigenschaften alle praktischen
Anwendungen bestimmen. Unser Ziel ist es, den Zusammenhang zu beschreiben
und optimale Metamaterialien vorzuschlagen.
Forschung im Kolleg
Im Kolleg werden die beiden Projekte „Vectorial beam coupling in fast photorefractive
crystals with AC-enhanced response (Oleg Filippov)“ und „Microwave interactions in
nonlinear metamaterials (Mikhail Lapine)“ durchgeführt (s. Berichte der Stipendiaten).
Kooperationen im Kolleg
Die Projekte werden in enger Zusammenarbeit mit B. Sturman (International Institute
for Nonlinear Studies, Novosibirsk, Russland) und E. Shamonina (Universität Oxford,
England) bearbeitet. Außerdem gibt es Kooperationen mit den Gruppen von K.
Betzler und E. Krätzig.
Publikationen in Zusammenhang mit dem Graduiertenkolleg
 V. P. Kamenov, Yi Hu, E. Shamonina, K. H. Ringhofer, and V. Ya. Gayvoronsky,
“Two-wave mixing in (111)-cut Bi12SiO20 and Bi12TiO20 crystals: Characterization
and comparison with the general orientation”, Phys. Rev. E 62, 2863 (2000).
 E. V. Podivilov, B. I. Sturman, S. G. Odoulov, S. Pavlyuk, K. V. Shcherbin, V. Ya.
Gayvoronsky, K. H. Ringhofer, and V. P. Kamenov, „Attractors and autooscillations for feedback controlled photorefractive beam coupling“, Opt. Comm.
192, 399 (2001).
 E. V. Podivilov, B. I. Sturman, S. G. Odoulov, S. Pavlyuk, K. V. Shcherbin, V. Ya.
Gayvoronsky, K. H. Ringhofer, and V. P. Kamenov, „Dynamics of feedback controlled photorefractive beam coupling“, Phys. Rev. A 63, 053805 (2001).
 V.P. Kamenov , E. Shamonina, K.H. Ringhofer, E. Nippolainen, V.V. Prokofiev,
and A.A. Kamshilin, „Photorefractive light scattering families in (111)-cut Bi12TiO20
crystals with an external electric ac field“, Phys. Rev. E 63 (1), 016607 (2001).

M.V. Gorkunov, E.V. Podivilov and B.I.Sturman, “Critical enhancement of nonlinear response in fast photorefractive crystals”, JETP 94, 470-481 (2002).
22
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E.V. Podivilov, B.I. Sturman, M.V. Gorkunov, V.P. Kamenov, and K.H. Ringhofer,
“Theory of critical enhancement of photorefractive beam coupling”, Phys. Rev. E,
65 046623 (2002).

E. Shamonina E, V.A. Kalinin, K.H. Ringhofer, L. Solymar, „Magneto-inductive
waveguide“, Electronics Lett. 38 (8), 371 (2002).

E. Shamonina, V.A. Kalinin, K.H. Ringhofer, and L. Solymar, „Magnetoinductive
waves in one, two, and three dimensions“, J. Appl. Phys. 92, 6252 (2002).

Gorkunov M., Lapine M., Shamonina E., Ringhofer K.H., “Effective magnetic
properties of a composite material with circular conductive elements”, Eur. Phys. J.
B 28, 263 (2002).

B. I. Sturman, V. Kamenov, M. Gorkunov, and K. H. Ringhofer, “Formation of
moving light domains during photorefractive feedback-controlled beam coupling”,
Opt. Comm. 216, 225 - 231 (2003)
Leider verlor Klaus Ringhofer im Dezember 2002 endgültig den Kampf gegen den
Krebs. Die Betreuung der Stipendiaten im Kolleg wird von M. Gorkounov, K. Betzler
und E. Krätzig weitergeführt.
23
Prof. Dr. Eckart Rühl,
Dr. Roman Flesch
Forschungsübersicht
Das Ziel der Forschungsvorhaben im Graduiertenkolleg bestand darin, neue Quellen
zur Erzeugung von kurzwelliger Strahlung im Bereich des Vakuum-UV zu entwickeln.
Dies sollte mit Hilfe von Clustern und Aerosolen erfolgen, die als Medium für
nichtlineare optische Prozesse dienen. Ausgangspunkt waren Arbeiten zur
Erzeugung der dritten Harmonischen in atomaren Gasen, wie z. B. Edelgasen [1, 2].
Komplementäre Arbeiten erfolgten im Rahmen des Graduiertenkollegs zur
Erzeugung von hochenergetischer XUV-Strahlung aus einer Laser-Plasma-Quelle.
Forschung im Kolleg
Die Experimente zur nichtlinearen Optik, die im Rahmen des Graduiertenkollegs
durchgeführt wurden, hat Herr Dr. A. Pramann maßgeblich vorangetrieben (s. Bericht
von Dr. A. Pramann). Ebenso war Herr J. Plenge im Graduiertenkolleg tätig, der
aufbauend auf ersten Arbeiten zur Frequenzverdreifachung an atomaren Gasen eine
XUV-Plasmaquelle unter Nutzung metallischer Targets zur Erzeugung von
durchstimmbarer XUV-Strahlung aufgebaut und genutzt hat [3-9] (vgl. Bericht von J.
Plenge). Während der Beschäftigungszeit von Herrn Dr. Pramann von November
2001 bis Januar 2003 hat er erfolgreich eine gepulste Gasexpansion zur Erzeugung
von freien Clustern in Verbindung mit nichtlinearen optischen Effekten aufgebaut.
Herrn Pramann ist es vor allem gelungen, einen kompakten Versuchsaufbau zu
realisieren, der es ermöglicht, ohne jegliche Reflexionsoptiken kurzwellige Strahlung
zu erzeugen und nachzuweisen. Dies geht über vorhergehende Arbeiten hinaus, in
denen ein komplizierter und verhältnismäßig ineffizient arbeitender Aufbau genutzt
wurde, in dem lange Wege und zahlreiche Reflexionen mit steilem Einfallswinkel
sowie ein Vakuum-UV-Monochromator genutzt wurden [2]. Das Experiment von
Herrn Pramann hat erste Funktionstests bestanden. Er konnte zunächst anhand von
atomaren Gasen zeigen, dass es Frequenzverdreifachung in bisher unbekannten
spektralen Regionen gibt. Der nächste Schritt besteht in der Kühlung der
Düsenstrahlexpansion, damit effizient Cluster entstehen und nichtlineare optische
Prozesse gemäß dem Projektantrag untersucht werden können.
Kooperationen im Kolleg
Die vorgeschlagenen Experimente sind als komplementär zu den übrigen Vorhaben
des Graduiertenkollegs anzusehen, die im Projektbereich Frequenzkonversion
angesiedelt sind. An Stelle von kristallinen Festkörpern standen Cluster und flüssige
Partikel im Vordergrund der Untersuchungen. Ebenso hatte das Vorhaben als
einziges zur Aufgabe, sehr kurzwellige Strahlung im XUV zu erzeugen. Daher fand
eine Kooperation innerhalb des Kollegs primär auf der Ebene eines intensiven
Erfahrungsaustausches sowie während der gemeinsamen Seminare statt. Zur
Intensivierung der Diskussion wurden gezielt Vortragende aus dem Umfeld des
Arbeitsgebietes in das Seminar des Graduiertenkollegs eingeladen, wie Prof. Dr. L.
Wöste (Berlin) und Prof. Dr. B. Wellegehausen (Hannover).
24
Literatur
1. R. Flesch, B. Wassermann, B. Rothmund und E. Rühl, J. Phys. Chem. 98, 6263
(1994).
2. R. Flesch, M.C. Schürmann, J. Plenge, M. Hunnekuhl, H. Meiss, M. Bischof und E.
Rühl, Phys. Chem. Chem. Phys. 1, 5423 (1999).
3. R. Flesch, M.C. Schürmann, M. Hunnekuhl, H. Meiss, J. Plenge und E. Rühl, Rev.
Sci. Instrum. 71, 1319 (2000).
4. R. Flesch, M.-C. Schürmann, H. Meiss, J. Plenge, M. Hunnekuhl und E. Rühl,
Phys. Rev. A 62, 52723 (2000).
5. J. Plenge, R. Flesch, M.-C. Schürmann und E. Rühl, J. Phys. Chem. A 105, 4844
(2001).
6. R. Flesch, J. Plenge, M.-C. Schürmann, S. Kühl, M. Klusmann und E. Rühl, Surf.
Rev. Lett. 9, 105 (2002).
7. R. Flesch, J. Plenge, S. Kühl, M. Klusmann und E. Rühl, J. Chem. Phys. 117, 9663
(2002).
8. J. Plenge, R. Flesch, S. Kühl, B. Vogel, R. Müller, F. Stroh und E. Rühl, Geophys.
Res. Lett., zur Veröffentlichung eingereicht (2002).
9. J. Plenge, Dissertation, Universität Osnabrück (2002).
Publikationen in Zusammenhang mit dem Graduiertenkolleg
1. R. Flesch, M.C. Schürmann, M. Hunnekuhl, H. Meiss, J. Plenge und E. Rühl, Rev.
Sci. Instrum. 71, 1319 (2000).
2. R. Flesch, M.-C. Schürmann, H. Meiss, J. Plenge, M. Hunnekuhl und E. Rühl,
Phys. Rev. A 62, 52723 (2000).
3. J. Plenge, R. Flesch, M.-C. Schürmann und E. Rühl, J. Phys. Chem. A 105, 4844
(2001).
4. R. Flesch, J. Plenge, M.-C. Schürmann, S. Kühl, M. Klusmann und E. Rühl, Surf.
Rev. Lett. 9, 105 (2002).
5. R. Flesch, J. Plenge, S. Kühl, M. Klusmann und E. Rühl, J. Chem. Phys. 117, 9663
(2002).
6. J. Plenge, R. Flesch, S. Kühl, B. Vogel, R. Müller, F. Stroh und E. Rühl, Geophys.
Res. Lett., zur Veröffentlichung eingereicht (2002).
6. J. Plenge, Dissertation, Universität Osnabrück (2002).
E. Rühl folgte im Herbst 2002 einem Ruf an die Universität Würzburg. Seine Projekte
im Graduiertenkolleg ‚Nonlinear optical processes in atomic and molecular clusters’
und ‘Frequency conversion, nonlinear optical processes in atomic and molecular
clusters’ schloss er im Januar 2003 erfolgreich ab (Berichte J. Plenge und A.
Pramann).
25
Prof. Dr. Hans Werner Schürmann
Forschung
Entsprechend dem Einrichtungsantrag sind untersucht worden:
Existenz- und Stabilitätskriterien für Solitonenlösungen der nichtlinearen
Schrödinger-Gleichung
Streuung am nichtlinearen Film mit unterschiedlichen Dielektrizitätsfunktionen
Nichtlineare Wellenleitung bei ortsabhängiger Dielektrizitätsfunktion
Elliptische Lösungen der Kadomtsev-Petviashvili-Gleichung
Semianalytische Lösungen der Wellengleichung bei photorefraktiver Nichtlinearität
Die Untersuchungen fanden in Kooperation mit den Professoren Serov und
Shestopalov (Lomonosov Universität Moskau) sowie innerhalb des Kollegs statt; die
Ergebnisse sind publiziert und auf internationalen Konferenzen vorgestellt worden.
Publikationen im Zusammenhang mit dem Graduiertenkolleg
H. W. Schürmann, V. S. Serov, Criteria for existence and stability of soliton solutions
of the cubic-quintic nonlinear Schrödinger equation, Phys. Rev. E 62,2, pp. 28212826 (2000).
H. W. Schürmann, V. S. Serov and Y. V. Shestopalov, Reflection and transmission of
a plane TE-wave at a lossless nonlinear dielectric film, Physica D, Vol. 158, pp. 197215 (2001).
H. W. Schürmann, V. S. Serov and Y. V. Shestopalov, Solutions to the Helmholtz
equation for TE-guided waves in a three-layer structure with Kerr-type nonlinearity, J.
Phys. A: Math. Gen., 35, 10789 – 10801 (2002).
H. W. Schürmann, V. S.. Serov and Y. V. Shestopalov, On the theory of TE polarized
waves guided by a lossless nonlinear three-layer structure, Proc. Progress in Electromagnetics Research Symposium (PIERS), Osaka, Japan, July 18-22, 2001, p.
670.
H. W. Schürmann, V. S. Serov and Y. V. Shestopalov, Waves in three-layer structures with Kerr-type nonlinearity and variable permittivity, Proc. Conf. on Mathematical Modelling of Wave Phenomena, Växjö University, Sweden, November 3 – 8, 2002
(in press; available on www.masda.vxu.se/bni/waephenomena.htm)
26
Apl. Prof. Dr. Manfred Wöhlecke
Forschungsübersicht
Die Arbeitsgruppe beschäftigt sich mit den optischen und dielektrischen Eigenschaften von Oxiden mit Niob-Sauerstoff oder Tantal-Sauerstoff Oktaederbausteinen
unter besonderer Berücksichtigung des Einflusses der Phasenübergänge mit
Relaxor-Charakter und arbeitet eng mit der Gruppe Nichtlineare Optik (Betzler)
zusammen. Ferroelektrische Phasenübergänge können in sehr engen (LiNbO 3 und
LiTaO3) und breiten (SBN) Temperaturbereichen auftreten und beeinflussen über
eine starke Änderung der Dielektrizitätskonstante alle damit zusammenhängenden
physikalischen Eigenschaften. Da die kongruente Schmelze der Kristalle nicht der
stöchiometrischen Zusammensetzung entspricht, werden viele Eigenschaften durch
die aktuelle Kristallzusammensetzung bestimmt. Im Berichtszeitraum wurden
untersucht:
 Einfluss der Dotierung auf die fotorefraktiven und ferroelektrischen Eigenschaften von SBN
 Dynamik ferroelektrischer Relaxoren
 Grundlegende optische Parameter wie Brechungsindex und Bandkante
 OH- - Streckschwingung in Oxiden
Forschung im Kolleg
Im Rahmen des Kollegs wurden verschiedene computergesteuerte Messplätze neu
aufgebaut oder aktualisiert. Dazu zählen eine Anordnung zur Raman-Streuung, eine
zur Dotierung von Kristallen mit Wasserstoff und ein Messplatz zur Bestimmung der
Dielektrizitätskonstanten und der pyroelektrischen Koeffizienten bei verschiedenen
Temperaturen. Eine systematische Untersuchung der Dotierung von SBN
unterschiedlicher Zusammensetzung mit Wasserstoff erlaubt eine Interpretation des
recht unstrukturierten OH-Streckschwingungsspektrums auf der Ebene von Details
der Kristallstruktur. Die Bandkante hängt in SrxBa1-xNb2O6 für x= 0,25 - 0,8 nur
geringfügig von der Zusammensetzung ab. Dagegen wird für LiTaO 3 eine ausgeprägte Abhängigkeit gefunden, die sich sehr gut zur zerstörungsfreien Bestimmung
der Zusammensetzung des Kristalls eignet. Dielektrische Charakterisierungen von
SrxBa1-xNb2O6 liegen für x-Werte oberhalb x=0,45 vor, für solche unterhalb muss der
Temperaturbereich des Messplatzes erweitert werden.
Kooperationen im Kolleg
Die Projekte werden in enger Zusammenarbeit mit den Gruppen von K. Betzler und
R. Pankrath (SBN-Kristalle) durchgeführt. Die LiTaO3 Kristalle unterschiedlicher
Zusammensetzung wurden von Ch. Bäumer (Kristallzucht H. Hesse) präpariert. Eine
intensive Zusammenarbeit besteht mit der Gruppe Th. Woike (Universität zu Köln).
Kooperationen im Rahmen eines INTAS-Projekts gibt es mit T. Volk und L. Ivleva
(Russian Academy of Science, Moscow). Ein bilaterales Projekt existiert mit L.
Kovács (Hungarian Academy of Sciences, Budapest).
27
Publikationen in Zusammenhang mit dem Graduiertenkolleg
Zeitschriftenartikel
 Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M.
Wöhlecke
Composition dependence of the ultraviolet absorption edge in lithium tantalate
J. Appl. Phys. March 1, 2003 issue
 T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau ,W.
Kleemann
Local electric-field-driven repoling reflected in the ferroelectric polarization of
Ce-doped Sr0.61Ba0.39Nb2O6.
Appl. Phys. Letters 80, 470 - 472 (2002)
 T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau ,W.
Kleemann
Evidence of random electric fields in the relaxor-ferroelectric Sr0.61Ba0.39Nb2O6
Europhysics Letters 57, 597 - 603 (2002)
 T. Granzow, Th. Woike, M. Wöhlecke, M. Imlau, W. Kleemann
Polarization-Based Adjustable Memory Behavior in Relaxor Ferroelectrics
Phys. Rev. Lett. 89, 127601 (2002)
 V. V. Gladkii, V. A. Kirikov, E. V. Pronina, T. R. Volk, R. Pankrath, M. Wöhlecke
Anomalies in the Slow Polarisation Kinetics of a Ferroelectric relaxor in the
Temperature Region of a Diffuse Phase Transition
Physics of the Solid State 43, 2140-2145 (2001)
 Th. Woike, T. Granzow, U. Dörfler, Ch. Poetsch, M. Wöhlecke, R. Pankrath
Refractive Indices of congruently melting Sr0.61Ba0.39Nb2O6
phys. stat. sol. (a) 186, R13 (2001)
 T. Volk, L. Ivleva , P. Lykov, D. Isakov, V. Osiko, M. Wöhlecke
Modification of the optical and photorefractive properties of Ce-doped strontium-barium niobate by co-doping with a nonphotorefractive La impurity
Appl. Phys. Letters 79, 854 (2001)
 T. Volk, L. Ivleva, P. Lykov, N. Pollok, V. Salobutin, R. Pankrath, M. Wöhlecke
Effects of rare-earth impurity doping on the ferroelectric and photorefractive
properties of strontium-barium niobate crystals
Optical Materials 18, 179 (2001)
 T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.
Kleemann
Influence of pinning effects on the ferroelectric hysteresis in cerium-doped
SrxBa1-xNb2O6
Phys. Rev B 63, 174101 (2001)
 Th. Woike, U. Dörfler, L. Tsankov, G. Weckwerth, D. Wolf, M. Wöhlecke, T.
Granzow, R. Pankrath, M. Imlau, W. Kleemann
Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration
Appl. Phys. B 72, 661 (2001)
28
1.2 Einzelberichte der in der vergangenen Periode geförderten
Kollegiat(inn)en
Dipl.-Phys. Calin Adrian David
Topic: Dielectric and optical properties of doped SBN
Abstract
The subject of the project has been extended to members of the ferroelectric lithium
niobate family, because such crystals were available when the project started with a
delay of nine months. In the course of the project new experimental set-ups have
been designed and realized, existing arrangements have been updated or partially
renewed including modern computer controlling using C++ or MatLab. Optical and
dielectric properties were measured in compounds of different composition of SBN
and LiTaO3. Special emphasis was placed on the optical band edge in SBN and
LiTaO3 with various composition and the OH-stretching vibration in SBN as well as
the phase transition properties.
Construction and rebuilding of set-ups
In the group existed an old double-grating spectrometer (Spex 14018) with some mechanical deficiency and an obsolete controlling system, but new uninstalled holographic gratings and freshly coated mirrors.
After mounting the new gratings in the old holders and pre-aligning them, the mirrors
were installed and then the whole system was aligned according to the instructions of
the manufacturer until a resolution better than that guaranteed by the Spex company
has been achieved. For this task basic procedures of a Visual C++ programme were
used to control the spectrometer and allow simple signal detection with a photomultiplier. Later on the programme has been considerably extended to perform Raman scattering including data accumulation and standard viewing of the spectrum.
The system will be used in the next future to measure the Raman scattering of various undoped SrxBa1-xNb2O6 crystals with x varying between 0.25 and 0.8.
We used the basics of a set-up to measure the spontaneous polarization as a function of temperature, which was designed in the group of Th. Woike (Cologne), to build
an improved version with state of the art computer controlling. The system is quite
versatile and can be equipped with different meters to measure the dielectric constant and the conductivity. The set-up is built up of an electrometer charge measuring
device (Keithley 6514), a temperature controller (PRO800 from Profile), a high voltage amplifier (610C from Trek). Again a Visual C++ programme with an IEEE 488
interface card was installed. With this setup poling of the sample, measuring the conductivity and hysteresis like properties is possible.
In the near future we will collaborate with the group of M. Imlau to extend the frequency range down to the sub-Hertz regime. Preliminary results in SrxBa1-xNb2O6
indicated that an extension of the set-up to more than 300 °C is necessary.
