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Titel: (Logo-m.eps ) Ers tellt v on: Adobe Illus trator(TM) 5.0.1 Vors chau: Dies e EPS-Grafik w urde nicht ges peic hert mit einer enthaltenen Vorsc hau. Kommentar: Dies e EPS-Grafik w ird an einen PostScript-Druc ker gedruc kt, aber nic ht an andere Druckertypen. 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 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 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 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 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 39 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 46 47 48 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. 83 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. 84 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. 85 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 97 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) 98 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 99 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. 100 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 101 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. 102 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: 103 - 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 104 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 105 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 106 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. 118 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 119 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” 01.07.02 120 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“ 04.02.03 121 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. 122