Download Fabrication of functional surfaces using ultrashort laser pulse ablation

JARNO KAAKKUNEN
Fabrication
of
functional surfaces using
ultrashort laser pulse
ablation
Publications of the University of Eastern Finland
Dissertations in Forestry and Natural Sciences
No 45
Academic Dissertation
To be presented by permission of the Faculty of Science and Forestry for public
examination in the Auditorium M100 in Metria Building at the University of
Eastern Finland, Joensuu, on November, 4, 2011,
at 12 o’clock noon.
Department of Physics and Mathematics
Kopijyvä Oy
Joensuu, 2011
Editors: Prof. Pertti Pasanen
Prof. Kai Peiponen, Prof. Matti Vornanen
Distribution:
University of Eastern Finland Library / Sales of publications
P.O. Box 107, FI-80101 Joensuu, Finland
tel. +358-50-3058396
http://www.uef.fi/kirjasto
ISBN: 978-952-61-0538-3 (printed)
ISSNL: 1798-5668
ISSN: 1798-5668
ISBN: 978-952-61-0539-0 (pdf)
ISSNL: 1798-5668
ISSN: 1798-5676
Author’s address:
University of Eastern Finland
Department of Physics and Mathematics
P.O.Box 111
80101 JOENSUU
FINLAND
email: [email protected]
Supervisors:
Professor Jari Turunen, Dr. Tech.
University of Eastern Finland
Department of Physics and Mathematics
P.O.Box 111
80101 JOENSUU
FINLAND
email: [email protected]
Kimmo Päiväsaari, Ph.D.
University of Eastern Finland
Department of Physics and Mathematics
P.O.Box 111
80101 JOENSUU
FINLAND
email: [email protected]
Reviewers:
Professor Duncan P. Hand, Ph.D.
Heriot-Watt University
School of Engineering and Physical Sciences
DB 1.54, Heriot-Watt University
EDINGBURGH EH14 4AS
UNITED KINGDOM
email: [email protected]
Laboratory Manager Timo Kajava, Dr. Tech.
Aalto University
Department of Applied Physics
P.O.Box 15100
00076 AALTO
FINLAND
email: [email protected]
Opponent:
Professor Olivier Parriaux, Ph.D.
Université Jean Monnet
Laboratoire Hubert Curien UMR CNRS 5516
18 rue du Professeur Benoı̂t Lauras
42000 SAINT-ETIENNE
FRANCE
email: [email protected]
ABSTRACT
This thesis considers ultrashort laser ablation of functional surfaces
using diffractive optics. Three different functionalities are studied:
optical absorption, wetting and diffraction. Various fast diffractive
optics based exposure systems are demonstrated, applying ultrashort laser pulse ablation, to realize these functionalities. Among
the benefits of ultrashort pulses is the fact that they are suitable for
direct processing of various materials.
First, a surface with high optical absorption is realized by interfering four ultrashort pulses using an optical setup based on
two lenses and a grating. Then a water repellant plastic surface
is demonstrated by applying a diffractive optical element (DOE)
together with ultrashort laser processing. Here the plastic is not directly laser processed to become super-hydrophobic, but ultrashort
laser pulses are used to fabricate a mould for mass-production processes such as injection moulding. Finally, ultrashort pulses are
used with a system composed of two gratings to process two dimensional (2D) diffractive structures with feature sizes less than
one micron.
Universal Decimal Classification: 53.084.85, 681.7.02, 535.3, 535.4, 535.8,
535-3, 544.537
PACS Classification: 52.38.Mf, 42.65.Re, 42.25.Hz, 42.40.Jv, 42.79.Dj,
78.40.-q, 68.08.Bc, 42.25.Fx
INSPEC Thesaurus: optics; micro-optics; optical elements; diffractive optical elements; diffraction gratings; surface texture; surface morphology;
metals; plastics; nanostructured materials; optical fabrication; high-speed
optical techniques; laser materials processing; laser ablation; light absorption; wetting; diffraction
Yleinen suomalaine asiasanasto: optiikka; optiset laitteet; laserit; lasertekniikka; mikrotekniikka; pintarakenteet; nanotekniikka; nanorakenteet; pinnat
- - ominaisuudet; pinnat - - tekstuuri
Preface
Let me start this acknowledgement part of the thesis from the people who had made this thesis possible. First, I would like to thank
the former head of the Department of Physics and Mathematics,
Dean Timo Jääskeläinen and the present one Prof. Pasi Vahimaa,
for the possibility to work at the department. I am also grateful
for Prof. Markku Kuittinen, who in the first place gave me an opportunity to work with the topic of my thesis and gave assistance
during my studies. Then, I would like to deeply thank my supervisor Ph.D. Kimmo Päiväsaari, who at practical level guided me
through the studies, and my other supervisor Prof. Jari Turunen
for his supervising and assistance to finalize my doctoral thesis.