29
Determination of the band edge of LiTaO3 of various compositions and
SrxBa1-xNb2O6
LiTaO3 has like LiNbO3 a congruently melting composition which does not coincide
with the stoichiometric one, but shows a Li-deficiency. The more perfect lattice of
stoichiometric samples minimizes the line broadening in many spectral features and,
from an application point of view even more important, makes the material less susceptible to optical damage. Furthermore, a reduction of the coercive field is obtained,
which is a significant parameter for the production of periodically poled nonlinear optical devices [1]. All available reports on composition controlled features of LiTaO 3
suffer from their very limited compositional resolution. A set of well characterized
plates of LiTaO3 with various compositions have been prepared by Ch. Bäumer. The
polarized absorption was measured with a Bruins Instruments Omega 10 spectrometer with a wavelength accuracy of 0.1 nm, using mercury emission lines for the wavelength calibration. Polished y- and z-cut samples of about 0.5 mm thickness with a
density of scratches not exceeding 1 % of the illuminated area were measured at
22 °C with high spectral resolution (0.1 nm). The absorption data have been corrected for reflection losses. The index of refraction has been taken from [2]. We neglected the variation of the index of refraction with composition and temperature, because
its influence on the reflection is very weak.
As in the case of LiNbO3, the position of the absorption edge is a very sensitive
measure for the composition of LiTaO3 crystals and thus can be used to determine
the composition of a crystal with a non-destructive method. In addition, the experimental data show that the Li-content is limited to 50.0 mol %, indicating that regular
Li sites can be occupied by Ta ions, but no Li can substitute regular Ta ions. The dependence of the polarized absorption can be described by an exponential fit function
with three parameters and interpreted with a simple model calculation using an appropriate overlay of an additional near ultraviolet absorption, caused by tantalum antisite ions, and the base absorption. The concentration of tantalum antisite ions increases for Li concentrations below 50 %.
Similar measurements have been carried out with ordinarily polarized light for
SrxBa1-xNb2O6 over the whole x-range. Only a very weak non-monotonic dependence
of the band edge energy for a given absorption was found (see Figure 1), thus indicating that for this light polarisation even for larger absorption coefficients no convenient composition determination will work.
The situation may change for extraordinary light polarization, because the index of
refraction data vary for this light polarization, see the report of A. Tunyagi. Such
measurements will be performed in the near future after preparing suitable a-cut
samples.
30
Figure 1: Wavelength dependence of the absorption as a
function of composition for certain absorption coefficients.
OH-stretching modes in SrxBa1-xNb2O6
Hydroxyl ions are often present in as-grown oxide crystals [3]. Their stretching vibrational mode at about 3495 cm-1 can be detected by infrared (IR) absorption spectroscopy. As-grown SBN crystals contain, however, only a small amount of hydroxyl ions.
Thus we had to use temperature treatments under wet atmosphere to increase the
hydroxyl ions. We adopted procedures reported in the literature [4,5] and optimized
them by treating the samples at about 900 °C for 10 h with oxygen flowing through a
water bottle held at 80 °C. These parameters guarantee a strong doping but avoid
significant reduction resulting in disturbing polaron absorption. Treating ten samples
of different composition (0.3 < x 0.8) in this manner yields a strong dependence of
the maximum absorption on composition. Sr-rich samples accept three times more
hydrogen than Ba-rich ones.
The IR absorption of the stretching mode had been measured in pure and Ce-doped
congruent (Sr0.61Ba0.39Nb2O6) crystals [4,5]. A relatively broad absorption band has
been detected which is composed of a main line at about 3495 cm -1 and a broad
shoulder at the low energy side expanding up to about 3000 cm -1. The shoulder was
assumed to contain of least two [5] or three [4] bands due to the existence of different
hydrogen environments in the unfilled tungsten bronze structure. The aim of our
study w
itions in order to obtain more information about the relation between the band components and the crystal structure.
We observed a significant influence of the composition on the OH-stretch mode absorption spectra. With rising x, the absorption of the main band at about 3495 cm -1
increases, the low energy shoulder decreases and an additional broad absorption is
built up. For a decomposition and comparison the spectra were normalized with re-
31
spect to the area. This clearly shows that the high energy wing of the main absorption
does not depend on the composition, see Figure 2.
Figure 2: Normalized absorption of the OH-absorption in
SrxBa1-xNb2O6 for (0.3 < x 0.8).
Spectra for compositions with x above the value of the congruently melting composition (0.61) intersect at about 3380 cm-1, while those for x below 0.61 at about
3430 cm-1. Sr-rich compositions cause spectra consisting of an almost symmetrical
main band and a broad low energy feature. In Ba-rich compositions this feature is
weak, but the main band is highly asymmetric. Extensive decomposition trials have
shown, that the spectra can be very well described with three transitions, if we neglect a weak transition at about 3222 cm-1. The transition I describing the main band
varies so weakly with composition that we fixed it at 3492.7 cm -1, the same is true for
second transition II at 3454.5 cm-1, while the third transition III shifts more than 50
cm-1 with composition. The three transitions can be ascribed to hydrogen bond to
oxygen in three different environments. Transition I is probably caused by a hydrogen
vibrating towards an oxygen which belongs to a niobium octahedron and is located in
the ab-plane. Transition II may be due to an OH-stretching mode influenced by Ba,
while type III is a mode related to Sr, which changes its position with respect to x thus
causing a frequency shift.
References
[1] K. Kitamura, Y. Furukawa, K. Niwa, V. Gopalan, T. E. Mitchell, Applied Physics
Letters 73, 3073-3075, (1998)
[2] K. S. Abedin, H. Ito, J. Appl. Physics 80, 6561-6563 (1996)
32
[3] M. Wöhlecke, L. Kovács,
Critical Reviews in Solid State and Materials Sciences, 26(1), 1 - 86 (2001)
[4] S. Hunsche, A. Gröne, G. Greten, S. Kapphan, R. Pankrath, J. Seglins, Physica
Status Solidi A-Applied Research 148, 629 (1995).
[5] M. Lee, H. S. Lee, R. S. Feigelson, J. Applied Physics 84, 1558 (1998).
Publications
Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke:
Composition dependence of the ultraviolet absorption edge in lithium tantalite:
J. Appl. Physics 93, in print (2003).
C. David, A. Tunyagi et al.: OH stretching modes in SrxBa1-xNb2O6 (in preparation)
Attended lectures
WS 01/02 : P. Hertel: Linear response theory.
SS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearity.
WS 02/03: H.-J. Schmidt: Nonlinear wave equations
WS 01/02: V. Trepakov: Optics and Spectroscopy of semiconductors and insulators
Workshop “Photorefractive Nonlinearities” (October 2001, Osnabrück).
Workshop “SBN - a typical relaxor?” (May 2002, University of Osnabrück).
Workshop “SBN: Crystal Growth and Details of the Structure” (July 2002, University
of Osnabrück).
Seminars of the Graduate College 695 (WS 01/02 SS 02 WS 02/03).
Seminars of the Research Group “Optical Materials” (WS 01/02, SS 02, WS 02/03 ).
Contribution to the Seminars
Seminary Talk on 27.01.2003 “Optical Properties of as grown and Hydrogen doped
SrxBa1-xNb2O6”
Various short talks in the seminar of the research group
External research stay
IR-absorption Measurements performed on SZFKI institute in Budapest (08.07.2002
– 19.07.2002)
Duration of the dissertation: Start 01.10.2001, termination expected 30.09.2004
Period of support in the College: 01.10.2001 – 31.12.2003
Supervisor: apl. Prof. Dr. Manfred Wöhlecke
33
Dipl.-Phys. Oleg Filippov
Topic: Vectorial beam coupling in fast photorefractive crystals with ACenhanced response
Results
Up to now, the main results of our investigations can be divided into two parts:
“Polarization properties of light-induced scattering in Bi12TiO20” (see Section I), “Photorefractive AC-enhanced nonlinear response in sillenites” (see Section II). The results obtained are of interest for the use of fast photorefractive materials for various
applications such as grating recording, linear detection of weak signals, and for characterization purposes. The work has been performed in close cooperation with Dr.
B.I.Sturman, International Institute for Nonlinear Studies, Novosibirsk, Russia.
I. Polarization properties of light-induced scattering in Bi12TiO20
Cubic crystals of the sillenite family (Bi12SiO20, Bi12TiO20, and Bi12GeO20) and semiconductors like GaAs, CdTe, InP are the fastest photorefractive materials, which
makes them attractive for numerous applications. Some techniques are used to enhance the weak nonlinear response of these materials. The most appropriate for
practical proposes is the AC-technique and therefore the photorefractive response
under this technique was considered.
Strong spatial amplification achieved in sillenite crystals manifests itself in pronounced light-induced (nonlinear) scattering [1,2]. The underlying mechanism of this
phenomenon is not complicated: Weak seed waves, arising due to the surface and
bulk crystal imperfections, experience then a strong spatial amplification at the expense of the pump.
Due to the vectorial character of beam coupling in cubic crystals it is not possible
to separate the spatial changes of the light energy and polarization. Furthermore, the
light-induced scattering in this case is highly sensitive to the orientation of the electric
field about the principal crystal directions. This means that one must use the vectorial
theory of beam coupling for the description of scattering light phenomenon in such a
system [3].
In contrast to the angular intensity distribution, the polarization states of the scattered light in cubic crystals were not yet analyzed theoretically. We have applied the
vectorial theory of beam coupling [3] to describe the polarization properties of smallangle, light-induced scattering in cubic AC-biased BTO crystals for different polarization states of the incident pump beam [A1]. The diagonal geometry, distinguished by
the strongest vectorial coupling, was chosen for comparison between theory and experiment.
We have found the angular intensity distribution for several representative cases
of the pump beam. We have revealed that in the case of a horizontal pump beam
polarization the scattered light has a pronounced horizontal right lobe. The maximum
rate of spatial amplification (increment) is max  48 cm-1. The light-induced scattering
is strongest in this case. For the vertical polarization of the pump beam the intensity
distribution is quite different: it possesses one tilted left lobe at the azimuth angle
  1500. The maximum value of the increment is noticeably smaller here,
max  27 cm-1.
To analyze the effect of pump polarization on the scattering characteristics in more
detail, we have considered the cases of right and left circular pump polarization and
34
also two cases of linear polarization with a polarization angle 450. Our theoretical
shown that the value of the increment in these cases can be represented as a halfsum of the increments for the cases of horizontal and vertical pump beam polarization. Hence the angular distribution consists of two lobes: the strongest one is tilted
by 150 to the horizontal axis, the weakest lobe is tilted 1600. The maximum values
of the increment are max  30 cm-1 and max  8 cm-1, respectively.
Coming to the polarization properties of scattered light we have found that they are
more sensitive to the choice of the experimental and material parameters than the
intensity distributions. This is especially true with respect to the weakest lobes.
In cases of the horizontal and vertical pump beam polarization we have shown that
for the horizontal lobe the scattering polarization has to be horizontal and for the tilted
lobe is vertical. Experimental polarization measurements confirm this prediction with
high accuracy.
For mixed pump polarization we have obtained that the polarization of the scattered light depends on the azimuth angle and does not depend on the polar angle.
Since the most important propagation directions correspond to the maximum of the
increment we have determined the scattering light polarization for the azimuth angles, which correspond to the maximal intensity of the scattered light. For the main
(strongest) scattering lobe, the polarization is almost horizontal for right-, left-circular
and 450-pump polarizations. Experimental polarization measurements confirm this
result. For the weakest lobe the intensity ratio of vertical/horizontal components of the
scattered light is sufficiently small and therefore cannot be considered as big enough
to expect a quasi-vertical polarization of the scattering lobe. This theoretical prediction has found only a qualitative experimental confirmation. Experiments show that
the polarization of the weakest lobe is vertical for the cases of mixed pump polarization.
II. Photorefractive AC-enhanced nonlinear response in sillenites
During the last decade, the enhancement of photorefractive response in fast photorefractive crystals by the application of external AC fields is the subject of numerous experimental and theoretical studies. Large applied fields (up to 50 kV/cm) have
become available for AC-experiments. It was established that the low-contrast range,
where the fundamental component of the space-charge field grows linearly with light
contrast (m), is very narrow. Furthermore, the region of large light contrast has become important in connection with the soliton propagation problem. Lastly, a number
of applications of fast photorefractive materials, such as detection of weak signals
and grating recording, are relevant to high-contrast effects.
Particularly, it was found that a square-wave shape of the AC-field provides for the
best enhancement [4]. The enhancement properties are closely related to the presence of weakly damped low-frequency eigenmodes (space-charge waves) and spatial subharmonics generation. The results of numerical simulations of the largecontrast effects in the AC-biased sillenites [5,6] are in good agreement with the experiments, but the understanding of physical background lacked.
An analytical approach has been employed recently for the AC-enhancement description [7]. It is based on averaging over the fast AC-oscillations. Using this procedure it is possible to come to a rather general equation for the space-charge field profile. It was shown that the low and high contrast effects in AC-enhanced space
charge formation could be uniformly described by a simple differential equation for
35
the space-charge field. This equation was used recently for the description of beam
propagation effects [7].
We have applied this promising approach to the analysis of space-charge field
formation during grating recording [A2]. We have obtained the contrast dependence
of the fundamental harmonic E1, the second E2 and third E3 harmonics of the spacecharge field in the whole range of light contrast. It was found that the photorefractive
response remains non-local within the whole contrast range. We have revealed that
the quality factor Q (introduced in the low-contrast limit) determines the photorefractive nonlinear response in the whole contrast range and the dependence of this response on Q is saturated for Q >> 1. This means that for different values of model
parameters corresponding to the same value of Q, the contrast dependence of the
fundamental harmonic E1(m) is the same within good accuracy. The limiting value of
the fundamental harmonic (for m close to unity) is also quite universal E 1  0.64E0
(where E0 is the amplitude of the external AC-field).
For application purposes one should distinguish between low and high contrast
regions. The region of small light contrast (m < 0.05) is the best for spatial amplification. For larger contrast, the amplification becomes smaller but this region is the most
appropriate one for grating recording. Redistribution of the light pattern is relatively
harmless here.
Apart from the fundamental harmonic E1, responsible for beam-coupling effects,
the first higher harmonics E2 and E3 are of practical interest. These harmonics can be
measured with the help of auxiliary Bragg-matched light beams. They are very important for characterization purposes. We have found that higher harmonics of the
space-charge field become sufficiently large (up to 0.2-0.4E0) already for relatively
small values of light contrast. Since they are not weak, their direct measurements
should not be difficult. We have revealed that the contrast dependence of the second
harmonic peaks at m  0.5 and then turns to zero at m = 1. We have also found that
the contrast dependences of higher harmonics of the space-charge field correspond
to the formation of the step-like field profile with increasing m.
III. Future plans
Our investigation of the space-charge field formation in the low- and the highcontrast regions allows to generalize on the whole contrast range the theory of vectorial beam coupling, which was developed up to now only for the low-contrast case [3].
The theory to be developed will describe fully the two-beam coupling in fast photorefractive crystals under AC-enhancement in the whole contrast range. Unlike the scalar theory, the vectorial one allows to define in addition to intensity distributions of
beams also their polarization properties. This is especially important for the cases
where the optical activity essentially influences the beam coupling process. The
planed generalization of the beam coupling theory would allow to find new conservation laws, including the polarization degrees of freedom, and new regimes of polarization-dependent beam coupling in cubic crystals of the sillenite family or in semiconductors.
A prominent application of such effects is the linear detection of weak oscillating signals by means of polarization filtering. This new effect is feasible exclusively
due to the vectorial character of beam coupling in cubic crystals. Its efficiency is expected to gain because of the saturation of the space-charge field fundamental harmonic in the region of large contrasts. On the basis of the generalized vectorial theory it is possible to optimize the detection technique.
36
Another apparent manifestation of the AC-enhanced beam coupling is the
light-induced (nonlinear) scattering in sillenites. Application of the vectorial theory to
the analysis of scattering characteristics was restricted to the low-contrast limit. Our
approach would allow explaining the effects of saturation, which are clearly seen in
experiments but are missing in existing theoretical considerations.
In summary, the main directions of our future investigations are:
1. (2003) The generalization of the vectorial beam coupling theory on the whole
light contrast range. Explanation of the saturation effects in photorefractive
scattering in sillenites.
2. (2004) Theoretical study of the optimization of the linear detection technique.
References
[1] E. Raita, A. A. Kamshilin, and T. Jaaskelainen, ”Polarization properties of
fanning light in fiberlike bismuth titanium oxide crystals”, Opt. Lett. 21, 18971899 (1996).
[2] A. A. Kamshilin, V. V. Prokofiev, and T. Jaaskelainen, ”Beam fanning and
double phase conjugation in a fiber-like photorefractive sample”, IEEE J.
Quant. Electron. 31, 1642-1647 (1995).
[3] B. I. Sturman, E. V. Podivilov, K. H. Ringhofer, E. Shamonina, V. P. Kamenov, E. Nippolainen, V. V. Prokofiev, and A. A. Kamshilin, ”Theory of photorefractive vectorial wave coupling in cubic crystals”, Phys. Rev. E 60, 33323352 (1999).
[4] S. I. Stepanov and M. P. Petrov, Opt. Commun. 53, 292, 1985.
[5] J. E. Millerd, E. M. Garmire, M. B. Klein, B. A. Wechsler, F. P. Strohkendl,
and G. A. Brost, J. Opt. Soc. Am. B ,1449, 1992.
[6] G. I. Brost, J. Opt. Soc. Am. B 9, 1454, 1992.
[7] G. F. Calvo, B. I. Sturman, F. Agull-Lpez, and M. Carrascosa, Phys. Rev.
Lett. 84, 3839, 2000.
Publications:
[A1] O. Filippov, K. H. Ringhofer, M. Shamonin, E. Shamonina, A. A. Kamshilin, E.
Nippolainen, B. I. Sturman, “Polarization properties of light-induced scattering
in Bi12TiO20 crystals: Theory and experiment for the diagonal geometry”, accepted by JOSA B.
[A2] O. Filippov, K. H. Ringhofer, B. I. Sturman, “Photorefractive ac-enhanced nonlinear response of sillenites: Low- and high-contrast effects”, accepted by European Physical Journal D.
37
Attended lectures, conference visits, research stays:
1. P. Hertel: Linear response theory
2. E. Krätzig, K. Ringhofer: The photorefractive nonlinearity
3. H.-J. Schmidt: Nonlinear wave equations
Seminars and workshops of the Graduate College 695
Duration of the dissertation: Start 15.11.2001, termination expected 15.11.2004
Period of support in the College: 15.11.2001 - 31.12.2003
Supervisors: Prof. Dr. K. H. Ringhofer , Dr. M. Gorkounov, Prof. Dr. E. Krätzig
38
Dipl.-Phys. Andreas Geisler
Topic: Properties of one-dimensional spatial solitons in photorefractive media
Results
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40
41
42
Attended lectures
WS 01/02: P. Hertel, Linear response theory
SS 02: E. Krätzig/K. Ringhofer, The photorefractive nonlinearity
WS 02/03: H.-J. Schmidt, Nonlinear wave equations
- Workshop "Photorefractive Nonlinearities" (October 2001, Osnabrück)
- AMOP (March 2002, Osnabrück)
Duration of the dissertation: start WS 2000, termination expected SS 2004
Period of support in the college: Supervisor: Prof. Dr. H. W. Schürmann
43
Dipl.-Phys. Airat Gubaev
Topic: Light-Induced absorption changes in the visible and infrared range in
ferroelectric crystals.
Results
Introduction
The photorefractive crystals SrxBa1-xNb2O6 (SBN) and Ba1-yCayTiO3 (BCT)
can be grown in a congruent composition (melt and crystal have the same composition in SBN for x-0,61 and in BCT for y=0,23), which allows to produce large, homogeneous samples ideally suited for applications. The photorefractive properties can
be enhanced by doping with polyvalent ions like Ce, Cr etc.. The light-induced charge
transport from the doping ions and trapping in shallow polaronic states has been
identified by photo EPR [1] and optical experiments [2] to constitute the underlying
process. The trapping of those photo-induced charge carriers in specific centers can
be studied especially well at low temperatures, where the centers display a rather
long lifetime and the build-up of the resulting space charge field, which is modifying
the refractive index, can be investigated with various spectroscopic techniques. The
polaronic NIR absorption centers have been studied already in some detail [3],
whereas the thermally more stable VIS centers are more difficult to describe by theoretical models and need further experimental investigation to clarify the situation.
Experimental techniques
For the detailed spectral measurements a Fourier spectrometer (Bruker IFS
120HR) and a Beckman Acta VII grating spectrometer are used to cover the spectral
range from UV to FIR. A liquid helium bath cryostat (Leybold) is employed in the absorption measurements and the crystals are immersed in superfluid helium (at 2K) or
in helium exchange gas.
As illumination source, we use a Ar+- and a Kr+- laser (spectra physics 171).
Experimental results.
In continuation of the work of I.Kislova we studied the light-induced (Ar+laser) absorption charges in SBN:Ce to yield a quantitative description of the nonlinear intensity and temperature dependence of the relevant physical parameters. The
specific cerium–related FIR bands in SBN:Ce have been investigated at first to yield
a quantitative description of the Cerium-center properties and its concentration and
temperature dependence.
As a next step we plan to investigate the light-induced dissociation (under
Kr+- laser light) of VIS centers in SBN:Ce. This dissociation under simultaneous buildup of a transient NIR-Polaron population has qualitatively be seen and described by
I.Kislova [4], but it needs further quantitative work to elaborate the underlying processes. Besides additional optical absorption measurements we plan to investigate
the photoconductivity and the photo-Hall effect under Kr+- laser illumination, to get
new information about the charge carriers created. We hope that these results will
then yield clear evidence for the nature of the VIS centers, which are currently being
discussed as either bipolarons (in analogy to such centers in LiNbO 3) or polarons
trapped at charged centers, or charge transfer vibronic exitons (CTVE) being trapped
at charged centers [5].
[1] A.Mazur, C.Veber, O.F.Schirmer, C.Kuper, and H.Hesse. J.Appl.Phys. 85, 6751
(1999)
44
[2] M.Gao, R.Pankrath, S.Kapphan, V.Vikhnin. Appl.Phys. B. 68, 849 (1999)
[3] M.Gao, S.Kapphan,R.Pankrath, X.Feng, Y.Tang, V.Vikhnin. J.Phys.Chem..Sol.,
61, 1775 (2002)
[4] I.Kislova report to Grad.Koll 695 and S.Kapphan, I.Kislova, M.Wierschem,
T.Lindemann, M.Gao, R.Pankrath, V.Vikhnin, A.Kutsenko. Rad.Eff. and Defects in
Solids, 2003 (in print)
[5] V.S.Vikhnin, S.Avanesyan, H.Liu, S.E.Kapphan. J.Phys. and Chem. of Solids, 63,
1677 (2002)
Duration of the dissertation: Start 05.12.02 , termination expected end 2005
Period of support in the College: 05.12.02 - 31.12.03
Supervisor: Prof. Dr. S. Kapphan
A. Gubaev continues the project of I. Kislova who returned to her home country Russia for personal reasons. Hence he started only in December 2002.
45
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49
50
51
Dr. Vladimir Kamenov
Topic: Critical phenomena in Optics
Results
The investigations are divided into two main fields: “Critical enhancement of photorefractive response” (see Section I) and “Critical phenomena for feedback-controlled
photorefractive beam coupling” (see Section II).
I.
Critical enhancement of photorefractive response
The cubic crystals of the sillenite family [e.g., Bi12SiO20 (BSO), Bi12TiO20 (BTO), and
Bi12GeO20 (BGO)] are attractive because of their fast photorefractive response. The
main disadvantage of sillenites is their weak photorefractive response. There are two
convenient methods (DC and AC) for enhancement of the photorefractive response
of cubic crystals [1, 2]. These methods employ a DC or AC external electric field and
a proper frequency detuning between the interacting light beams. The enhanced exponential gain factor reaches the values of a few tens of cm -1.
It is well known that the enhanced two-beam coupling is often accompanied by subharmonic generation owing to a parametric excitation of weakly damped lowfrequency space-charge waves (SCWs) [3, 4]. In the most important case, the fundamental space-charge grating with grating vector K, recorded by a pair of pump
beams, becomes unstable against the spontaneous growth of a SCW with the spatial
frequency K/2 (the subharmonic grating). This instability is a threshold phenomenon:
the light contrast m has to exceed a certain threshold value mth  3/ Q , where Q is the
quality factor of the fundamental SCW. An important feature of the subharmonic generation is that the K/2-grating becomes very pliable to any driving force when approaching the threshold of subharmonic generation.
Recently, it was proposed to use this pliancy of the subharmonic grating for an additional (critical) enhancement of the photorefractive response [5]. In contrast with the
above enhancement methods [1,2], the exponential gain factor for the critical enhancement can basically be arbitrary large. Approaching the threshold of the subharmonic generation, the gain factor grows infinitely. This novel critical effect has
been missed in the previous studies because some important terms related to the
effect of the material nonlinearity had been omitted in the initial equations.
Unfortunately, the model considered in [5] does not include such important attributes
of the coupling experiments in cubic photorefractive crystals as the vectorial character of the beam coupling and the longitudinal inhomogeneity of the pump intensity
owing to light absorption. The first factor produces spatial oscillations of the coupling
strength and the second one makes the resonance value of the frequency detuning
dependent on the propagation coordinate (i.e., broadens the resonance) [6,7]. Therefore, there was a gap between the basic idea of the critical enhancement expressed
in cite [5] and the capability of the theory to indicate the necessary conditions for detection and possible utilization of this novel phenomenon.
Our work [A1, A2] aims for an extended analysis of the critical enhancement by taking into account the above attributes. This includes the formulation of a vectorial
52
model of the critical enhancement incorporating the effect of spatial inhomogeneity,
an analytical treatment of this model, and a numerical characterization of the critical
spatial amplification. We have shown that the real attributes of subharmonic experiments affect considerably the apparent characteristics of the critical enhancement but
do not suppress this effect. Our analytical and numerical results have allowed to optimize the conditions for detection of the critical enhancement in BSO crystals and to
predict the main observable features including polarization, spectral, and orientation
properties. The possibility to achieve a very strong spatial amplification in thin crystals ( d  1mm ) and to avoid in this way numerous extraneous effects is an important
prediction of our theory.
II.
Critical phenomena for feedback-controlled photorefractive beam coupling
When phase volume holograms are recorded in photorefractive crystals, a 100% diffractivity of the recorded grating is often desirable. It has been shown [8] that when
an active feedback stabilization is applied to LiNbO3 crystals, a diffraction efficiency
of unity can be achieved for a wide range of experimental parameters. This fact
opens new possibilities for thermal fixing [9] and for reducing the light scattering [10].
The main function of the feedback loop is to keep the phase difference between the
transmitted signal beam and the diffracted pump beam in the direction of the signal
beam equal to  / 2 . This is realized by a proper phase modulation of the input signal
beam.
In our work [A3, A4], we investigate the dynamics of the feedback-assisted beam
coupling. We show that two qualitatively different modes of operation are possible
when feedback stabilization is applied to photorefractive crystals with local response.
If the initial intensity ratio,   1 , is bigger than some threshold value,  th , the feedback changes the phase of the signal beam linearly in time. The corresponding diffraction efficiency of the photorefractive grating is less than 100%. For   th , the
initial signal phase consists of an oscillation periodic in time superimposed on linear
growth. In this case, the diffractivity of the recorded photorefractive grating is 100%.
We show that the transition between these two modes of operation is similar to a
phase transition, with a critical slowing down of the periodic phase variations. For the
case with periodic phase variations of the signal beam, the system undergoes several additional phase transitions: we have found a variety of qualitatively different periodic modes and non-trivial transitions between them. Good qualitative agreement
between theory and experiment is obtained for LiNbO3 crystals.
[1]
P. Refregier, L. Solymar, H. Rajenbach, and J. P. Hiugnard, „Two-beam coupling in photorefractive Bi12SiO20 crystals with moving grating: theory and experiment”, J. Appl. Phys. 58, 45-57 (1985).
[2]
S. I. Stepanov and M. P. Petrov, „Efficient unstationary holographic recording
in photorefractive crystals under alternating electric field” Opt. Commum. 53,
292-295 (1985).
[3]
B. I. Sturman, M. Mann, J. Otten, and K. H. Ringhofer, “Space-charge waves
in photorefractive crystals and their parametric excitation”, J. Opt. Soc. Am. B
10, 1919-1932 (1993).
53
[4]
L. Solymar, D. J. Webb, and A. Grunnet-Jepsen, “The Physics and Applications of Photorefractive Materials “, Claredon, Oxford, 1996.
[5]
E. V. Podivilov, B. I. Sturman, H. C. Pedersen, and P. M. Johansen, “Critical
enhancement of photorefractive beam coupling”, Phys. Rev. Lett. 85, 18671870 (2000).
[6]
D. J. Webb and L. Solymar, “The effects of optical activity and absorption on
two-wave mixing in Bi12SiO20”, Opt. Commun. 83, 287-294 (1991).
[7]
B. I. Sturman, A. I. Chernykh, V. P. Kamenov, E. Shamonina, and K. H. Ringhofer, “Resonant vectorial wave coupling in cubic photorefractive crystals“ J.
Opt. Soc. Am. B 17, 985-996 (2000).
[8]
A. A. Freschi and J. Frejlich, “Stabilized photorefractive modulation recording
beyond 100% diffraction efficiency in LiNbO3:Fe crystals”, J. Opt. Soc. Am. B
11, 1837-1841 (1994).
[9]
S. Breer, K. Buse, K. Peithmann, H. Vogt, and E. Krätzig, “Stabilized recording
and thermal fixing of holograms in photorefractive lithium niobate crystals”,
Ref. Sci. Instrum. 68, 1591-1594 (1998).
[10]
P. M. Garcia, A. A. Freschi, J. Frejlich, and E. Krätzig, “Scattering reduction for
highly diffractive holograms in LiNbO3 crystals”, Appl. Phys. B 63, 207-208
(1996).
Publications
[A1]
E. V. Podivilov, B. I. Sturman, K. H. Ringhofer, M. V. Gorkunov, and V. P.
Kamenov, “Theory of critical enhancement of the photorefractive response”,
Phys. Rev. E, 65 046623 (2002).
[A2]
E. V. Podivilov, B. I. Sturman, K. H. Ringhofer, M. V. Gorkunov, V. P. Kamenov, H. C. Pedersen, and P. M. Johansen “Modeling of critical enhancement
of photorefractive response in cubic crystals”, OSA TOPS 62, p. 230 (2000).
[A3]
E. V. Podivilov, B. I. Sturman, S. G. Odoulov, S. M. Pavlyuk, K. V. Shcherbin,
V. Ya. Gayvoronsky, K. H. Ringhofer, and V. P. Kamenov, “Wealth of dynamic
regimes for feedback-controlled photorefractive beam coupling”, OSA TOPS 62,
p. 221 (2000).
[A4]
K. V. Shcherbin, S. M. Pavlyuk, S. G. Odoulov, K. H. Ringhofer, V. P. Kamenov, E. V. Podivilov, and B. I. Sturman, “Critical phenomena for feedbackassisted phase grating recording”, OSA TOPS 62, p. 616 (2000).
Duration of the dissertation: Postdoc
Period of support in the College: 01.01.01 - 30.08.2001
Supervisors: Prof. Dr. K. H. Ringhofer, E. Krätzig
Dr. Kamenov left the Graduate College 30.08.01 to start an activity at the Carl Zeiss
AG, Oberkochen.
54
Dipl.-Phys. Inna Kislova
Topic: Light-induced absorption changes in ferroelectric crystals
I. Results
Introduction
Our research project is aimed at investigating the optical and dielectric properties of
the crystals SrxBa1-xNb2O6 (SBN, x=0.61) pure, doped with Ce, Cr ions or doubly
doped with Ce and Cr and of the Ba1-yCayTiO3 (BCT, y=0,23) crystals doped with Fe.
Both promising photorefractive crystal systems Ba1-YCaYTiO3 and SrxBa1-xNb2O6
possess a congruently melting mixture (for SBN x=0,61 and for BCT y=0,23) [1,2].
This allows to grow large, homogeneous crystals of excellent optical quality, which is
the basis for a wide range of optical applications [3]. Due to the statistical distribution
of the constituents and a partially unfilled (tungsten bronze) structure for SBN, the
ferroelectric phase transition (Tc≈373 K for congruent BCT pure and Tc≈353 K for
congruent SBN pure) shows a relaxor type character with polar contributions well
above Tc. The electro-optical coefficients of the pure crystals are already large and
can be enhanced considerably by suitable doping with polyvalent ions like those
mentioned above [4,5]. For some of the dopants (like Ce and Cr in SBN) a majority
charge state 3+ has been determined [6,7], however with an individual site occupancy, Ce3+ replacing Sr2+ ions and Cr3+ sitting on the Nb5+ sites [8,9]. These dopants
can be identified by their broad impurity induced absorption bands in the visible
range, a shift of the UV- absorption edge to longer wavelength in the case of Cr doping and additional Far-IR bands (near 2000cm-1) in the case of Ce-doping [6]. A lightinduced charge transport from these doping ions and trapping in shallow polaronic
states (Ti3+ in BCT respectively Nb4+ in SBN) has been identified by photo-EPR [10]
and optical experiments [11] to constitute the underlying processes for the enhanced
photorefractive properties in doped crystals. The majority of photo-excited charge
carriers have been determined by laser beam coupling experiments [12] and Halleffect [13] measurements to be electrons. The trapping of these photo-induced
charge carriers in certain centers can be considered as the first step in the build-up of
space charge fields which modify the refractive index and are the basis of the photorefractive effect under non-uniform spatial illumination. The properties and the physical nature of the centers created under illumination have been identified so far only to
some extent and are investigated further in this study with several techniques.
Experimental techniques
A Fourier spectrometer (Bruker IFS 120 HR) and a Beckman Acta VII grating spectrophotometer were used to measure the absorption spectra from the UV to the FIR
region. A Helium bath cryostat (Leybold) was employed in absorption measurements
and the crystals were immersed in superfluid helium (2 К) or in Helium exchange gas.
A Ar+- and Kr+ - laser (spectra physics 171) were used as illumination sources.
Photoluminescence and excitation spectra were measured using a photon counting
system. A high pressure Xenon lamp was used as the excitation source. A closed
cycle cryostat (Leybold) was used for the Photoluminescence and Thermoluminescence measurements.
Dielectric susceptibility near Tc was measured in a temperature variable set-up with a
HP 4270A automatic bridge.
55
Experimental results
a) Variation of doping-dependent properties in photorefractive SrxBa1-xNb2O6 : Ce, Cr,
Ce+Cr
Doping SBN crystals with Ce and Cr induces broad dichroic absorption bands. The
absorption coefficients in the visible region (at 514 nm) for SBN single crystals increase linearly with the Ce or Cr (up to ~ 20 000 ppm., p.f.u., (per Nb 2)) concentration. In Ce-doped crystals the integral FIR absorption of the Ce 3+ bands near 2000
cm-1 also vary linearly with the Ce concentration in the crystal, providing an independent method to estimate the Ce3+- content in double doped crystals even where
the UV-VIS absorption bands of Cr and Ce in SBN overlap. Comparison of individual
concentrations determined in double doped SBN:Ce+Cr and of single doping cases
shows no increase in the respective built-in coefficients of Ce and Cr for co-doping,
giving no evidence of a self compensation of Cr by Ce centres. For the Cr-doped
crystals a shift of the UV-absorption edge to longer wavelength with increasing Crdoping is observed as well.
Fe2+/ 3+ centers in BCT have been detected in photo-EPR experiments with absorption bands at 2eV and 3.5eV, respectively [10].
b) Dielectric measurements in the SBN crystals doped with Cr and Ce
The dielectric measurements show for both Ce and Cr doping about the same shift of
the phase transition temperature Tc, decreasing with increasing dopant concentration. The concentration dependence of the transition temperature for co-doped SBN:
Ce+Cr appears to be nearly the same as for single doping cases taking into account
the total impurity centre concentrations [publ.3]. For values of about 20000 ppm
(p.f.u.) Tc reaches about room temperature. The width at half maximum of the dielectric permitivity (33) versus temperature increases considerably with increasing concentration.
c) Photo- and thermoluminescence in the SBN crystals
One can observed a broadband green (em about 490nm) and a redphotoluminescence (red-PL) band (em about 765nm) with UV excitation (ex=350
nm). The green-photoluminescence (green-PL) can be excited only at the UV-band
edge, whereas the red-PL can be excited also at longer wavelength. The red
(765nm) emission intensity is increasing linearly with the Cr-doping for concentrations
up to about 5000 ppm Cr, with increasing deviations at higher concentrations. The
excitation spectra of the red luminescence, follow closely the shape of the Cr or Ceinduced absorption band and the Cr-induced shift of the UV-band edge. The time
dependence of the decay of the PL emission after excitation shut-off is nearly monoexponential. The decay time constant is about 3msec at 20 K, getting shorter at
higher temperatures. In single doped Cr,Ce SBN as well as in SBN:Ce+Cr two well
separated Thermoluminescence-peaks can be observed at about 90 K and at about
220 K, after low temperature (10K) excitation with a Xenon-lamp and a subsequent
waiting period of about 10 minutes before measurement with a heating rate of 5K/min
from 10 to 320 K. The spectral distribution of the line shape of the PL and of the TL
emission are at first sight the same, indicating a similar emission process after the
liberation of the respective charge carriers. However, a closer inspection yields a
spectral fine-structure with at least three strong emission subbands at 766nm, 775nm
and at 830nm. These sub bands decay with different time constants (em = 766nm
with =3,7ms and em= 775nm with =4,4 ms at 10K). The sub band at 766nm can be
preferentially excited in two spectral regions ex =350nm and ex=650nm, whereas
56
the subband at 775nm is more prominent for excitations at wavelength ex=470nm.
This longer wavelength subband also is getting more intensive with increasing Cr
doping in the crystals. Both Thermoluminescence emission peaks, at 90 K and at 220
K, show roughly the same spectral distribution in agreement with the Photoluminescence emission band, pointing at only slightly different recombination processes even
for the doubly doped SBN:Ce+Cr crystals.
d) Light-induced absorption changes
Under illumination with Ar+ - laser light (488 nm) at low temperature (2K, crystal immersed in superfluid liquid He) two broad dichroitic light-induced absorption bands
can be observed in BCT:Fe and similarly in SBN:Cr,Ce. The first absorption band
(VIS centers) is observed around 2eV and the second in the NIR around 0.7eV (6000
cm-1). The NIR absorption has been identified previously by photo-EPR as belonging
to Ti3+ small polarons in BCT or to Nb4+ small polarons in SBN [10]. The centers responsible for the VIS absorption have not been identified yet, but obviously are produced simultaneously with the NIR polarons. Both, the VIS and the NIR light-induced
absorption bands depend on polarization and nonlinearly on illumination intensity.
The temperature dependence in the production of these centers shows a steep
change at about 100K for the NIR polarons in SBN (at about 40 K for BCT) and at
about 200K for the VIS centers (at about 80 K for BCT). These characteristic temperatures are also revealed in thermoluminescence studies of SBN:Ce, Cr as intensity
peaks, where charge carriers (electron-polarons) are thermally excited in shallow
traps, followed by a hopping mobility of the liberated polarons till radiative recombination with deep trapping centers is occurring. The steady state of the light-induced absorption under illumination and the kinetics of its decay after a switch-off of the illumination , can be described by a simple model of charge transport from doping centers
(Fe2+ + Ti4+  Fe3+ + Ti3+ in BCT, Ce3++ Nb5+Ce4+ + Nb4+ in SBN) with subsequent
recombination as reported previously for SBN [11, 14].
e) Light-induced dissociation of VIS centers
The Ar+ light-induced VIS centers are rather stable at 2 K, whereas the NIR centers
decay rather fast and disappear completely within less than 50 sec in BCT:Fe (100
sec in SBN:Ce) [14, 15]. This allows to perform experiments in the following way. After creating a sizeable population of NIR polarons and of VIS centers at 2 K, the Ar +laser was switched-off. After waiting about 7 min. to let the NIR-polarons decay completely, then a Kr+ - laser was switched-on. First a build-up of NIR polaron absorption
and then a transient decay of this NIR absorption (depending strongly on the Kr +laser intensity) with a simultaneous decay in the VIS-absorption is observed.
After switching-off the Kr+-laser, the NIR polaron absorption decays with its own,
characteristic recombination decay time. This clearly demonstrates the dissociation of
the VIS centers into small polarons and has been observed both, in BCT:Fe and in
SBN:Ce.
Conclusions
The nature of the NIR centers as small polaron centers is well established in crystals
like BaTiO3 [16] or LiNbO3 [17], and similarly in SBN [11] and BCT [10]. Their characteristic NIR-absorption exhibits in SBN and BCT a temperature- and intensity dependent behaviour, the details of which are not fully understood yet and warrant further studies.
The VIS-centers are discussed as possibly being either bipolarons (in analogy to
such centers in LiNbO3), or polarons trapped at charged centers, or charge transfer
57
vibronic excitons (CTVE) being trapped at charged centers [15]. The present experiments do not yet allow to draw unambiguous conclusions – but one of the dissociation products must be an electron polaron.