During my studies, I spent one year in the Laser-Laboratorium
Göttingen (LLG), Germany. There I got familiar with many excellent people to whom I am grateful, not only professionally, but also
personally. First, I would like to thank my supervisor, the head of
Ultrashort Pulse Photonics group, Dr. Simon Peter for the possibility to work at LLG and his practical help in Lab and also outside
the work. I am also grateful to Jürgen Ihlemann, Tamas Nagy, JanHendrik Klein-Wiele, Jörg Meinertz and all the other co-workers in
LLG, for their assistance during my stay in Göttingen. Last, but
absolutely not the least, I am especially grateful for my second supervisor Jozsef Békési, who supported and guided me at the LLG
and outside the work.
Acknowledgements of my colleagues in Joensuu I have to start
from the current and former people of my office. Ismo, Kalle,
Petri and Jussi have helped me practically and we have had many
rewarding discussions (professional, not so professional, and in
something between). Similar mind expanding (morning) conversations I have had also with Ville K. and Kalle K., which have
helped me through my studies. I am also grateful to all former
and current members of our department for their professional and
non-professional assistance during these years. From the former
colleagues, I have to mention Birgit P., who was always willing to
help when it was needed. Specially I am also grateful to the second
member of the laser ablation-team Martti S. for all of his help.
I am grateful to the reviewers Prof. Duncan P. Hand and Dr.
Tech. Timo Kajava for their comments and statements. I also appreciate the Finnish Foundation for Technology Promotion and Emil
Aaltonen Foundation for their personal grants.
In the end, I would like to thank my parents Jouni and Raija,
who have supported (both financially and personally) and buoyed
me, naturally during my whole life, but especially during my studies. I am also grateful to all my relatives and friends, specially to
Ville, Osku and my siblings, Jyri, Saara, Riikka and Roni, for their
support. Especially I am also thankful to my godmother Sari E.,
who has always encouraged me in my studies. Last and the most
important thanks I would like to dedicate to my loving wife AnniKaisa, for her understanding and support during my studies.
Joensuu September 2, 2011
Jarno Kaakkunen
LIST OF PUBLICATIONS
This thesis consists of the present review of the author’s work in the
field of ultrashort optics and the following selection of the author’s
publications:
I K. Päiväsaari, J.J.J. Kaakkunen, M. Kuittinen and T. Jääskeläinen, “Enhanced optical absorptance of metals using interferometric femtosecond ablation,” Optics Express 15, 13838–13843
(2007).
II J.J.J. Kaakkunen, K. Päiväsaari, M. Kuittinen and T. Jääskeläinen, “Morphology studies of the metal surfaces with enhanced
absorption fabricated using interferometric femtosecond ablation,” Applied Physics A: Materials Science & Processing 94, 215–
220 (2009).
III J. Bekesi, J.J.J. Kaakkunen, W. Michaeli, F. Klaiber, M. Schoengart, J. Ihlemann and P. Simon, “Fast fabrication of superhydrophobic surfaces on polypropylene by replication of shortpulse laser structured mold,” Applied Physics A: Materials Science & Processing 99, 691–695 (2010).
IV J.J.J. Kaakkunen, J. Bekesi, J. Ihlemann and P. Simon, “Ablation of microstructures applying diffractive element and UV
femtosecond laser pulses,” Applied Physics A: Materials Science
& Processing 101, 225–229 (2010).
V J.J.J. Kaakkunen, K. Päiväsaari and P. Vahimaa, “Fabrication
of large area hole arrays using high efficiency two-grating
interference system and femtosecond laser ablation,” Applied
Physics A: Materials Science & Processing 103, 267–270 (2011).
Throughout the overview, these papers will be referred to by Roman numerals. In addition, the author has also participated in
preparation of other peer-reviewed papers [1, 2].
AUTHOR’S CONTRIBUTION
The ideas leading to the papers I, II and V are the author’s. Theoretical calculations, design of the gratings, all measurements, characterization and laser ablation has been mainly done by the author.