References
1.C.Kuper, R.Pankrath, H.Hesse, Appl.Phys. A65, 301 (1997)
2.R.Neurgaonkar, W.Cory, J.Oliver, H.Ewbank, W.Hall, Opt.Eng.26, 392 (1987)
3.P.Guenter, J.P.Huignard, Top. In Appl. Phys.:Photorefr. Mat. 61/62(Springer-Berlin)
(1988)
4.C.Kuper, K.Buse, U.v.Stevendaal, M.Weber, T.Leidlo, H.Hesse, E.Krätzig,
Ferroelectrics,208/209,213 (1998)
5.Y.Tomita, A.Suzuki, Appl.Phys., A 59, 579 (1994)
6.G.Greten, S.Hunsche, U.Knuepfer, R.Pankrath, U.Siefker, N.Wittler, S.Kapphan,
Ferroelectrics 185,289 (1996)
7.R.Niemann, K.Buse, R.Pankrath, M.Neuman, Sol. St. Commun.98, 209(1996)
8.T.Woike, G.Wekwerth, H.Palme, R.Pankrath, Solid St. Comm.102, 743 (1997)
9.T.Woike, U.Doerfler, L.Tsankov, G.Weckwerth, D.Wolf, M.Woelecke, T.Granzow,
R.Pankrath, M.Jmlau, W.Kleemann, Appl.Phys.B72, 661 (2001)
10.A.Mazur, C.Veber, O.Schirmer, C.Kuper, H.Hesse, J.Appl.Phys.,85,6751 (1999)
11.M.Gao, R.Pankrath, S.Kapphan, V.Vikhnin, Appl.Phys. B68, 849 (1999)
12.M.Ewbank, R.Neurgaonkar, W.Cory, J.Feinberg, J.Appl.Phys.62, 374 (1987)
13.A.Gerwens, K.Buse, E.Kraetzig. J. Opt. Soc. Am.B15, 2143 (1998)
14.M.Wierschem, T.Lindemann, R.Pankrath, S.Kapphan, Ferroelectrics 264, 315
(2001)
15.M.Gao, S.Kapphan, R.Pankrath, X.Fenq, Y.Tang, V.Vikhnin, J. Phys. Chem. Sol.
61, 1775 (2002)
16. S.Koehne, O.F.Schirmer et.al., J.Supercond., 12, 19 (1999)
17.E.Krätzig, O.F.Schirmer in “Photorefractive Materials and their Applications”
(Ed.P.Guenter,J.P.Huignard) Topics in Appl.Phys., Vol. 61, Springer Berlin (1988)
III.
Publications
1. “Photo- and thermoluminescence in congruent SBN crystals doped with Ce and
Cr.” I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A. B. Kutsenko, V. S.Vikhnin.
Ferroelectrics, 2002, Vol.273, pp.187-192.
2.”Charge transfer vibronic excitons and excitonic-type polaron states: photoluminescence in SBN.” Vikhnin V.S., Kislova I., Kutsenko A.B., Kapphan S.E.
Solid State Communications 121 (2002) 83-88.
3. “Variation of doping-dependent properties in photorefractive SrxBa1-xNb2O6 : Ce,
Cr, Ce+Cr.” S. Kapphan, B. Pedko, V. Trepakov, M. Savinov, R. Pankrath and I. Kislova. Rad. Effects and Defects in Solids, 2002 (in print).
4. “Congruent Sr0.61Ba0.39Nb2O6 doubly doped with Ce and Cr: photo- and thermoluminescence investigations.” I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A.B.
Kutsenko, V.S.Vikhnin. Rad. Effects and Defects in Solids, 2002 (in print).
5. “Light-induced plaronic absorption at low temperature in pure and (Fe, Ce, Cr)
doped SrxBa1-xNb2O6 or Ba1-yCayTiO3 crystals and photodissociation of VIS centers
into small polarons.” S. E. Kapphan, I. Kislova, M. Wierschem, T. Lindemann, M.
Gao, R. Pankrath, V. S. Vikhnin ,A. B. Kutsenko. Rad. Effects and Defects in Solids,
2002 (in print).
58
IV.
Attended lectures
WS 01/02 : P. Hertel: Linear response theory.
SS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearity.
WS 01/02: V.Trepakov: Optics and Spectroscopy of semiconductors and insulators
Workshop “Photorefractive Nonlinearities” (October 2001, Osnabrueck).
Workshop “SBN - a typical relaxor?” (May 2002, University of Osnabrueck).
Seminars “Laser Optics” (WS 01/02 SS 02).
Seminars of the Graduate College 695 (WS 01/02 SS 02).
V.
Conference visits
1. 10th International Meeting on Ferroelectricity (IMF-10) (September 2001, Madrid,
Spain). (2 posters).
2. 9th Europhysical conference on defects in insulating materials (EURODIM 2002,
July 2002, Wroclaw, Poland). (2 posters and 1 oral contribution).
3. 16th Russian Conference on Physics of Ferroelectrics (September 2002. Tver,
Russia). (1 report).
VI.
Duration of the dissertation: Start 01.08.01 - The candidate was leaving for
personal reasons 31.10.02 (Serious illness of her father in Tver, Russia).
VII.
Period of support in the College: 01.08.01 - 31.10.02
Supervisor: Prof. Dr. Siegmar Kapphan
59
Dipl.-Phys. Mikhail Lapine
Topic: Microwave interactions in nonlinear metamaterials
Results
The work was performed in close collaboration with Prof. L. Solymar and Dr.
E. Shamonina from the Dept. of Engineering Science of the University of Oxford.
Metamaterials are artificial structures, composed as a regular lattice of identical elements. They attracted growing interest in the recent years. This was motivated
by increasing attention to the microwave range in electromagnetics, as metamaterials
offer new possibilities for manipulations with microwaves.
Common principles of structural organization make metamaterials similar to
crystals. However, the scale is different, and this shifts the applicable range of electromagnetic radiation to microwaves.
In most cases the suggested applications of metamaterials (e.g., magnetic
field guides [1] are concerned with the linear properties. Analogy between crystals
and metamaterials encourages us to consider also the nonlinear properties.
The most promising metamaterial among the suggested ones [2], which was
also experimentally studied [3], is based on circular conductive elements. However,
in the current literature no proper theory describing metamaterials was given and prior to the analysis of nonlinear properties we had to develop a linear theory for the
response of a similar metastructure, which allows for analytical consideration [A1, A2,
A3]. The metamaterial we consider is assembled as a regular lattice of split conductive rings. The developed theory is based on the same principles as the theory of optical linear response in crystal optics. The microscopic problem on the level of structure elements is solved with the help of the impedance matrix, assuming that the response is local and the mutual interaction is described in the quasi-static limit. Then
macroscopic averaging yields the effective parameters. The obtained permeability
shows frequency dispersion with the resonance frequency of the metamaterial being
shifted from the resonance of a single element. This shift depends on the lattice constants and type. The effect is very remarkable, but it was not taken into account by
other authors due to rather rough approximations and doubtful assumptions they followed. Above the resonance the real part of permeability is negative. The frequency
range of negative values depends strongly on the lattice type. We found that this
range is most extended for a hexagonal arrangement of rings in a plane with the
neighboring layers being maximally shifted with respect to each other. We supported
the analytical consideration with numerical calculations. These were performed by ab
initio solving Maxwell's equations for a finite structure consisting of a few thousand
elements and subsequent numerical averaging. The permeability obtained in this way
is independent of the shape of the sample and appears to be in an excellent agreement with the analytical results.
In order to provide nonlinearity to the response of the structure element it was
suggested to use diode inclusions [4]. The arising multi-wave interactions allow to
affect the wave propagation directly in a convenient “all-optical” manner, i.e., without
conversion into electronic signals.
To calculate the nonlinear susceptibility of the metamaterial with the diode insertions we generalize the approach, which we developed for the linear case. We
consider [A4] a weak nonlinearity, for which the current-voltage characteristic of a
diode includes a quadratic nonlinear term. This leads to the coupling of the Fourier
components of the fields and currents at different frequencies. A detailed analysis
shows that a three-wave interaction occurs. We finally obtain the magnetization of the
60
metamaterial in a form analogous to the polarization of an optical medium with a
quadratic dielectric nonlinearity, and we derive an analytical expression for the quadratic nonlinear susceptibility. It is determined by the properties of a single element as
well as by the linear properties of the metamaterial. Like the optical nonlinearity, the
nonlinearity of the magnetic metamaterial increases resonantly as one of the frequencies involved approaches the resonance of the linear susceptibility.
The general symmetry of Maxwell's equations with respect to the magnetic
field - electric field transposition allows to expect that one can deal with the nonlinear
interaction of electromagnetic waves in the proposed metamaterial using the welldeveloped apparatus of nonlinear optics. The whole variety of known nonlinear optical processes can have the corresponding analogues in metamaterials.
For practical estimations we consider an example of metastructure made of
rings with radius r0 = 2mm, arranged with the density n ~ r0-3. Choosing backward
diodes as nonlinear insertions, as they possess the best sensitivity and the highest
nonlinearity, we estimate that a nonlinear contribution to the susceptibility of the order
of 0.001 can be achieved. However, this is accompanied by significant losses. To
make use of a nonlinear metamaterial we have to ensure that the ratio of the nonlinear contribution to the damping (the latter being determined essentially by the imaginary part of the linear susceptibility) is as high as possible. This figure of merit appears to be proportional to the ratio of the impedance of the diode to its resistance,
|Z()|/R. For backward diodes it is of the order of unity, and their usage can be limited by losses. A promising opportunity is offered by varactors. For varactors the capacitive impedance can be much higher than the resistance and it is only necessary
to ensure that this condition is fulfilled in the desired frequency range.
It is clear that nonlinear metamaterials open vast possibilities for the applications taking the advantage of “all-optical” manipulations with microwaves. The developed theory covers an important particular case of weak nonlinearity, which allows
for comprehensible theory, analogous to nonlinear optics. However, the use of diodes in the mode of strong nonlinearity is quite desirable for the applications. This
case cannot be described in a way similar to optics, and requires an extended theory,
which we plan to develop in 2003. The detailed analysis of practically interesting nonlinear processes with microwaves, such as parametric amplification, frequency conversion, phase conjugation, etc., will be carried out in 2004.
[1] E. Shamonina, V. A. Kalinin, K. H. Ringhofer and L. Solymar, J. Appl. Phys. 92,
6252 (2002).
[2] J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, IEEE Trans. Microwave Theory Techn. 47, 2075 (1999).
[3] R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 77 (2001).
[4] V. A. Kalinin and V. V. Shtykov, Sov. J. Commun. Technol. Electron. 36, 96
(1991).
Publications:
[A1] M. Lapine, M. Gorkunov, E. Shamonina, and K. H. Ringhofer, “Permeability of a
metamaterial made of conductive rings”, Proc. of 9th Int. Conf. on Electromagnetics of Complex Media, 65 (2002).
61
[A2] M. Gorkunov, M. Lapine, E. Shamonina, and K. H. Ringhofer, “Effective magnetic properties of a composite material with circular conductive elements”, Eur.
Phys. J. B, 28, 263 (2002).
[A3] E. Shamonina, L. Solymar, V. A. Kalinin, M. Lapine, and K. H. Ringhofer, “Flux
distributions in a non-resonant magnetic metamaterial”, Proceedings of the
Progress in Electromagnetics Research Symposium PIERS 2002, July 1-5
2002, Cambridge, Massachusetts, USA, p. 249
[A4] M. Lapine, M. Gorkunov, and K. H. Ringhofer, “Nonlinearity of a metamaterial
arising from diode insertions into resonant conductive elements”, (submitted to
Phys. Rev. E, 2003).
Visited conferences
9th Int. Conf. on Electromagnetics of Complex Media (“Bianisotropics-2002”),
8-11 May 2002, Marrakech, Morocco (poster presentation).
Attended lectures
P. Hertel: Linear response theory (WS 01/02)
E. Krätzig, K. Ringhofer: The photorefractive nonlinearity (SS 02)
H.-J. Schmidt: Nonlinear wave equations (WS 02/03)
Seminars of the Graduate College 695
Workshops of the Graduate College 695
Duration of the dissertation: Start 01.11.01, termination expected 31.10.04.
Period of support in the College: 01.11.01 - 31.12.03
Supervisors: Prof. Dr. Klaus Ringhofer, Prof. K. Betzler, Dr. M. Gorkounov
62
Dipl.-Phys. Manfred Müller
Improvement of lithium niobate crystals for frequency conversion
Lithium niobate (LiNbO3) is the material of choice for many integrated-optical devices.
Due to large nonlinear-optical effects it is also now widely used for second-harmonicgeneration (SHG), for optical parametrical oscillation (OPO), as well as for optical parametrical amplification (OPA). This has been made possible through quasi phase
matching (QPM) with periodically-poled lithium niobate (PPLN), which allows frequency
conversion over a wide range of light wavelengths and enables utilization of the nonlinear optical coefficient d33, which is especially high in LiNbO3 [1].
However, most nonlinear-optical devices using LiNbO3 crystals operate in the near to
middle IR region. Extension of this technology to smaller wavelengths is impeded by the
emergence of photo-induced refractive index and absorption changes (so called “optical
damage”) and the difficulty to produce PPLN with an adequately short period length.
Periodically-poled components were be fabricated with period lengths down to 4 m
using special lithographic techniques [2]. An alternative method for fabrication of domains with shorter period lengths was presented for lithium tantalate crystals (LiTaO 3).
There the direct transfer of a light pattern into a domain structure is demonstrated. After
reversal of the domains the coercive field is transiently reduced. In LiTaO3 it was shown
that illumination can accelerate the recovery of the coercive field to the original value.
Thus illumination with a light pattern causes for some time a spatially modulated coercive field, and application of a homogeneous external electrical field of proper strength
during this time yields the desired domain pattern [3,4]. However, no such effects have
been reported for LiNbO3, although LiNbO3 and LiTaO3 are isomorphic.
Within the scope of this project Dipl.-Phys. Manfred Müller has investigated methods to
avoid optical damage as well as the poling characteristics of LiNbO 3 crystals while illuminating them with intense laser light over a wide spectral range. Most of the results
that are obtained so far are related to the properties and physics behind "domain engineering" of LiNbO3. The goal is to find a way to control optically the resulting domain
structure and hence the nonlinear optical properties. Furthermore, since the domain
structure of LiNbO3 is not directly visible, new techniques were developed that enable
improved monitoring of the poling process.
Experimental setup: Figure 1 shows the standard setup used in the experiments. The
electric field is applied to the crystal with transparent liquid electrodes (water), which
allow the necessary illumination. During the experiments an external electric field is continuously increased with a rate of 30 V/(s mm) up to values well above the coercive field
(about 20 kV/mm). The displacement current due to the change of the spontaneous polarization is used to monitor the poling process in time. To get spatially-resolved information the holder is integrated into a Mach-Zehnder interferometer. In LiNbO3 the
63
orientation of a ferroelectric domain
determines the sign of the electrooptic coefficient and therefore, if a
homogeneous electric field is applied,
the sign of the electro-optic refractive
index change. This leads to a noticeable discontinuity in the interference
pattern.
LiNbO3 crystal
c-axis
HV
Liquid
electrodes
DM
Pump light
BS
Guard
ring
All LiNbO3 crystals described in this
report are congruently melting, undoped, z-cuts with a thickness of 0.5
mm (supplier: Crystal Technology
Inc.).
O ring
Test light
DM
BS
Fused silica slabs
Fig. 1. Schematic representation of the poling setup
(BS: beam splitter, DM: dielectric mirror)
Influence of illumination on the poling characteristics of lithium niobate crystals: LiNbO3
shows like LiTaO3 a transient reduction of the coercive field immediately after a poling
event. However, full recovery of the coercive field takes only 20-30 s and is thus much
faster than in LiTaO3. It was found that unlike in LiTaO3 the relaxation of the coercive
field in LiNbO3 is independent of illumination (except for light-induced thermal effects).
Coercive field [kV/mm]
20.0
19.5
19.0
18.5
351 nm 351 nm 334 nm
2
2
2
I = 3 W/cm I = 6 W/cm I = 3 W/cm
18.0
17.5
Inside laser beam
Outside laser beam
17.0
0
10
20
30
40
50
60
Number of poling cycle
Fig. 2. Coercive field versus number of poling cycles for the forward poling direction measured interferometrically inside (circles) and outside (triangles) the illuminated area. For the time periods indicated by the gray bars the sample is illuminated by a laser beam with wavelength  and intensity I.
There is always a 6 min waiting time between two poling processes to avoid measurement of transient effects. Illumination at the wavelength  = 351 nm changes the coercive field only temporarily
because the crystal temperature increases. Illumination at the wavelength  = 334 nm, however,
yields a strong change of the coercive field, which is significant even after one hour without illumination. Missing data points indicate that the interferometer couldn’t clearly resolve the phase jump during poling.
64
However, it was also found that if the crystal is not illuminated between but during the
poling events with light of the wavelength 334 nm or shorter a considerable reduction
of the coercive field occurs (see Fig. 2). Even after the laser illumination is stopped, a
significant quasi-permanent decrease of the coercive field of
about 800 V/mm persists and
remains for hours without appreciable change. Therefore it can
be ruled out that the observed
change of the coercive field is of
thermal origin. Furthermore, this
effect is present only if illumination takes place during the poling
process. Illumination before or
after the poling has no impact on
the coercive field. The origin of
the effect is still under investigation. Possibly the intense UV illumination induces defects in the
crystal that assist domain nucleation or otherwise lower the coerFig. 3. Photo-induced domain pattern illuminated with two
cive field.
approximately 40 m wide stripes of UV-light as indicated
on the right side.
The observed effect is used to
realize light-controlled domain patterning in lithium niobate. A crystal is illuminated
through a binary grating for four poling cycles. The laser is turned off, and a following
forward poling process is aborted immediately after the domains start to switch. Etching of the crystal with hydrofluoric acid reveals the presence of a domain pattern,
which is approximately a replica of the illumination pattern. Figure 3 shows a magnified photograph of the crystal: it can clearly be seen that ferroelectric domains have
begun growing in the illuminated areas where the coercive field is lowest.
By optimization of the effect one should be able to generate domain patterns on the
micrometer scale utilizing an interferometrical light pattern and homogeneous electric
fields. In doing so it is especially important to time the abortion of the final poling process precisely, so that the domains in the illuminated regions had time to enough to
coalesce but that no extension into the unilluminated parts of the crystal occurred. In
order to do so, a monitoring technique able to resolve even such small domain sizes
must to be developed.
Monitoring of the poling process through light diffraction at domain boundaries: To
monitor the poling process, the crystal is placed into the holder and is illuminated
along the z-axis with a plane wave of light from an Ar+ laser. The generated light pattern is observed on a screen that is positioned behind the crystal holder. Figures 4 ah show for ultraviolet light (wavelength  = 351.1 nm) the light pattern that is observed during the various stages of the poling process. Well below the coercive field,
only diffuse scattering is present. When the poling starts (i.e. a displacement current
arises) a distinct ring structure at an 8° opening angle appears. With increasing voltage the ring turns into 6 dots, which transform into a star with a 6-fold symmetry. Inside the star a fine structure is distinctly visible. The star disappears abruptly with
65
completion of the poling process, and only the simple transmitted plane wave remains present.
Fig. 4. Light pattern observed on a screen behind the crystal during various stages of the poling process while the applied field was increased at a rate of 30 V/(s mm). The pictures were taken at a)
E = 19.08 kV/mm; b) E = 19.30 kV/mm; c) E = 19.38 kV/mm; d) E = 19.53 kV/mm; e) E = 19.57 kV/
mm; f) E = 19.67 kV/ mm; g) E = 19.69 kV/ mm; h) E = 19.72 kV/mm, respectively.
If the poling process is aborted and the voltage turned off while the star is visible, the
domain pattern is frozen. Therefore, it is possible to study the resulting star pattern
that arises from the same domain pattern for different light wavelengths, different incident angles of the light and different external electrical fields. For example Figures
5 b and c show the wavelength dependence of the star for the same external electric
field, while Fig. 5 a shows a part of the corresponding domain pattern. In accordance
with the crystal symmetry of LiNbO3, domain boundaries appear along six preferential
directions with multiples of 60° between them.
To determine whether diffraction at domain boundaries is responsible for this effect, a
single domain wall of macroscopic length is illuminated through a pin hole. Figure 6
shows the light pattern that is observed on a screen behind the sample. Diffraction by
the wall is clearly present with a maximum diffraction angle similar the one observed
for the star. However, a surprising feature is that the direction of the diffracted beams
depends on the sign of the applied field.
Fig. 5. a) Frozen domain pattern revealed after etching the crystal in 48 % hydrofluoric acid for 90 min
b) The corresponding light pattern for an external electric field of -14 kV/mm seen at a wavelength of
351.1 nm and c) seen at a wavelength of 501.7 nm
66
sin(  )
0,14
0,12
0,10
0,08
350
400
450
500
 [nm]
Fig. 6 Light pattern created by illumination of a single Fig. 7. Sine of the opening angle of the
domain boundary through a circular aperture for various star versus wavelength . The line serves as
electrical fields E a) E = -12 kV/mm; b) E = -8 kV/mm; a guide for the eye.
c) E = -4 kV/mm; d) E = 0 kV/mm; e) E = 4 kV/mm;
f) E = 8 kV/mm; g) E = 12 kV/mm.
This diffraction effect, together with the hexagonal symmetry of the domain structure,
can qualitatively explain the emergence of the star pattern during the poling process,
although the physical reason behind the diffraction at the domain boundaries is still
unclear. We suppose that the domain walls have a certain thickness and that the
phase jump of the transmitted wave is continuous and not abrupt. Therefore shortwavelength light, where the phase jump is larger, should be diffracted with a larger
angle, which agrees with the experimental observation (see Fig. 7).
However, it is clear that the star gives us information about the directions of the domain boundaries, irrespective of the domain size. This can be a very valuable tool for
monitoring of the light-controlled poling process explained above. — Based on the
achievements we are pretty optimistic that the PhD thesis of Dipl.-Phys. Manfred Müller will help to realize improved optical parametric oscillators.
1. M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, "Quasi-phased-matched
second harmonic generation: tuning and tolerances", IEEE J. Quant. Electron. 28,
2631-2654 (1992)
2. R. G. Batchko, V. Y. Shur, M. M. Fejer, and R. L. Byer, "Backswitch poling in lithium niobate for high-fidelity domain patterning and efficient blue light generation",
Appl. Phys. Lett. 75, 1673-1675 (1999)
3. S. Chao and C. Hung, "Large photoinduced ferroelectric coercive field increase
and photodefined domain pattern in lithium-tantalate crystal", Appl. Phys. Lett. 69,
3803-3805 (1996)
4. P. Brown, G. Ross, R. Eason, and A. Pogosyan, "Control of domain structures in
lithium tantalate using interferometric optical patterning", Opt. Comm. 163, 310316 (1999)
67
Publications
1. I. Nee, M. Müller, K. Buse, E. Krätzig, "Role of iron in lithium-niobate crystals for
the dark-storage time of holograms", J. Appl. Phys. 88, 4282-4286 (2000)
2. I. Nee, M. Müller, and K. Buse, "Development of thermally fixed photorefractive
holograms without light", Appl. Phys. B 72, 195-200 (2001)
3. M. Wengler, M. Müller, E. Soergel, and K. Buse, "Dynamics of ferroelectric domain reversal in lithium niobate crystals", Appl. Phys. B, accepted
4. M. Müller, E. Soergel, M. Wengler, and K. Buse, "Star-shaped light diffraction
from ferroelectric boundaries", submitted
5. M. Müller, E. Soergel, M. Falk, J. Hukriede, and K. Buse, "Reduction of optical
damage in lithium niobate crystals by hydrogen loading", in preparation
Attended lectures, conference visits, research stays
Conferences:
1. 10th European Conference on Integrated Optics (ECIO), Paderborn
04.-06.04.2001
2. 4th Annual Meeting of the COST Action P2, Budapest, 16.-19.05.2001
3. Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Osnabrück,
04.-08.03.2002
4. Conference on Lasers and Electro-Optics '03, Baltimore, 01.-06.06.2003
5. 9th International Conference on Photorefractive Effects, Materials, and Devices,
Nice, 17.-21.06.2003
Lectures:



Atomphysikalisches Kolloquium (4 courses)
Physikalisches Kolloquium (4 courses)
Seminar über angewandte Optik (4 courses)
Research stay (scheduled):

Stay in the group of Prof. Dr. R. W. Eason, University of Southampton, Optoelectronics research laboratory, Light-induced patterning of ferroelectric domains, fall
2003 for 4 weeks.
Duration of the dissertation: Start 01.10.2000, termination expected 31.03.2004
Period of support in the College: 01.01.2001-31.12.2003
Supervisor: Prof. Dr. Karsten Buse
68
69
70
71
72
73
Dr. Axel Pramann
Topic: Frequency conversion,
Nonlinear optical processes in atomic and molecular clusters
Results
The generation of line-tunable, coherent light of high resolution in the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) spectral range (>> 10 eV) in the laboratory
is an experimental task of high importance with respect to both applications such as
high resolution spectroscopy in the vacuum ultraviolet and related subjects. The
availability of a compact light source with such properties is of interest to fields such
as molecular spectroscopy, photochemistry, photoionization, and state selective reaction dynamics. The lowest electronically excited states of small molecules (e. g. N 2,
H2O …) are found below 185 nm (7 – 10 eV). Therefore, it is important to have access to these states by one photon excitations, which are induced by VUV radiation.
The current project - and its results - are divided into several parts:
A set up for the generation of high energetic VUV- and XUV-light was developed.
The general experimental procedure makes use of nonlinear effects of frequency tripling of tunable laser radiation in the ultraviolet regime, incident on the gaseous tripling medium. In the following step, the operation conditions and parameters of the
frequency tripling were tested and characterized using well-known systems such as
N2 and Kr.
Moreover, these gases were also used to investigate so far unknown regions of frequency tripled radiation in the VUV regime.
The experiment consists of at least three main components (Fig. 1). A pulsed dye
laser system (operated at 10 Hz repetition rate) is used for the generation of a fundamental frequency, which is tripled in a jet. The dye laser is pumped by an excimer
laser (308 nm, 300 mJ/pulse). The fundamental of the laser radiation was typically
520-560 nm (20 – 25 mJ/pulse), which is subsequently frequency doubled by a BBO1 crystal. With this standard method, laser light in the wavelength range of 260 – 280
nm (3 – 5 mJ/pulse) is produced, which is used for the generation of the third harmonic light in the VUV. The resolution of this fundamental light is better than 0.09
cm-1. The frequency doubled light is crossed by a pulsed molecular beam of the tripling gas in the center of a vacuum chamber.
It is important to focus the laser light as near as possible to the orifice of a pulsed
valve (General Valve). Frequency tripling can only be achieved when the phase
matching condition is fulfilled. The frequency tripled light is propagating collinear to
the initial direction of the ultraviolet laser light. Formation of a supersonic jet is accomplished by expanding the gas at a high stagnation pressure p 0 (10 bar) through a
pulsed nozzle with a small orifice into a vacuum chamber. In the case of molecular
gases, strong cooling of the internal degrees of freedom occurs, which leads to cluster formation. Subsequently, the frequency-tripled light ( < 100 nm) enters a second
vacuum chamber, which is filled with a gas with high ionization potential (e. g. acetone: IP = 9.7 eV). It is important to note that the detection gas exhibits no multiple
photon processes, so that exclusively one photon processes lead to cation formation.
74
Fig. 1. Experimental setup
As a result, one can easily distinguish between the fundamental and THG light. The
ionized molecules are detected with an ion detector located in a third vacuum chamber. With this set up, time-of-flight (ToF) spectra of the ion bunches are measured.
The integral of the respective ion signal is proportional to the intensity of the generated frequency tripled light. The power L3 of the frequency tripled light is given by
3 2
2
3
(1)
L3  2 2 4 N 2 [  3 (3 )] 2 L1 
 0 c 1
Because of the quadratic relation between L3 and the particle density N and the cubic
relation between L3 and the power of the incident light L1, it is evident that for THG
the key parameters are both a high laser power of the frequency doubled light and a
high density of the tripling medium.
The experimental setup is tested and characterized using molecular nitrogen (N 2) as
the tripling medium, because it is known that N2 has a high tripling conversion efficiency in the energy range > 10 eV.1 With this reference gas, the wavelength dependence of THG is monitored in the range  < 93 nm. In general, with the new setup
75
the same two- and three-photon resonance-enhanced lines (highly populated rotational lines) similar to previous work of Lee and co-workers1 are observed. For the
characterization of the new machine it is useful to select an intense rotational line of
high intensity (Fig. 2).
Fig. 2. Time-of-flight spectrum of C3H6O+ generated at THG = 90.26 nm using N2 as
the tripling medium (p0 = 9.5 bar; Plaser = 2.6 mJ/pulse).
Here, the line at  = 90.26 nm is used for characterization and optimization, because
this line has been identified with a similar setup using a VUV-monochromator for
wavelength detection in a previous work.2 After the generation of THG light at this
wavelength, the operating conditions of the machine are optimized. First of all, the
laser power dependence of the intensity of the frequency tripled light is measured.
Usually, an optimum conversion efficiency of 10 -6 between the UV-laser light and the
frequency tripled light is achieved. For N2, THG light is generated with laser powers
between 2.4 and 3.5 mJ/pulse. As stated above (eq. 1), the THG intensity is proportional to the cube of the UV- laser pulse. Another important parameter is the stagnation pressure dependence of the THG light intensity. The phase matching condition
for THG is strongly dependent on the stagnation pressure p 0 of the gas prior to expansion. The stagnation pressure of N2 is varied between 0 and 10 bar. THG signals
are detected in the range between p0 = 4 and 10 bar. Maxima in THG are observed
at p0 = 7 – 8 and 9 – 10 bar corresponding to a high particle density at the point of
frequency tripling. Because of the pulsed character of the experiment extensive care
must be taken not only for the geometrical adjustment of the laser and the molecular
beam. Additionally, a proper timing between the opening of a pulsed valve and the
laser pulse is of importance to fulfill the phase matching condition (Fig. 3).
The highest signal intensity is found for a delay time of 500 – 600 s between the
nozzle opening and the laser shot.
76
Fig. 3. THG signal intensity as a function of the time delay between the opening of
the valve and the firing of the UV-laser (tripling gas: N2 (p0 = 9.5 bar, THG = 90.26
nm).
Fig. 4. ToF spectrum of C3H6O+ measured at the Kr resonance line at 92.30 nm
(Plaser = 3.5 mJ/pulse).
After the characterization of the experimental setup frequency tripling of gases at unknown wavelength ranges was investigated. For this purpose, a jet of krypton is
77
used. In the work of Lee and co-workers1 line tunable THG of Kr has been measured
for the first time down to 90.4 nm. Some 4p – ns and 4p – nd Rydberg series in that
study are reproduced with our setup. As an example, we optimized the operating
conditions for Kr jets (as described for N2) at the prominent line at 92.3 nm (Fig. 4).
However, compared to the THG signals of N2, the intensities of the Kr signals are
about one order of magnitude weaker. For beams of Kr, tunable THG is observed for
the first time in the VUV wavelength range down to 86 nm (14.4 eV). As an example,
Fig. 5 shows a time-of-flight spectrum of C3H6O+ molecules obtained at a tripling
wavelength of 90.3 nm using a Kr beam.
Kr
90.3 nm
Fig. 5. ToF spectrum of C3H6O+ obtained at a tripling wavelength of 90.3 nm using a
krypton beam.
The intensities and structures of the TOF signals in the wavelength range between
90.3 and 89.1 nm exhibit all very similar structures and almost the same intensities
within the experimental error. Thus, a continuum of the THG signals in this spectral
region without prominent lines is observed. This behavior is in contrast to the sharp
Rydberg states in the wavelength range above 90.4 eV.
This spectral pattern without detectable resonance structures is similar to that of xenon in the wavelength range between 90 and 92 nm.
The second reason for using beams of Kr is the known ability of Kr to form clusters
after a supersonic expansion applying proper expansion parameters. This represents
another area of interest in the project, where size-dependent third-harmonic generation in clusters will be investigated. Currently, experiments on cluster production can
be performed with the new setup, so that new regions of frequency tripling are expected to occur.
78
References:
1
R. H. Page, R. L. Larkin, A. H. Kung, Y. R. Shen, and Y. T. Lee, Rev. Sci. Instrum.
58 (1987) 1616.
2 J. Plenge, diploma thesis, University of Osnabrück, 1999.
Attended lectures, conference visits, research stays
- Seminars of the graduate college 695
- Workshops of the graduate college 695 during 2002
Duration of the dissertation: Postdoc
Period of support in the college: 15.11.2001 - 31.01.2003
Supervisors: Prof. Dr. E. Rühl, Dr. R. Flesch
Dr. Pramann left the Graduate College 31.01.03 to start an activity at the
Physikalisch-Technische Bundesanstalt (PTB), Braunschweig.
79
Dipl.-Math. Dipl.-Phys. Florian Rahe
Topic: Space-charge waves in photorefractive crystals
Results
Space-charge waves (SCW) are the eigenmodes of charge oscillation in a system of
traps and free carriers in semi-insulating solids, when carriers move in an electric
field. They were initially named trap recharging waves, because their nature is associated with trap charging and discharging by free carriers which are excited thermally
or by illumination. Their propagation is due to the influence of an applied electric field.
These waves have very specific properties. For instance they are strongly attenuated, because their free path length is typically limited by the carrier drift length. Moreover, the propagation direction of the SCW is determined by the direction of the applied field. Due to the fact that their wave vector is inversely proportional to their frequency, phase and group velocities are oppositely directed.
Space-charge waves are of great interest in photorefractive crystals, especially for
the sillenite family Bi12MO20 (where M = Ge, Ti or Si), because the dynamic properties of these crystals in the presence of an external electric field are very often determined by these waves. This especially applies for the process of holographic recording, hologram relaxation and oscillations of holographic gratings. For example,
the SCW excitation can provide an increase of the sensitivity of devices, which are
based on the principles of dynamic holography. SCW can also play an important role
in further semi-insulating semiconductors, i.e., GaAs, InP:Fe, CdTe:V and other materials. It can be supposed that some transient phenomena in photoreceivers are associated with SCW as well.
There are several methods of SCW excitation, electrical or optical. The electrical
methods encounter serious experimental difficulties in the selective excitation of
SCW with a desired set of parameters. Much more flexible is the optical excitation of
SCW by illuminating the crystal with a periodic interference pattern. Optical methods
are pulse detection, a moving interference pattern or an oscillating interference pattern. A careful selection of the experimental method for the investigation of SCW is
important, because the obtained information depends critically on the technique used
for SCW excitation and detection.
I investigated SCW in photorefractive crystals of the sillenite family (namely
B12GeO20, B12TiO20 and B12SiO20). An optical method for SCW excitation was used.
The crystals were illuminated with an interference pattern, oscillating near a mid position. If the grating spacing and the oscillation frequency of the interference pattern
coincide with the spatial period and temporal eigenfrequency of a space charge
wave, resonance excitation occurs. The use of electro-optic crystals makes it easy to
detect the SCW, because their space charge field can be detected via diffraction of a
test laser.
In the case of optical excitation, two main regimes can be considered. The first one is
the linear regime, when only effects proportional to the first power of the contrast ratio
m of the interference pattern are taken into account. The second is the nonlinear regime, in which effects proportional to m2 (or a higher power of m) become important.
This situation can be compared with effects in nonlinear optics. In the case of effects
80
proportional to m2, one can expect to observe phenomena similar to those known
from nonlinear optics, like second-harmonic generation and rectification.
The main subject of my investigations is the nonlinear regime. The illumination of the
sample with an oscillating interference pattern results in a simultaneous excitation of
SCW and the formation of a static space-charge grating, whose spacing is equal to
the spatial period of SCW. The interaction of the static space-charge grating and the
SCW lead to new nonlinear effects, which don’t exist in nonlinear optics. They are
spatial doubling, where doubling of the SCW wave vector occurs without frequency
doubling, and spatial rectification, where a spatially homogeneous electric field oscillating with frequency of the SCW arises. During my investigations I detected the effects of second harmonic generation, where doubling of the SCW wave vector and
doubling of the SCW frequency occur, and rectification.
Second harmonic generation was observed with the help of a test laser. The laser
was adjusted to read out the space charge grating with the doubled wave vector via
the electro optic effect. The diffracted beam was detected by a photodiode connected
to a lock in amplifier. The signal P2f was detected at the second temporal harmonic to
observe second harmonic generation. In this case theory predicts three resonance
peaks, which arise due to different forced excitations of SCW. All three peaks were
detected (see figure 1) and their relative positions fit well to the theory. They also fulfil
the dispersion relation of SCW.
Figure 1. Frequency dependence of the output signal P2f
at 2f for different applied
fields E0. For Bi12GeO20, m =
0.43, W0 = 130 mW/cm2,  =
1 rad,  = 13 m: ■: E0 = 10
kV/cm, ●: E0= 8 kV/cm, ▲: E0
= 6kV/cm. The lines are
guides to the eye. The
change of the resonance frequencies follows the dispersion relation for SCW.
For overall rectification the theory predicts a change in the current in the external circuit. This also implies a change of the static homogeneous field inside the crystal.
Consequently, there are two possibilities to measure the rectification effect. First by
measuring the DC current in the external circuit. This was realized by measuring the
DC voltage over a loading resistance. A strong effect could be observed. A decrease
of the current up to 25% was observed. In figure 2 one can see the effect for different
oscillation amplitudes  of the interference pattern. The resonance frequencies fulfil
the dispersion relation of SCW. Again theory fits well with the measured data (figure
3). To detect the change of the internal field of the crystal I used the electro-optic effect. The change of the internal field results in a change of one of the refractive indices. It results in a change of the polarisation of a test beam propagating through the
crystal. This can easily be detected with the help of a polarizer. The change of the
81
internal field can be detected and the resonance frequencies coincide with the resonance frequencies of the current measurements.
Figure 2: DC current I0 as a
function of the phase modulation frequency f for different oscillation amplitudes : For Bi12GeO20, E0
= 8 kV/cm,  = 13.1 m, m
= 0.43 and W0 = 130
mW/cm2; -■-:  = 0.2 , -●:  = 0.2 , -▼-:  = 0.6 ,
--:  = 0.8 , -○-:  = 1.0
 . The lines are guides to
the eyes.
Figure 3: Comparison between theory (lines) and
experiment (symbols) for
overall
rectification
in
Bi12GeO20. I0 is the dc current, f is the phase modulation frequency:  = 0.2 
(solid line, ●)  = 0.9 
(dashed line, ◄).
In further experiments all nonlinear interactions have been investigated simultaneously. It turned out that not all resonance frequencies coincide, especially for high
oscillation amplitudes . The resonance frequencies for the nonlinear effects are
shifted to lower frequencies. This seems to be in contradiction to the theory, however
the theory is only valid for small . For high amplitudes terms of higher orders of 
have to be taken into account, which results in a shift of the resonances to lower frequencies, especially in the case of nonlinear effects.
With these measurements, especially by measuring the rectification effect, one can
determine crystal parameters like the product  of the charge carrier mobility and
the charge carrier lifetime or the “real” internal field, taking losses at the electrodes
into account.
The experiments are not confined to photorefractive crystals. For measuring the rectification effect one doesn’t need the electro-optic effect. Therefore, this is a simple
82
method for the characterisation of semi-insulating semiconductors. It is planned to
investigate further semiconductors. Promising are also semi-insulating quantum-dot
semiconductors, because some interesting results can be expected.
Publications




M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Optically Induced Nonlinear Wave Processes in Photorefractive Crystals, Technical Digest IQEC 2002,
375 (2002)
S. Schwalenberg, F. Rahe, E. Krätzig, Recording Mechanisms of Anisotropic Holographic Scattering Cones in Photorefractive Crystals, Optics Commun. 209, 467
(2002)
M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Overall Rectification and
Second Harmonic Generation of Space Charge Waves, Phys. Rev. B 66, 085107
(2002)
M. P. Petrov, V. V. Bryksin, F. Rahe, C. E. Rüter, E. Krätzig, Space Charge Rectification Effects in Photorefractive Bi12TiO20 Crystals, Optics Commun., submitted
Attended lectures
Linear response theory (P. Hertel)
The photorefractive nonlinearity (E. Krätzig and K. H. Ringhofer)
Nonlinear wave equations (H.-J. Schmidt)
Seminars and workshops of the Graduate College
Conference visits
June 22–27, 2002 IQEC/LAT 2002 in Moscow; contribution: Optically Induced Nonlinear Wave Processes in Photorefractive Crystals
Research stays
 September 17 – December 02, 2001 National Academy of Sciences, Institute of
Physics, Kiev, Ukraine (Prof. Dr. Serguey Odoulov).
 June 14 – 21, 2002 IOFFE Physico-Technical Institute Russian Academy of Sciences, St. Petersburg, Russia (Prof. Dr. Mikhail P. Petrov).
Duration of the dissertation: Start 01.01.01, termination expected end 2003
Period of support in the college: 01.01.01 to 31.12.03
Supervisor: Prof. Dr. E. Krätzig; in cooperation with Prof. Dr. M. P. Petrov, A. F. Ioffe
Physico-Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia.
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Prof. Dr. Marika Schleberger
Topic: Scanning force microscopy to image ferroelectric domains
Results
The main goal of our project was to image ferroelectric domains in doped and undoped SBN single crystals by means of scanning force microscopy [1]. We wanted to
find out what the topography of the domains looks like and whether the size of the
domains depends significantly on the concentration of the dopant or not. The atomic
force microscope (AFM) is ideally suited for such investigations since the instrument
is capable of measuring the topography as well as electrostatic interactions with a
spatial resolution of a few nanometers.
Our first experiments with the AFM showed images which we interpreted as ferroelectric domains. The measurements were done in the contact-mode with a Si3N4 tip in
air. The domain structure of the planes normal to the c-axis typically show a coral-like
pattern of troughs which are about 1.5 nm deep and roughly 100 x 100 nm 2 in size.