The ideas of papers III and IV were originated by the co-authors
Dr. Peter Simon and Dr. Jozsef Békési. In these publications, fabrication of the DOEs, laser ablations and characterization of the samples have been mainly done by the author. The author has written
the manuscripts to the papers II, IV and V; in papers I and III the
author has participated in the writing.
Contents
1 INTRODUCTION
1
2
ULTRASHORT PULSES
5
3
ULTRASHORT LASER PULSE
ABLATION
9
3.1
9
3.2
Fundamentals of ultrashort pulse ablation . . . . . .
3.1.1
Benefits of ultrashort pulses . . . . . . . . . . .
11
3.1.2
Ultrashort pulse ablation phenomena . . . . .
12
Ultrashort pulse lasers . . . . . . . . . . . . . . . . . .
13
3.2.1
CDP TISSA-50 and MPA-50 . . . . . . . . . . .
15
3.2.2
Dye/Excimer hybrid laser system with KrFamplifier . . . . . . . . . . . . . . . . . . . . . .
15
Quantronix Integra-C . . . . . . . . . . . . . .
16
3.2.3
4
5
DIFFRACTIVE OPTICS
19
4.1
Interference . . . . . . . . . . . . . . . . . . . . . . . .
19
4.2
Diffraction gratings . . . . . . . . . . . . . . . . . . . .
23
4.3
Diffractive optical elements . . . . . . . . . . . . . . .
26
4.3.1
28
Fabrication of the DOE using SiOx method . .
ULTRASHORT LASER PULSE
ABLATION USING DIFFRACTIVE
OPTICS
33
5.1
33
Interference laser ablation . . . . . . . . . . . . . . . .
5.1.1
Interfering ultrashort pulses using a grating
and imaging system . . . . . . . . . . . . . . .
35
Two-grating interferometer . . . . . . . . . . .
36
Ultrashort laser ablation using DOE . . . . . . . . . .
37
5.1.2
5.2
6 ULTRASHORT PULSE ABLATION OF THE FUNCTIONAL
STRUCTURES
39
6.1 High absorption structures . . . . . . . . . . . . . . . 39
6.2 Super-hydrophobic surfaces . . . . . . . . . . . . . . . 42
6.3 Diffractive structures with sub micron
features . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7 CONCLUSIONS
47
REFERENCES
49
1 Introduction
The development of lasers has been fast during the past few decades
and, in addition to research, nowadays they are widely used also in
industry. Added to the increase of laser power, the decreasing duration of optical pulses has been one of the major aims in laser development. Although the first pulsed lasers with pulse length in the
femtosecond (fs) range were manufactured a couple of decades ago,
they are still widely studied. Nowadays fs-lasers are commercially
available and they are used in many fields like terahertz radiation
generation and detection [3], spectroscopy [4], nano-particles generation [5] etc.. These lasers with ”ultrashort” pulse duration can
also be used for material removal, which is called laser ablation [6].
With ultrashort laser ablation, various materials and shapes can
be machined ensuring its applicability in many applications [7]. For
example, ultrashort pulses can be used for direct drilling of the high
aspect ration hole in various materials like silicon [8], glasses [9]
and metals [10]. Added to surface structuring of materials, ultrashort pulses can be applied for volume structuring inside transparent materials [11]. One application of the volume structuring
is Bragg grating ablation inside the fibers [12]. Because of the facility to ablate various materials, fs-laser ablation can also be used
for materials coating, or pulsed laser deposition, with such materials that can not be done with other methods [13]. One remarkable
commercial application of the ultrashort laser ablation is femtosecond laser surgery of the eye, also called femto-LASIK (laser in situ
keratomileusis) [14]. In this thesis, ultrashort laser pulse ablation is
used to fabricate various functional surface structures in different
materials, using diffractive optics based methods.
It is possible to realize various functionalities by using microand/or nano-size surface structures. Some of them are related to
behavior of the surface, e.g. reflection, transmission and absorbtion,
when it is illuminated with various light sources. In this thesis min-
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
imizing the optical reflectance of non-transparent materials is studied. This means that these materials must absorb all the light that is
not reflected. Various methods and materials have been studied to
realize this property. Generally silicon has been studied extensively,
because it has potential applications in solar cell technology [15–17].
In case of silicon, etching based methods have been used to enhance optical absorption. Nowadays also femtosecond based methods have been studied in this area [18, 19]. Recently femtosecond
laser based methods to enhance absorption of other materials than
silicon have been studied by A.Y. Vorobyev and C. Guo [20–24] and
also other groups [25, 26]. All of these methods are based on structures which are randomly formed, although they have certain characteristic shapes and feature sizes. Generation of high absorption
surfaces have been studied by the author in papers I and II, using
an interferometric method. This method facilitates fabrication of
high absorption structures in a controlled way.