The typical domains on the planes that are parallel to the c-axis are much smaller.
They are elongated and exhibit the same depth of 1.5 nm.
We had problems with a few of the crystals we looked at since we could not get the
tip into contact with the surface. An effect that is most likely due to strong electrostatic interactions. These crystals could only be measured in the non-contact mode
where the tip is oscillating at its resonance frequency some distance away from the
sample. In this mode of measurement we make use of the frequency shift as the
feedback signal. The frequency shift is due to the charges present on the surface.
We found basically the same domain pattern, however, the images appeared somewhat blurry.
A clear improvement of the images could be achieved by using the damping of the
cantilever instead. Since the damping - unlike the frequency shift - varies exponentially with the tip-sample distance heights can be measured even more exactly then
with conventional AFM. However, this method was used for the first time, and therefore, we know little about the origin of the contrast in those images. The images must
thus be interpreted with some care and cannot be simply regarded as pure topography data.
We could not influence the domain structure by neither poling nor depoling the crystals. This can be easily explained if we assume the following: The domain structure is
already present after the growth of the crystals. The basic etch that is subsequently
used for polishing is more aggressive in the domains with a corresponding polarization, i.e., there will be more material removed in these areas. The ferroelectric domains are thus “written” into the crystal surface. This process is of course limited to
the immediate surface and has no influence on the ferrolectric properties of the bulk.
With the AFM we see only the topography of the original ferroelectric domains. New
domains or domain structures that are influenced by electric fields are obviously not
formed on the surface or are to weak to be detected by the AFM. In order to test this
theory the crystals should be depoled before the polishing process. Unfortunately,
these experiments could not be performed anymore in the frame of this short project.
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All experiments were done in close collaboration with Martin Görlich and Monika
Wesner.
References
[1] P. Lehnen, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric nanodomains in
the uniaxial relaxor system Sr0.61-xBa0.39Nb2O6:Ce-x(3+). Phys. Rev. B 6422, art. no.
224109 (2001).
Period of support in the college (postdoc): 01.01.01 to 30.04.01
Supervisor: Prof. Dr. E. Krätzig
Dr. Schleberger left the graduate College 30.04.2001 to continue research within the
scope of the Heisenberg-Programm of the DFG. Now she is working as a professor
at the University of Essen.
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86
87
88
Attended lectures
The photorefractive nonlinearity (E. Krätzig and K. H. Ringhofer)
Nonlinear wave equations (H.-J. Schmidt)
Seminars and workshops of the Graduate College
Duration of the dissertation: Start 01.12.01, termination expected end 2004
Period of support in the college: 01.12.01 - 31.12.03
Supervisor: PD Dr. J. Schnack, Prof. Dr. K. Bärwinkel, apl. Prof. Dr. H.-J. Schmidt
89
Dipl.-Math. Elena D. Svetogorova
Topic: Reflection and transmission of a plane TE-wave at a lossless nonlinear
dielectric film with a permittivity depending on the transverse coordinate
90
91
92
93
Attended lectures, conference visits, research stays
WS 01/02: P. Hertel, Linear response theory
SS 02: E. Krätzig/K. Ringhofer, The photorefractive nonlinearity
WS 02/03: H.-J- Schmidt, Nonlinear wave equations
Workshop "Photorefractive Nonlinearities" (October 2001, Osnabrück)
Seminars of the Graduate College 695 (WS 01/02, SS 02, WS 02/03)
Duration of the dissertation: Start 01.09.2001, termination expected 01.09.2004
Period of support in the college: 01.09.2001 - 31.12.2003
Supervisors
Prof. Dr. H. W. Schürmann, Department of Physics, University of Osnabrück,
Prof. Dr. V. S. Serov, Department of Mathematical Sciences, University of Oulu, Finland
94
Dipl.-Phys. Arthur Tunyagi
Topic: Nonlinear optical and dielectric properties of undoped StrontiumBarium-Niobate near the phase transition.
Results
Strontium-Barium-Niobate (SBN), SrxBa1-xNb2O6, can be grown in a wide composition
range of x=0.25…0.8 (for details of the crystal growth process see report of M. Ulex).
The crystals undergo a structural phase transition from a ferroelectric lowtemperature to a paraelectric high-temperature phase at temperatures above room
temperature. The phase transition is of relaxor type – more or less broadened in
temperature. Broadening and transition temperature depend on composition, dopants, and inhomogeneities. The aim of our project is the measurement of linear and
nonlinear optical and of dielectric properties around the phase transition temperature
of SBN. For undoped crystals in the whole composition range, the influence of the
phase transition on these properties is studied . On the other hand, these properties
– especially those which are very sensitive for the structural change at the phase
transition – are used to investigate the phase transition itself.
Construction and rebuilding of set-ups
During the first period of the project it was necessary to design and build-up several
new experimental set-ups. Furthermore, existing arrangements had to be updated or
partially renewed, redesigned computer control using C++ or Matlab had to be added.
Due to the main topic of the project – the temperature dependent study of nonlinear
optical properties – a set-up for the investigation of the second harmonic generation
(SHG) of light was constructed. It offers now the possibility to measure the second
harmonic generated from a Nd:YAG laser (1064 nm) as a function of the temperature
of the sample. The control program consists of several C++ routines for heater control, temperature measurement, data acquisition using either single pulse detection
up to several kHz repetition rate or averaging, plotting, and more. The SHG measurements described were performed using this set-up.
For measuring the permittivity a set-up using an LRC bridge (Hewlett-Packard
4284A) and a commercial temperature controller (Profile PRO 800) has been built. A
control program in C++ was developed which allows to measure the permittivity as a
function of temperature and frequency. With this set-up the dielectric measurements
on the crystals were performed.
Furthermore, a data acquisition interface for a Fabry-Perot spectrometer has been
renewed, which now consists of a photon counting card controlled by a C++ program.
95
Refractive index measurements on SrxBa1-xNb2O6
The ordinary and the extraordinary refractive index for available compositions have
been measured using a goniometer and the prism method. For the visible region a
mercury lamp was used, for the infrared region two laser-diodes of 790 nm and
1550 nm. The infrared light was detected using an IR-sensitive video camera. The
experimental points could be consistently described by Sellmeier relations. While the
ordinary refractive index is practically independent of the Sr/Ba ratio, the extraordinary index decreases with decreasing Sr content thus increasing the birefringence.
The results are shown in Figure 1.
Figure 1: Refractive index as a function of the wavelength for SrxBa1-xNb2O6 with x =
0.52 . . . 0.8.
Second Harmonic Generation on SrxBa1-xNb2O6
The results of the refractive index measurements (Fig. 1) show that in SBN phasematched second harmonic generation is not possible using a Nd:YAG laser as the
fundamental light source. Yet, non-phase-matched SHG can be efficiently used to
study the structural phase transition of SBN: From the crystal structure of SBN [1,2]
with point symmetry 4 mm for the ferroelectric and 4 / mmm for the paraelectric
phase one can derive that second harmonic light can be generated only in the ferroelectric phase. When the temperature is increased, the decay in the second harmonic
intensity around the phase transition temperature reflects the transition from the noncentrosymmetric low-temperature to the centrosymmetric high-temperature phase.
96
A typical measurement of the second harmonic intensity as a function of temperature
is shown in Figure 2.
Figure 2: The result of the SHG measurement for SrxBa1-xNb2O6 with x=0.52 (upper
curve: SHG due to the tensor element d33, lower: tensor element d31).
Two different polarization geometries are chosen, polarization of the fundamental
beam parallel or perpendicular to the polar axis of SBN (c-axis), respectively. In both
cases the second harmonic polarization was parallel to the c-axis. Thus, the tensor
elements d33 and d31 can be derived from the measurements. As general trends for all
compositions investigated up to now we can derive:
– d33 generally is larger than d31 throughout the whole composition range,
– both d33 and d31 increase with increasing Ba content,
– the difference between d33 and d31 increases with increasing Ba content.
In the future we intend to make various measurements clarifying the correlation between poling state and second harmonic intensity. The second harmonic intensity
then could be used as a sensitive measure for studying the poling dynamics.
Very sensitive SHG measurements revealed a new, to date unknown, noncolinear
SHG process which becomes visible when the laser beam is directed parallel to the
c-axis of the crystal. The effect is closely connected to the domain geometry of the
crystals for which a needle-like structure had been postulated [3] and seems to be
present in crystals of all compositions investigated up to now. Further, thorough
measurements are necessary to assure the features of this new effect and to derive
at least a simple physical model for it. Explanations developed for noncolinear SHG
found e. g. in lithium niobate [4,5] can not be adopted for SBN.
Permittivity Measurements on SrxBa1-xNb2O6
More information about the phase transition characteristics can be derived from the
electric permittivity. At different frequencies, the capacitance of the sample was
measured as a function of temperature. From these measurements we were able to
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determine the relaxor-typical broadening of the phase transition in SBN [6,7] as a
function of the composition. A typical result is presented in Figure 3.
Figure 3: A typical result for an electric permittivity measurement as a function of
temperature and frequency (SrxBa1-xNb2O6 with x = 0.52 ).
Analysing all results we can conclude that samples with higher strontium content
show more expressed relaxor properties, whereas in the Ba-rich samples this feature
is only weakly expressed. Because the relaxor features are more pronounced at low
frequencies we intend to extend our measurements to that region in future.
OH-stretching modes in SrxBa1-xNb2O6
In cooperation with C. David the behaviour of the OH-stretching modes in SBN was
measured. We observed a significant influence of the composition on the OHstretching mode absorption spectra. With rising x, the absorption of the main band at
about 3495 cm-1 increases, the low energy shoulder decreases and an additional
broad absorption is built up. This shows that hydrogen ions can occupy several different positions in the unfilled tungsten bronze structure of SBN which are energetically non-equivalent. A thorough evaluation is presently being developed. More details about the sample treatment and the measurements are given in the report of C.
David.
References:
[1] T.S. Chernya, B.A. Maksimov, I.V. Verin, L.I. Ivleva, V.I. Simonov ; Cryst. Reports,
42, 375-380 (1997)
[2] T.S. Chernya, B.A. Maksimov, I.V. Verin, L.I. Ivleva, V.I. Simonov ; Physics of the
Solid State 42, 1716-1721 (2000)
[3] S. Kawai, T. Ogawa, H. S. Lee, R. C. DeMattei, R. S. Feigelson : Appl. Phys. Lett.
73, 6 (1998)
[4] A. Reichert, K. Betzler : J. Appl. Phys. 79, 2209 (1996).
[5] K.-U. Kasemir, K. Betzler: Appl. Phys. B 68, 763 (1999).
[6] L.E. Cross Ferroelectrics 76, 241-267 (1987)
[7] I.A. Santos, J.A. Eiras : J.Phys, Cond. Matter 13, 11733-11740 (2001)
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Publications
Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke:
Composition dependence of the ultraviolet absorption edge in lithium tantalate.
J. Appl. Physics, in print (2003)
C. David, A. Tunyagi et al.: OH stretching modes in SrxBa1-xNb2O6 (in preparation)
Attended lectures
WS 01/02 : P. Hertel: Linear response theory
SS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearity
WS 02/03: H.-J. Schmidt: Nonlinear wave equations
WS 01/02: V. Trepakov: Optics and Spectroscopy of semiconductors and insulators
Workshop “Photorefractive Nonlinearities” (October 2001, Osnabrueck)
Workshop “SBN - a typical relaxor?” (May 2002, University of Osnabrueck)
Workshop “SBN: Crystal Growth and Details of the Structure” (July 2002, University
of Osnabrueck)
Seminars of the Graduate College 695 (WS 01/02 SS 02 WS 02/03)
Seminars of the Research Group “Optical Materials” (WS 01/02, SS 02, WS 02/03 )
Contribution to the Seminars
Seminary Talk on 21.10.2002 “Second Harmonic Generation on SBN Crystals”
Various short talks in the Research Group seminar
External research stay
IR-absorption Measurements performed on SZFKI institute in Budapest (08.07.2002
– 19.07.2002)
Duration of the dissertation: Start 01.09.2001, termination expected 31.08.2004
Period of support in the College: 01.09.2001 – 31.12.2003
Supervisor: Apl. Prof. Dr. Klaus Betzler
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Dipl.-Phys. Michael Ulex
Topic : Growth and characterization of SrxBa1-xNb2O6 crystals with x ranging
from 0.2 to 0.8
Results
Abstract
Fifteen different compositions of SrxBa1-xNb2O6 with crystal compositions from x =
0.32 to 0.79 were grown and investigated. Their quality allows investigations of optical properties. A preliminary phase diagram has been determined. The lattice constants and the densities were measured. In addition, using the lattice constants, the
densities were calculated; good agreement with the measured ones was found. Further properties have been reported by C. David and A. Tunyagi.
Introduction
SrxBa1-xNb2O6 solid solutions (SBN) with compositions ranging from x = 0.25 to 0.75
are known since 1960 and studied by many groups.
For the binary system SrNb2O6-BaNb2O6 the phase diagram was determined by Carruthers et al. [1] in 1970, indicating a wide range of solid solution and a congruently
melting composition at x = 0.5. Later Megumi et al. [2] found a value of the congruently melting composition of x = 0.61 (1977). However, Carruthers et al. did only determine the liquidus curve, but not the solidus one. Therefore the exact composition
range of the solid solution is unknown. Additionally, preliminary results of our crystal
growth experiments showed, that the variation of the liquidus temperature with composition is much smaller than determined by Carruthers et al.
Studies of almost all important properties of SBN have been reported, but a systematic study of selected properties as a function of the composition is still missing and is
besides the crystal growth the subject of this project. These investigations will be
done in co-operation with the Ph.D.-students C. David und A. Tunyagi (see their reports).
Crystal growth and first optical assessment
The crystals are grown in a resistance-heated furnace with the Czochralskitechnique. Because of resistance-heating the temperature gradient ΔT within the
melt is about 1 °C/cm while the temperature stability is better than 0.1 °C. The crystals are grown in [001]-direction with a pulling-rate of 0.8 mm/h for compositions xcr ≥
0.5 and with 0.4 mm/h for compositions xcr < 0.5. During the growth process the crystal rotates with 38 cycles per minute. The experiments are performed in the temperature range 1484 °C ... 1496 °C.
Crystals with a composition xcr ranging from 0.32 to 0.79 have been grown. The crystals of good optical quality are transparent and colourless and have a length up to 80
mm and a diameter of about 5 mm.
For the investigations of the physical properties the crystals were cut into eight different objects with shapes like plates, cubes and prisms and finally grinded and polished.
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Figure 1: SBN crystals grown within this study (xcr = 0,34: below, xcr = 0,61: above)
The crystals have good quality suitable for optical measurements. However, tests
with crossed polarizers have shown inhomogeneities for xcr ≠ 0.61, which increase
with compositions more off the congruently melting one.
It is assumed, that these inhomogeneities arise from an accumulation or reduction of
Sr or Ba at the phase boundary. Experiments to reduce this kind of inhomogeneities
by variations of the rotation rate and the vertical temperature gradient in the crystal
growth apparatus are in progress.
Figure 2: Photo of an SBN-crystal (xcr = 0.79, c-cut) taken with crossed polarizers
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Determination of the phase diagram
To improve the phase diagram, the compositions of the crystals were determined by
X-ray fluorescence analyses. For this purpose 500 mg of the crystal to be analyzed
or of a standard with a well-known composition are solved in 5 g of Spectromelt A12
(Merck) at 950 °C in a Pt/Au-crucible. The X-ray fluorescence analysis was powered
by a copper-source and analyzed by a LiF grating. The lines Lα1 (Ba) and Kα (Sr and
Nb) were measured with a statistical uncertainty of less then 0.13 %.
With the help of the standard the composition of the crystal xcr can be determined
with a reproducibility of Δx = ± 0.005.
Table 1: Composition of the melt (xm) and of the crystals grown from this melt (xcr):
xcr
0.788
0.787
0.779
0.736
0.688
0.644
0.613
0.563
xm
0.805
0.812
0.787
0.753
0.700
0.650
0.610
0.550
xcr
0.511
0.477
0.446
0.404
0.382
0.341
0.322
xm
0.492
0.431
0.373
0.319
0.300
0.243
0.194
Figure 3 shows the measured compositions of the crystals and of the melt. The y-axis
shows the growth-temperature at a crystal length of about 60 mm. The lines represent a preliminary phase diagram. This phase diagram shows a small difference between the solidus curve (solid line) and the liquidus curve (dotted line) at the Sr-rich
range (Δx = 0.02 for xcr = 0.79) and a large difference at the Ba-rich range (Δx = 0.13
for xcr = 0.32). The data include an error of about ± 3 °C relative to each other and
have an absolute uncertainty of ± 20 °C.
1505
Temperature (°C)
1500
1495
1490
1485
xmelt
1480
xcrystal
1475
1470
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
Composition
Figure 3: Preliminary phase diagram
Density and lattice constants
The lattice constants were measured in co-operation with Prof. Dr. Schmahl at the
University of Bochum. The measurements of the density were performed in cooperation with Prof. Dr. Bohatý at the University of Cologne. The results of these
measurements are shown in figure 4 and 5.
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3,96
12,47
Lattice constants a (Å) .
a
12,46
3,95
12,45
3,94
12,44
3,93
12,43
3,92
12,42
3,91
c
3,90
12,41
0,50
0,55
0,60
0,65
xcr
0,70
0,75
0,80
Figure 4: Lattice constants a (•) and c (■) as a function of composition xcr
Density (mg/mm3)
5,40
5,35
5,30
5,25
5,20
0,35
0,45
0,55
xcr
0,65
0,75
Figure 5: Density of SBN-crystals as a function of composition xcr
The density was also calculated by using lattice constants. The result of the calculation agrees quite well, a constant difference of Δ ρ = 0.039 mg/mm3 ± 0.006 mg/mm3
or Δ ρ/ρ = 0.8 % was found. The origin of this difference is still unclear and will be
subject of further investigations.
The following properties have been measured by my colleagues C. David and A.
Tunyagi, who are members of the graduate school, too. They are concerned with:
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- Absorption measurements of the band edge and of the OH-stretching vibration (C.
David)
- Refractive indices (A. Tunyagi)
- Second harmonic generation (A. Tunyagi)
- Dielectric constants (C. David, A. Tunyagi).
Future plans
For the construction of the phase diagram liquidus temperatures still have to be
measured. For this purpose an existing furnace was modified [3].
Further properties like thermal expansion of the crystals or the determination of the
distribution of Ba and Sr on the different lattice sites along the a- and b-direction will
be done in co-operation with other groups (Prof. Schmahl, University of Bochum,
Prof. Bohatý, University of Cologne).
References
[1] J. R. Carruthers, M. Grasso: “Phase Equilibria Relations in the Ternary System
BaO-SrO-Nb2O5“. Journal Electrochemical Society 117, 1426 (1970).
[2] K. Megumi, N. Nagatsuma, Y. Kashiwada, Y. Furuhata: “The congruent melting
composition of SBN”. Journal of Materials Science 11, 1583 (1977).
[3] Ch. Kuper, R. Pankrath, H. Hesse: “Growth and dielectric properties of congruently melting Ba1-xCaxTiO3 crystals”. Applied Physics A, 65, 301 (1997).
Publications
C. David, A. Tunyagi, M. Ulex et al.: “OH stretching modes in SrxBa1-xNb2O6” (in
preparation)
Attended lectures, conference visits, research stays
Attended lectures
WS 01/02: P. Hertel: Linear response theory
SS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearity
WS 02/03: H.-J. Schmidt: Nonlinear wave equations
Seminars
Seminars of the Graduate College 695 (WS 01/02, SS 02, WS 02/03)
Conference visits
Working group „Kristalle für Laser und nichtlineare Optik“ of DGKK:
- 27.-28.9.2001 in Köln
- 26.-27.9.2002 in Bonn
Annual conference of the DGKK:
- 20.-21.3.2002 in Idar-Oberstein
Contribution to the seminars
Seminary talks on 11.11.2001 and 2.12.2002
Various short talks in the seminar of the research group
External research stays
30.11.2001
Institute of Crystal Growth Berlin-Adlershof, Dr. Reiche
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22.-26.4.2002
Institute of Physics of the Russian Academy of Science, Laboratory
for Crystal growth, Dr. Ivleva
Institute of Crystallography of the Russian Academy of Science,
Prof. Dr. Volk
13.9.2002
University of Cologne, Institute of Crystallography, Prof. Dr. Bohatý
5.11.2002
University of Bochum, Institute of Crystallography, Prof. Dr. Schmahl
19.-20.12.2002 University of Cologne, Institute of Crystallography, Prof. Dr. Bohatý
Duration of the dissertation
Start: 1.5.2001, determination expected 30.4.2004
Period of support in the College
1.5.2001 to 31.12.2003
Supervisors
Dr. Rainer Pankrath, apl. Prof. Dr. Klaus Betzler
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Dr. Monika Wesner
Topic: Nonlinear optical properties of photorefractive strontium-barium niobate
crystals
Results
During the period of support in the graduate college I was able to finish the research
on nonlinear optical properties of photorefractive strontium-barium niobate crystals
(SBN), and to write the dissertation “Nichtlineare optische Effekte im Ferroelektrikum
Strontiumbariumniobat”. Oxide ferroelectric strontium-barium niobate crystals are the
basis of my investigations. They are favorably suited for experiments in nonlinear
optics. Their special features are large nonlinear, for example pyro- and piezoelectric,
electro- and thermooptic coefficients, robustness and a high optical quality. Moreover, excellent barrier-waveguides can be produced by ion-implantation. The ionimplantation is performed in collaboration with Dr. P. Moretti from the University of
Lyon, France. During my dissertation, several other oxide crystals have been
checked as an alternative for SBN, too [D, I ]. However, for the performed nonlinearoptical experiments in the field of thermooptic beam self-focusing, photorefractive
modulational instability, pattern and soliton formation, other crystals rarely prove to
be a comparable alternative to SBN volume crystals or SBN waveguides.
SBN crystals of the congruently melting composition, grown by Dr. R. Pankrath at
the University of Osnabrück, possess an internationally acknowledged quality. The
dominating electrooptic coefficient r33 could be shown to be larger than 200 pm/V
even for near infrared wavelengths up to  = 1.5 µm [E]. Due to the large electrooptic
coefficients, photorefractive effects are possible even in the infrared, though ferroelectric insulator crystals are seldom investigated in this interesting (telecommunication) wavelengths range. After the polishing of the crystals, the surfaces are optically
flat. Checks of the surface quality were performed with an atomic force microscope
(AFM) in collaboration with Prof. Dr. M. Schleberger, now University of Essen. She
could prove the existence of worm- or leavelike structures on the (001)-surfaces of
the SBN-crystals. The depth of these structures does not exceed 2 nm. We have
clear hints, that the structures are indeed ferroelectric domain patterns, which are
conserved during the polishing process. Up to now, there exist only few figures of the
domain structure of SBN throughout the literature [1].
By thermooptic self-focusing effects, so-called thermal lenses could be induced in
SBN-waveguides for the first time. In contrast to usual observations in volume oxide
crystals, the induced thermal lenses maintain their spherical properties over a large
range of laser powers. For thermooptic effects, the laser power determines the magnitude of the nonlinearity. An example is demonstrated in Fig. 1. Here, a focused
beam of an Ar+-laser is coupled into the SBN-waveguide. Shown is the laser beam
profile at the crystal’s endface. In the figure, the changes of the beam profile with
increasing input laser power Pin, i. e., with increasing nonlinearity, are shown in a
contour plot. Due to thermooptic refractive index changes, the beam self-focuses up
to laser powers of about 50 mW. By this way, spherical lenses with focal lengths
around 1 mm can be induced. Thermooptic beam filamentation occurs, if the focal
point of the lens reaches the inner part of the waveguide. This can also be seen in
Fig. 1 for laser powers larger than Pin = 50 mW. The filamentation can be shown to
be as well explainable with the model of spherical aberration of the thermal lens [2] or
with modulational instabilities [3]. Thermooptic refractive index changes build up
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comparatively fast (100 µs-magnitude) [A] and are shown to be useful to switch and
focus light beams and divide them into different channels.
Fig. 1: Example of thermooptic nonlinear effects in
SBN-waveguides. Shown is
a contour plot of the beam
profile of an incoupled Ar+laser beam at the crystal’s
endface for increasing laser
power Pin.
A further part of my doctoral thesis is devoted to modulational instabilities, which
manifest in the filamentation of an initially homogeneous beam profile. Modulational
instabilities are induced by perturbations of the beam profile which grow exponentially. The instabilities occur, sometimes inevitable, as side effect during experiments of
other nonlinear phenomena, especially at the photorefractive soliton formation. A detailed knowledge of them is therefore of importance. However, this subject is seldom
treated experimentally for the photorefractive nonlinearity. Just as rare are theoretical
considerations concerning this nonlinearity which can be approximated as a nonlinear Schödinger equation with saturable nonlinearity (see report F. Homann). Experimentally, input beam configurations are found, which have a high resistance against
modulational instabilities. Following, this finding proves helpful in experiments with
photorefractive solitons. If, instead of a single laser beam, two counterpropagating
beams are used, the irregular filaments of the modulational instability order, for instance to hexagonal patterns. The additionally observed more complicated structures
point to the fact that in photorefractive crystals even patterns with higher than hexagonal symmetry can be induced, provided that the saturation grade of the nonlinearity
is large enough.
An important part of my dissertation is devoted to photorefractive spatial solitons.
Spatial solitons are stable light beams propagating with a constant beam shape. Due
to their energy-conserving properties they are promising for applications, for example
in telecommunications. Besides the usefulness for applications, the special solitons’
features make them interesting for theoretical physicists and mathematicians since
the first investigations in the 1870th. Since 1993 it is known that solitons can be induced in photorefractive crystals [4]. Following, a research boom set in, because now
physicists had found an easily accessibly optical system, where diverse aspects of
soliton formation could be studied experimentally. The experiments done in Osnabrück were the first measurements in SBN-waveguides [ B, C, E, G - I]. Part of the
work has been done in collaboration with Prof. Dr. V. Shandarov from the University
of Tomsk, Russia, and Prof. Dr. J. Xu, Nankai University, China. The experimental
proof of the existence of a soliton is difficult as a matter of principle. We could verify
107
for the first time that photorefractive solitons exist in SBN-waveguides up to wavelengths of 1.5 µm, i. e., up to the telecommunication wavelengths region. One of the
most convincing experiments is demonstrated in Fig. 2. Shown are the intensity profiles at the endfaces of samples of the same SBN-crystal, but with different propagation lengths. The input conditions, especially the input beam width of 44 µm, are kept
constant. In Fig. 2 obviously the output beam profiles are almost equal despite the
different propagation lengths – which only can be explained by soliton formation. The
soliton-like behavior is in striking contrast to the usual one, which is a “normal” nonlinear lens as demonstrated in Fig. 1.
Fig. 2 Experiment to verify
the existence of photorefractive solitons in SBNwaveguides at a wavelength  = 1310 nm. Shown
are beam profiles at the
endfaces of three samples
of the same SBN-crystal
with different propagation
lengths z = 1.7, 5.2, and
7.8 mm. The input beam
width is kept constant at 44
µm.
We further found that the input beam configuration mostly used in experiments with
photorefractive solitons is not ideal. With a changed beam geometry we were able to
induce photorefractive solitons over an intensity range of five orders of magnitude,
which has to be compared with two orders of magnitude demonstrated in the literature so far. Moreover, the easily inducible solitons also prove to be well suited as a
measurement method, for instance to determine the so-called dark intensity (a ratio
of photo- and dark conductivity) or to investigate the poling state of a ferroelectric
crystal spatially-resolved.
In investigations of the temporal development of the solitons we were able to prove
the experimental value of a theoretical model of Fressengeas et al. [5]. This model
allows far reaching predictions, however, at the expense of rough approximations.
Because of that, at the beginning, the experimental applicability of the model was
doubtful. This work is, to our knowledge, the only one where the complicated temporal development of solitons is extensively classified. One of the most remarkable
results of the theoretical and experimental investigations is that the build-up of photorefractive solitons is not markedly influenced by the wavelength. This is in striking
contrast to other photorefractive phenomena, e. g., the common photorefractive twobeam coupling [6], which has a distinctly slower development time in the infrared than
at visible wavelengths.
A large section of my thesis covers problems of the switching of the polarization in
SBN. Despite the long time of research in ferroelectrics (since the 1940 th), the
switching behavior of ferroelectrics still pose questions. This applies the more for relaxor-ferroelectrics like SBN, which are characterized by a broadened phase transi108
tion. Important information about the poling state of the crystal can be gained with
self-focusing methods developed during the dissertation (see above), as well as by
frequency-doubling microscopy. The frequency-doubling measurements have been
performed in collaboration with A. Rosenfeldt, University of Münster, and PD Dr. M.
Flörsheimer, INE Forschungszentrum Karlsruhe GmbH. In frequency-doubling microscopy, ferroelectric domain walls are made visible. In our investigations we found
that if a SBN-crystal is poled in opposite directions by application of external electric
fields at room temperature, the switchable polarization decreases (“ages”) from the
positive towards the negative electrode. In the literature, “aging” is mostly measured
in the volume of the crystal. For this reason there are only few hints, that “aging” can
indeed be a spatially inhomogeneous effect.
Moreover, for the first time systematically depolarized stripes with widths in the micron range have been produced by intense illumination and application of electric
fields [F]. The depolarized stripes could be shown to possess an enlarged refractive
index compared to the poled material [7]. Because of that the depolarized stripes
serve as waveguides for light. This can be seen in Fig. 3. Here, each image shows
an about 80 x 80 µm² wide area of the crystal’s endface. The lateral position of the
crystal is changed relatively to the illuminating stripe laser beam. The relative position
is mentioned below each picture. Row a) shows the initial intensity distribution, which
is the same for all positions. After the production of the stripes, in row b), obviously
two waveguiding channels appear at the positions +/-0.03 mm. The channels are
stable for at least a month if the crystal is kept in the dark, and for at least a week if
the crystal is illuminated with intense read-out light.
Fig. 3: Each picture shows an about 80 x 80 mm² wide area of the crystals endface.
The crystal position is changed relative to an illuminating stripe laser beam. The position is mentioned below each picture. (a) inititial intensity distribution (b) after fabrication of two waveguides at x = +/- 0.03 mm.
More complicated structures can be formed with this new method of electrical fixing,
too. Promise have produced patterns of poled and unpoled stripes, with stripe widths
below 1 micron, which can be used for quasi-phase-matched frequency doubling.
References:
[1] P. Lehnen, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric nanodomains in
the uniaxial relaxor system Sr0.61-xBa0.39Nb2O6:Ce-x(3+). Phys. Rev. B 6422, art. no.
224109 (2001).
[2] S. A. Akhmanov, D. P. Krindach, A. V. Migulin, A. P. Sukhorukov, R. V. Khokhlov.
IEEE J. Quantum Electron. QE-4, 568 (1968).
109
[3] V. I. Bespalov and V. I. Talanov. JETP Lett. 3, 307 (1966).
[4] G. C. Duree, J. L. Shultz, G. J. Salamo, M. Segev , A. Yariv, B. Crosignani, P.
DiPorto, E. J. Sharp, R. R. Neurgaonkar. Phys. Rev. Lett. 71, 533 (1993).
[5] N. Fressengeas, J. Maufoy, G. Kugel. Phys. Rev. E 54, 6866 (1996).
[6] D. L. Staebler, J. J. Amodei. J. Appl. Phys. 43, 1042 (1972).
[7] Th. Woike, T. Granzow, U. Dörfler, C. Poetsch, M. Wöhlecke, R. Pankrath.
phys. stat. sol. (a) 186, R13 (2001).
Publications
[A] D. Kip, M. Wesner, E. Krätzig, V. Shandarov, P. Moretti, "All-optical beamdeflection and switching in planar strontium-barium niobate waveguides“. Appl. Phys.
Lett. 72, 1960 (1998).
[B] D. Kip, M. Wesner, V. Shandarov, P. Moretti, “Observation of bright spatial photorefractive solitons in a planar strontium-barium niobate waveguide”. Opt. Lett. 23, 821
(1998).
[C] D. Kip, M. Wesner, C. Herden, V. Shandarov, "Interaction of spatial photorefractive solitons in a planar waveguide”. Appl. Phys. B 68, 971 (1999).
[D] V. Shandarov, M. Wesner, J. Hukriede, D. Kip, "Observation of dark spatial photovoltaic solitons in planar waveguides in lithium niobate”. J. Opt. A: Pure Appl.
Opt. 2, 500 (2000).
[E] M. Wesner, C. Herden, D. Kip, E. Krätzig, P. Moretti, "Photorefractive steadystate solitons up to telecommunication wavelengths in planar SBN waveguides”. Opt.
Commun. 188, 69 (2001).
[F] M. Wesner, C. Herden, D. Kip, "Electrical fixing of waveguide channels in strontium-barium niobate crystals”. Appl. Phys. B 72, 733 (2001).
[G] M. Wesner, C. Herden, R. Pankrath, D. Kip, P. Moretti, "Temporal development
of photorefractive solitons up to telecommunication wavelengths in SBN”. Phys. Rev.
E 64, 36613 (2001).
[H] D. Kip, C. Herden, M. Wesner, "All-optical signal routing using interaction of mutually incoherent spatial solitons”. Ferroelectrics 274, 135 (2002).
[I] J. Xu, V. Shandarov, M. Wesner, D. Kip, "Observation of two-dimensional spatial
solitons in iron-doped barium-calcium titanate crystals”. phys. stat. sol. (a)
189, R4 (2002).
[J] D. Kip, M. Wesner, E. Krätzig, V. Shandarov, P. Moretti, "Bright photorefractive
spatial solitons in optical waveguides on SBN“. Proc. SPIE 3733, 155 – 162 (1998).
[K] D. Kip, M. Wesner, C. Herden, V. Shandarov, P. Moretti, “Spatial photorefractive
solitons in planar strontium-barium niobate waveguides”. OSA TOPS 27, 479 – 482
(1999).
[L] M. Wesner, D. Kip, V. Shandarov, P. Moretti, “Thermally-induced all-optical beam
steering and switching properties of SBN waveguides”. OSA TOPS 27, 441 – 446
(1999).
[M] D. Kip, J. Hukriede, M. Wesner, E. Krätzig. "Photorefractive waveguides“. Proc.
SPIE 3801, 9 – 23 (1999).
[N] V. Shandarov, D. Kip, M. Wesner, J. Hukriede, "Development and collapse of
dark spatial optical solitons in planar waveguides in lithium niobate”. Technical Digest
CLEO Europe, CMG5 (2000).
110
[O] D. Kip, C. Herden, M. Wesner, "Electrical fixing of waveguide channels using
dynamic self-focusing in strontium-barium niobate crystals”. Technical Digest CLEO
Europe, CFF1 (2000).
[P] M. Wesner, D. Kip, P. Moretti, "Infrared photorefractive effects in ion-implanted
SBN waveguides”. Technical Digest CLEO Europe, CFF6 (2000).
[Q] M. Wesner, C. Herden, D. Kip, "A new method of electrical fixing in strontiumbarium niobate crystals”. OSA TOPS 62, 152 - 157 (2001).
[R] D. Kip, C. Herden, M. Wesner, "All-optical signal router based on the interaction
of mutually incoherent solitons”. OSA TOPS 62, 685 - 689 (2001).
[S] V. Shandarov, D. Kip, M. Wesner, “Distinctions of the characteristics of bright
spatial solitons in SBN crystals form existence curve predictions”. OSA TOPS 62,
690 - 695 (2001).
Attended lectures
 P. Hertel, Linear response theory, WS 01/02
 E. Krätzig, K. Ringhofer, The photorefractive nonlinearity, SS02
 Seminars of the Graduate College SS 01, WS 01/02, SS 02, and WS 02/03
 Workshop on Photorefractive Nonlinearities, Oct. 4 – 5, 2001, Contribution: talk “IR
photorefractive solitons in SBN”.
 Workshop “SBN: Crystal Growth and Details of the Structure” July 1, 2002
 Workshop “Strontium-Barium-Niobate (SBN) – a typical relaxor?” May 6, 2002.
Conference visits
 Topical Meeting on Photorefractive Materials, Effects, and Devices, Elsinore,
Denmark, June 25 – 27, 1999; contribution: poster “Thermally-induced all-optical
beam steering and switching properties of SBN waveguides”.
 Spring Conference of the German Physical Society, Bonn, Germany, April 3 – 7,
2000; contribution: talk "Infrarote photorefraktive Solitonen in SBN”.
 CLEO Europe 2000, Nice, France, Sept. 10 – 15, 2000; contributions: talk “Electrical fixing of waveguide channels using dynamic self-focusing in strontium-barium
niobate crystals” and talk “Infrared photorefractive effects in ion-implanted SBN
Waveguides”.
 Spring Conference of the German Physical Society, Hamburg, Germany, March
26 – 30, 2001; contribution: talk “Zeitliche Entwicklung photorefraktiver Solitonen
in planaren SBN-Wellenleitern”.
 Topical Meeting on Photorefractive Materials, Effects, and Devices, Delavan,
USA, July 8 - 12, 2001; contribution: talk “A new method of electrical fixing in
strontium-barium niobate crystals”.
Duration of the dissertation: 01.12.1998 – 07.03.2003
Period of support in the College: -Supervisors: Prof. Dr. E. Krätzig, Prof. Dr. D. Kip
111
Dipl.-Phys. Albert Wirp
Topic: Optical Frequency Conversion in Oxide Waveguides
Results
1. Introduction
Green and blue laser sources are very interesting for many applications like data
storage, medical applications, printing, laser TV and material treatment.
Semiconductor lasers of these wavelengths are rarely available. One solution is
frequency conversion, especially Second Harmonic Generation (SHG). Near infrared
laser light is frequency doubled to blue and green light, respectively.
Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) crystals are promising
candidates, because these materials are commercially available and they possess
large nonlinear coefficients. The transmission ranges are up to 320 nm for LiNbO3
and up to 270 nm for LiTaO3. To achieve SHG the phase matching condition for the
fundamental and the frequency doubled waves must be fulfilled. Due to the small
birefringence, conventional phase matching is not possible. Thus, a Quasi Phase
Matching (QPM) condition is needed. This is realised by periodic inversion of the
ferroeletric domains. The period length of the domain inversion depends on the
wavelength of the fundamental light. This means all wavelengths the crystal is
transparent for are useable. Here, LiTaO3 is favourably suited due to its broad
transparency range.
The efficiency of the SHG depends on the square of the intensity of the fundamental
light. In channel waveguides light is guided in a small area, which implies a high light
intensity over the whole conversion or waveguide length.
Under illumination LiNbO3 and LiTaO3 show photorefractive index changes, which
have to be minimized for efficient SHG. This effect can be reduced by doping the
crystal with, e.g. Mg, Zn or by using stoichiometric crystals fabricated for example by
a vapour transport equilibration technique.
2. Simulations
2.1 The Waveguides
The number of modes guided in a waveguide fabricated by diffusion of metal ions
depends on the refractive index profile and the wavelength. Important parameters are
diffusion temperature and the time of indiffusion. Furthermore, the effective refractive
index depends on temperature. To optimize these values the profiles of the
waveguides are simulated for several sets of parameters. Useful parameters are a
50 nm-thick titanium layer in a 4 m-wide stripe diffused into LiNbO3 for 20 hours at
1000 °C. This results in an index profile shown in Fig. 1. The waveguide is
sinlgemode for infrared light, but multimode for visible light. This is the best set of
parameters found so far.
112
Fig 1:The index profile of a typical
waveguide: a 4 m-wide and 50 nmthick layer of titanium is diffused into
LiNbO3 for 20 hours at 1000 ° C.
2.2 Temperature Dependence of the Refractive Index
The refractive index varies with temperature due to the thermooptic effect. The
temperature dependence can be described by a Sellmeier equation. Furthermore, an
increase of the crystal length due to an increased temperature has to be taken into
account. The thermooptic effect proves to be the major effect, but the thermal
expansion is also measured.
3. Experimental Methods
3.1 Preparation of the Waveguides
The crystals are cut from a 0.5 mm-thick wafer (z-cut orientation) to pieces of
approximately 8 x 10 mm2. The crystals are carefully cleaned and evapored with
layers of 50 nm titanium. The whole samples are covered with photoresist and stripes
are formed using lithographic techniques. The uncovered titanium is etched with an
appropriate acid and the photoresist is removed afterwords, too. Then the titanium is
diffused into the crystal for 20 hours at 1000 °C and 1250 °C for LiNbO 3 and LiTaO3,
respectively. Finally, the front and rear faces of the waveguides are polished to
optical quality. For this step a glass plate is clambed onto the crystal to get a sharp
and rectangular edge, to allow endface-coupling into the 10 m-thick waveguides.
3.2 Fabry-Perot Interferometer
The temperature dependence of the refractive index of our waveguide samples is
measured with a fabry-perot interferometer and the results are compared with theory.
This is necessary to distinguish between thermal refractive-index changes and lightinduced refractive index changes due to the photorefractive effect. The experimental
setup is shown in Fig. 2. Different wavelengths are used: 1064 nm of a Nd:YAGlaser, 633 nm of a He-Ne laser and several lines of an argon ion laser (different lines
from 514 nm to 456 nm). The light is coupled into the waveguide by a microscope
objective. The endface of the waveguide is imaged onto a photodetector. The crystal
is mounted on a peltier element, which allows to vary the temperature between room
temperature and 50 °C. Lower temperatures are not possible because of water
condensation on the crystal surface. An interference pattern is formed by the
113
transmitted beam and the beam which is reflected at the rear and at the front face of
the waveguide. On both sides only a few percent of the incident light is reflected, and
including the effect of light damping in the waveguide only small modulation on a
large background intensity is detected.
Fig 2: Experimental setup. The
detection of fabry perot interferences allows to determine the
thermal expansion and thermal
and light induced refractive index
changes at different wavelength.
4. Results and Outlook
Figure 3 shows the fabry-perot interferences at 1064 nm. The thermal expansion in
this measurement is calculated to = (1.1 ± 0.4) x 10-5 K-1. This is in good agreement
with the published value of = 1.5 x 10-5 K-1 [1]. The growing period length with
increasing temperature is caused by an increasing temperature gradient inside the
crystal, because the temperature is measured directly at the peltier element.
If the temperature is kept constant, one period of the oscillation corresponds to a
refractive-index change of 4 x 10-5. This means that light-induced refractive-index
changes as small as 2 x 10-6 should be detectable with this fabry-perot
interferometer.
In the future, light-induced refractive-index changes will be investigated in
waveguides with different lithium concentrations and dopings. Furthermore, the
poling behaviour of these waveguides will be examined.
Fig 3: Transmitted intensity vs. temperature for a wavelength of 1064
nm. The fabry perot interferences
are modulations on an offset.
114
Literature:
[1] Y. S. Kim and R. T. Smith, “Thermal expansion of lithium niobate single crystals,”
J. Appl. Phys. 40, 4637-4641 (1969).
Publications
J. Imbrock, A. Wirp, D. Kip, E. Krätzig, and D. Berben, “Photorefractive properties of
lithium and copper in-diffused lithium niobate crystals,” J. Opt. Soc. Am. B 19, 18221829 (2002)
Attended lectures, conference visits
Linear response theory (P. Hertel)
The photorefractive nonlinearity (E. Krätzig and K. H. Ringhofer)
Nonlinear wave equations (H.-J. Schmidt)
Seminar of the Graduate College
Quantum Optic School, Universities of Bonn and Potsdam, 02.04.- 12.04.2002
Research stays
University of Bonn, Group of Prof. Buse, 03.08.2001
University of Colone, “Institut für Mineralogie und Geochemie”, Group of PD Dr.
Woike, 27.11.2001
Duration of the dissertation: Start August 2001, termination expected July 2004
Period of support in the College: 01.08.2001 – 31.12.2003
Supervisors: Prof. Dr. D. Kip, Prof. Dr. E. Krätzig
115
2. Auflistung aller Kollegiat(inn)en
Doktorand(inn)en
Name
Zeitpunkt bzw.
voraussichtlich
er Zeitpunkt
der Promotion
David, Calin
September 04
Alter
bei
Eintritt
in das
Kolleg
25
Filippov, Oleg
Oktober 04
23
Goubaev, Airat
Dezember 05
23
Geisler,
Andreas
Homann, Felix
Oktober 03
32
März 04
29
Kislova, Inna
Rückkehr nach 25
Twer (Krankheit
des Vaters)
Oktober 04
25
Lapine, Mikhail
Müller, Manfred Dezember 03
25
Plenge, Jürgen Dezember 02
28
Rahe, Florian
Dezember 03
25
Shelokovskyy,
Pavlo
November 04
24
Svetogorova,
August 04
Elena
Tunyagi, Arthur August 04
21
Ulex, Michael
April 04
36
Wesner, Monika März 03
32
Wirp, Albert
26
Juli 04
24
Zeitpunkt und Ort
des ersten
berufsqualifiziere
nden
Abschlusses
Juni 99 in
Klausenburg
(Rumänien)
Januar 99 in
Moskau (Russland)
Förderzeitr Betreuer
aum im
Graduierte
nkolleg
01.10.01 31.12.03
Wöhlecke
15.11.01 31.12.03
Ringhofer,
Gorkounov,
Krätzig
Kapphan
Juni 02 in Kazan
01.12.02 31.12.03
Juli 94 in Hannover
-August 94 in
Osnabrück
02.04.01 31.12.03
August 99 in Twer
(Russland)
01.08.01 31.10.02
Juni 97 in Moskau
(Russland)
01.11.01 31.12.03
März 00 in
Osnabrück
Juli 99 in
Osnabrück
Oktober 98 in
Osnabrück
Dezember 00 in
Kharkiv (Ukraine)
01.01.01 31.12.03
--
Juni 01 in Moskau
(Russland)
Juni 00 in
Klausenburg
(Rumänien)
April 99 in Berlin
11.09.01 31.12.03
01.09.01 31.12.03
November 98 in
Osnabrück
Juli 01 in
Osnabrück
116
01.01.01 31.12.03
01.12.01 31.12.03
02.05.01 31.12.03
-01.08.01 31.12.03
Schürmann
Schmidt,
Bärwinkel,
Schnack
Kapphan
Ringhofer,
Gorkounov,
Betzler
Buse
Rühl, Flesch
Krätzig
Schnack,
Bärwinkel,
Schmidt
Schürmann
Betzler
Betzler,
Pankrath
Krätzig,
Kip
Kip,
Krätzig
Postdoktorand(inn)en
Name
Zeitpunkt der Alter bei
Promotion
Eintritt
in das
Kolleg
Dr. Kamenov,
Vladimir
Prof. Dr.
Schleberger,
Marika
Dr. Pramann,
Axel
Oktober 00
28
Juni 93
35
Juli 00
34
Zeitpunkt und Ort
des ersten
berufsqualifiziere
nden
Abschlusses
Juli 95 in Rousse
(Bulgarien)
Oktober 90 in
Osnabrück
Förderzeitr Betreuer
aum im
Graduierte
nkolleg
01.01.01 31.08.01
01.01.01 30.04.01
Ringhofer,
Krätzig
Krätzig
Oktober 94 in
Braunschweig
15.11.01 14.11.03
Rühl,
Flesch
3. Auswahl der Kollegiat(inn)en
Alle 13 Stipendien wurden national und international ausgeschrieben. Es gingen
insgesamt 82 Bewerbungen ein, darunter waren 8 interne Bewerbungen und 15
Bewerbungen von Frauen. Die Auswahl der Stipendiat(inn)en wurden von den
einzelnen Projektleitern unter Beachtung der Regeln der DFG vorgenommen, wobei
häufig Kollegen zu Rate gezogen wurden. Zusätzlich wurden von der
Mitgliederversammlung 3 weitere Kollegiat(inn)en aufgenommen.
Die insgesamt 19 Kollegiat(inn)en (das Postdoktorandenstipendium wurde
nacheinander 3-mal vergeben, ein Doktorandenstipendium 2-mal) teilen sich wie folgt
auf: 8 intern, 11 von außerhalb; 4 Frauen, 15 Männer; 10 Deutsche, 9 Ausländer.
Weiter ist noch anzumerken, dass sich unter den internen Kollegiaten die beiden
Kandidaten (Plenge, Müller) befinden, die in den Jahren 1999 und 2000 für die
besten Studienleistungen im Fachbereich Physik der Universität Osnabrück
ausgezeichnet wurden.
4. Durchführung des Studienprogramms
Im Antrag waren als Studienprogramm (zusätzlich zu den üblichen Veranstaltungen
des Fachbereichs) eine integrierte Ringvorlesung 'Nichtlinearitäten optischer
Materialien', ein regelmäßig stattfindendes Seminar und mindestens einmal im Jahr
ein Workshop vorgesehen. Diese Veranstaltungen sind alle in der vorgesehenen
Weise in englischer Sprache durchgeführt worden.
Die Ringvorlesung erstreckt sich über 4 Semester und begann im WS 01/02, als
nahezu alle Stipendien vergeben waren. P. Hertel begann mit der Einführung 'Linear
response theory'. Hier wurden zunächst die Grundlagen linearer Theorien
besprochen, ein Schwerpunkt lag jedoch auch auf der Erweiterung in Bezug auf
nichtlineare Effekte. Das Vorlesungsskriptum liegt als Anhang 1 im Teil 2 dieses
Arbeits- und Ergebnisberichtes bei. Im SS 02 folgte die Vorlesung 'The
photorefractive nonlinearity' von E. Krätzig und K. Ringhofer. Die Grundlagen der
117
Photorefraktion wurden von einem experimentell und einem theoretisch arbeitendem
Physiker beleuchtet. Wichtige Experimente wurden von Hilfskräften vorbereitet und in
den Forschungslabors den Kollegiat(inn)en vorgeführt. Das Skriptum ist in Anlage 2
im Teil 2 dieses Berichts zu finden. Im WS 02/03 behandelte H.-J. Schmidt 'Nonlinear
wave equations'. Nach einem Überblick standen solitäre Lösungen in verschiedenen
Bereichen im Mittelpunkt, z. B. in der Hydrodynamik, der nichtlinearen Optik oder der
Plasmaphysik. In Anlage 3 ist das Skriptum aufgeführt. Im SS 03 werden K. Betzler,
M. Imlau und M. Wöhlecke über ’Frequency conversion and wave mixing’ vortragen.
Weiter haben wir den Kollegiat(inn)en noch folgende spezielle Veranstaltungen
angeboten: Writer's Workshop (P. Hertel) und Graphic Workshop (K. Betzler).
Das Seminar des Graduiertenkollegs startete im SS 01 und wurde regelmäßig
weitergeführt. Kollegiat(inn)en, Gäste und Betreuer stellten ihre Ergebnisse vor. Die
Seminarprogramme liegen als Anhang 4 bei. Neben diesem Hauptseminar gab es
noch zahlreiche Seminare kleinerer Gruppen und Arbeitsbesprechungen.
Bisher wurden 3 Workshops zu Kernfragen des Graduiertenkollegs durchgeführt.
Zahlreiche auswärtige Gäste nahmen teil. Die Themen der Workshops lauteten:
'Photorefractive Nonlinearities' (04. – 05.10.01), 'Strontium-Barium-Niobate (SBN) – a
typical relaxor?' (06.05.02) und 'SBN: Crystal Growth and Details of the Structure'
(01.07.02). Einzelheiten sind aus Anhang 5 zu ersehen. – Im SS 03 ist ein
Sonderkolloquium in Memoriam Klaus Ringhofer geplant.
5. Angaben zur Vergabe der Koordinationsmittel
Die Koordinationsmittel wurden einmal für Hilfskräfte eingesetzt, die folgende
Aufgaben für das Kolleg durchführten: Hilfe beim Schreiben der Skripten, vor allem
bei der Erstellung der Bilder; Einrichtung der Rechner; Vorbereitung von
Laborexperimenten für die Vorlesung; Durchführung numerischer Rechnungen. Dann
wurde die halbe Stelle der Sekretärin auf eine dreiviertel Stelle aufgestockt.
Kopierkosten des Kollegs wurden bezahlt und Programme von allgemeinem
Interesse beschafft.
6. Interne Erfolgskontrolle des Kollegs
Zunächst übernahm jeder Betreuer die Aufgabe, den Stand der Arbeiten seiner
Kollegiat(inn)en ständig zu verfolgen und gegebenenfalls steuernd einzugreifen.
Dazu dienten auch regelmäßige Arbeitsbesprechungen. Weiter haben alle
Kollegiat(inn)en mindestens einmal im Jahr im Seminar des Kollegs über die
Fortschritte vorgetragen. Diesen Vorträgen folgte stets eine Diskussion, die weitere
Schlüsse auf den Erfolg der Arbeiten zuließ. Der Gesamtstand des Kollegs wurde
auch bei 8 Mitgliederversammlungen erörtert und überprüft.
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7. Gastwissenschaftlerprogramm
7.1 Gastvorträge
Prof. Dr. V. Serov, Moskau State Univ., Russland
“Some mathematical aspects of soliton stability”
23.07.01
Prof. Dr. V. Vikhnin, Ioffe Inst. St. Petersburg, Russland
“Report about IMF-10 Meeting results and recent theoretical
developments in the field of defects in oxidic crystals”
10.09.01
Dr. V. Matusevich, Univ. Jena
“Application of wave mixing processes in BCT”
04.10.01
Prof. Dr. C. Denz, TU Darmstadt
“Storage of volume holograms in photorefractive materials”
04.10.01
Prof. Dr. W. Lange, Univ. Münster
“What can we learn from spontaneous optical
patterns and from localized in atomic vapors?”
04.10.01
Dr. A. Kießling, Univ. Jena
“Soliton-like structures in BTO”
04.10.01
Prof. Dr. R. Kowarschik, Univ. Jena
“Application of wave mixing processes in BCT”
04.10.01
Dr. M. Goulkov, Academy of Sciences, Kiev, Ukraine
“Nature and applications of light scattering in
photorefractive crystals”
04.10.01
Prof. Dr. R. Rupp, Univ. Wien
“Holographic scattering: angular and spectral properties”
04.10.01
Prof. Dr. M. P. Petrov, Ioffe Institute, St. Petersburg, Russia
Nonlinear interactions and scattering of space charge waves
05.10.01
Prof. Dr. T.Tschudi, Univ. Darmstadt
“Novelty filters in photorefractive crystals”
05.10.01
Prof. Dr. T. Volk, Institute of Crystallography, Moscow, Russia
Ferroelectricity-driven holographic properties of RE-doped SBN
05.10.01
Dr. U. Dörfler, Univ. Köln
“Holographic studies of SBN doped with Ce and Cr”
19.10.01
Dr. T. Granzow, Univ. Köln
“Relaxor ferroelectrics: Ce-doped SBN as an example”
19.10.01
PD Dr. T. Woike, Univ. Köln
“Holographic scattering in SBN, LiNbO3:Fe and LiTaO3:Fe”
26.10.01
Dr. A. Pramann, FU Berlin
“Anion photoelectron spectroscopy of size-selected
bimetallic clusters in molecular beams”
Prof. Dr. V. Serov, Moskau State Univ., Russland
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29.10.01
“Particular solutions of nonlinear wave equations”
and “Lie groups and the sine-Gordon equation”
06.11.01
Prof. Dr. Jingjun Xu, Nankai University Tianjin, P. R. China
“Research at the Photonics Research Center at the
Nankai University Tianjin”
09.11.01
Prof. Dr. B. Wellegehausen, Univ. Hannover
“Generation of coherent VUV- and XUV-radiation
by high intensity laser-matter interaction”
19.11.01
Dr. M. Fally, Universität Wien
“10 years neutron diffraction from light-induced gratings“
23.11.01
Dr. E. Chamonina, Oxford University, UK
“Photorefractive scattering”
03.12.01
Dr. M. Ellaban, Universität Wien
„Holographic scattering in LiNbO3“
14.12.01
Prof. Dr. L. Wöste, Freie Universität Berlin
“Perspectives of femtosecond spectroscopy:
from clusters to clouds”
28.01.02
Prof. Dr. V. Serov, Moskau, Russland
“Some mathematical results of nonlinear wave guiding structures”
and ”Nonlinear evolution equations: elliptic solutions”
28./29.1.02
Prof. Dr. M. P. Petrov, Ioffe Institute St. Petersburg, Russland
“Overall rectification and second harmonic generation
of space charge waves”
08.04.02
Prof. Dr. W. Schmahl, Univ. Bochum
“Elements of the Landau theory of phase transitions”
06.05.02
Dr. T. Granzow, Univ. Köln
“Experimental overview of relaxor-type properties of SBN”
06.05.02
Prof. Dr. W. Kleemann, Univ. Duisburg
“Disordered polar systems-an overview of concepts”
06.05.02
PD Dr. M. Flörsheimer, Univ. Karlsruhe
“Second-harmonic and sum frequency imaging of
interfaces in materials science, biophysics and
environmental geochemistry”
13.05.02
Prof. Dr. N. Hansen, Univ. Henri Poivcare, Nancy, Frankreich
“Crystal structure and electron density distribution
from Bragg scattering-What can be learned in
general and what about SBN”
01.07.02
Dr. V. Petricek, Academy of Sciences, Prag, Tschechien
“Determination of the Modulated Structure of SBN”
01.07.02
Dr. J. Schefer, Lab. for Neutron Scattering, ETHZ & PSI, Villingen
“Structure measurements of SBN by neutrons”
01.07.02
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Prof. Dr. V. Serov, Moskau State Univ., Russland
„Appoximative solutions of nonlinear integral equations
via iteration procedure"
22.07.02
Prof. Dr. Smirnov Moskau State Univ., Russland
"Propagation of Electromagnetic Waves in Open
Cylindrical Waveguides with Nonlinear Media"
22.07.02
Dr. M. Goulkov, Academy of Sciences, Kiev, Ukraine
“Insight into the nature of light-induced scattering in
photorefractive crystals with dominating local response”
26.07.02
Prof. Dr. S. Odoulov, Academy of Sciences, Kiev, Ukraine
“Photorefraction in periodically poled lithium niobate”
02.08.02
Dr. Boris Sturmann, Academy of Sciences, Novosibirsk, Russland
"Singular nonlinear response and soliton-like beam propagation in
fast photorefractive crystals"
25.10.02
PD Dr. T. Woike und Dipl.-Phys. P. Herth, Univ. Köln,
“Holographische Streuung und Polaronen“
13.11.02
Prof. Dr. T. Volk, Academy of Sciences, Moskau, Russland
“Polarization kinetics and the domain structure of SBN crystals”
26.11.02
PD Dr. M. Flörsheimer, Univ. Karlsruhe
„Beobachtung der Domänenstruktur ferroelektrischer Kristalle
durch nichtlineare Mikroskopie“
29.11.02
Prof. Dr. V. Vikhnin, Ioffe Inst. St. Petersburg, Russland
“Two types of CTVEs and two types of recombination
luminescence in ferroelectric oxides”
17.12.02
Prof. Dr. M. P. Petrov, Ioffe Institute, St. Petersburg
Nonlinear Interactions of Space Charge Waves
17.01.03
Dr. B. Briat, Laboratoire d’Optique Physique, ESPCI, Paris
A combined Optical/ Magneto-Optical / ODMR approach to
the identification of defects and charge transfer processes
24.01.03
Dipl.-Phys. F. Meier, Universität Basel
“Magnetische Moleküle und MQC“
04.02.03
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7.2 Gastaufenthalte
Dr. Alexey Kutsenko
Dr. Vladimir Shandarov
Prof. Dr. Michael Petrov
Prof. Dr. Michael Petrov
Dr. Boris Sturmann
Prof. Dr. Tatjana Volk
Prof. Dr. Valeri S. Serov
Prof. Dr. Vladimir Trepakov
St. Petersburg, Russland
Tomsk, Russland
St. Petersburg, Russland
St. Petersburg, Russland
Novosibirsk, Russland
Moskau, Russland
Moskau, Russland
St. Petersburg, Russland
01.04.01 – 30.04.01
07.04.01 – 27.05.01
23.09.01 – 15.12.01
01.04.02 – 14.05.02
27.04.02 – 14.07.02
07.11.02 – 15.12.02
04.12.02 – 11.12.02
27.11.02 – 05.12.02
8. Zwischenbilanz des Kollegs
Aus unserer Sicht sind die im Antrag formulierten Ziele bisher voll erreicht worden. In
der Forschung haben wir bisher drei Themenkreise behandelt: Photorefraktive
Nichtlinearitäten, Frequenzkonversion und Wellenmischen sowie Nichtlinearitäten bei
der Wellenleitung. Auf diesen Gebieten sind in Osnabrück bereits umfangreiche
Vorarbeiten geleistet worden. Die Grundausstattung, die vor allem im Rahmen der
Programme des Sonderforschungsbereichs 225 'Oxidische Kristalle für elektro- und
magnetooptische Anwendungen' beschafft wurde, konnten wir effizient nutzen.
Ein kohärentes, auf Integration ausgerichtetes Studienprogramm wird durchgeführt,
um einerseits einer allzu einseitigen Spezialisierung vorbeugen und den Blick für
umfassende Zusammenhänge zu schärfen und um andererseits zu helfen, unnötig
lange
Promotionszeiten
zu
verringern.
Durch
Zusammenarbeit
mit
Gastwissenschaftlern, durch Forschungsaufenthalte an anderen wissenschaftlichen
Einrichtungen und durch die Teilnahme an Fachtagungen ist die Ausbildung der
Kollegiat(inn)en vertieft sowie die Mobilität, die Diskussions- und die
Präsentationsfähigkeit gefördert worden. Wir gehen davon aus, dass der
überwiegende Teil der Kollegiat(inn)en die Promotion in drei Jahren abschließen
wird.
Der Fachbereich Physik hat eine Graduiertenschule eingerichtet. Dabei fällt dem
Graduiertenkolleg 'Nichtlinearitäten optischer Materialien' eine wesentliche Rolle zu.
Die gewonnenen Erfahrungen sollen auch auf andere Bereiche übertragen werden.
Die neue Promotionsordnung (gültig seit dem 26.11.2002) ist bereits stark von den
Graduiertenkollegs beeinflusst worden.
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