Another interesting functionality of materials is their wettability
and methods to control this have been developed by mimicking the
nature. The best-known example of a natural water-repellent surface is the leaf of lotus. This property of hydrophobicity is therefore
known as the lotus-effect, although many other plants have similar properties [27–29]. The water-repellence effect has been widely
studied by several scientists during last decades and a number of
chemicals [30] and surface structures have been developed to mimic
the lotus-effect [31–34]. Lately also femtosecond laser ablation has
been applied to fabricate hydrophobic surfaces directly [35, 36] and
indirectly [37–39]. Materials such as silicon, ablated in an appropriate gas environment [40, 41], can be turned super-hydrophobic
with randomly formed surfaces. Papers III and IV introduce a controlled, fast parallel method to fabricate super-hydrophobic surface
structures via mass production methods.
The third subject studied in this thesis is generation of lightdiffracting structures, more precisely diffraction gratings with sub
micron-size features. With ultrashort laser pulses, these structures
can be directly ablated into such materials that can not be handled
2
Dissertations in Forestry and Natural Sciences No 45
Introduction
with replication methods. It is well known that sufficiently small
surface structures reflect and deflect light in a way that can not be
handled with geometrical optics. Naturally, when considering materials like metals, only reflected waves are observed. If the periodicity of the structures is in the visual wavelength range, white light
is split into visually colorful spectra. In addition to spectroscopy
and decorative applications, these structures are suitable for security marking, because copying of them is not easy. There are several
method to fabricate these surface structures and most conventional
methods are based on optical lithography [42]. With such methods,
it is possible to fabricate almost arbitrary two-dimensional structures, but the selection of usable materials is limited. Secondly,
these methods are also time consuming since they require multiple
processing steps, facilities and machines. Therefore there is a demand for alternative methods and especially direct methods. Femtosecond laser ablation is suitable for this because it can be used for
fast direct structuring of an extensive selection of materials. In this
thesis (paper V), a fast method to fabricate two-dimensional diffractive surface structures in various materials using fs-laser ablation is
demonstrated.
This thesis is organized as follows. First there is a short introduction to the physics of ultrashort pulses in Chapter 2. Here
the principles of handling ultrashort optical pulses are discussed.
The interaction of ultrashort pulses with matter is presented in
Chapter 3, where also the lasers that are used in experiments are
described. Chapter 4 covers the basics of interference, diffraction
gratings and diffractive optical elements, which are applied in functional surfaces fabrication. Particularly, Section 4.3.1 describes the
fabrication method used for preparation of diffractive optical elements (DOEs). Functional surfaces that are fabricated in this thesis
are presented in Chapter 6. Here elements with the above mentioned three functionalities are manufactured in various materials
using methods introduced in Chapter 5. The last Chapter, contains
a short conclusion of this thesis.
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
4
Dissertations in Forestry and Natural Sciences No 45
2 Ultrashort pulses
Usually the term ”ultrashort” stands for pulses with temporal length
of less than few picoseconds (10−12 s) and generally in order of femtosecond (10−15 s). Hence the energy of these pulses is packed into
a very short time window. Although, compared to longer pulses,
the energy of ultrashort pulses is small, in the range of mJ, the peakpower (P peak ) can be very high, from Giga- (109 ) to Terawatts (1012 ).
This is because pulse peak-power is proportional to pulse energy
(E pulse ) and pulse length (τ),
Ppeak ∼
E pulse
.
τ
(2.1)
Hence also the peak intensities of ultrashort pulses can reach enormous values, of the order of 1021 W/m2 .
Ultrashort light pulses have large spectral bandwidth, because
the temporal duration of a laser pulse and spectral width are related to each other, meaning that shorter the pulse is temporally,
the wider its spectral width [43, 44]. This can be theoretically realized by considering general time and frequency Fourier transforms
of any scalar components of a completely coherent optical planewave pulse:
V (t) =
Z ∞
V (ω ) =
−∞
V (ω ) exp (−iωt) dω,
Z ∞
1
2π
0
V (t) exp (iωt) dt.
(2.2)
(2.3)
Further, the power spectrum S(ω ) and the temporal intensity I (t)
can be defined as
S(ω ) = |V (ω )|2 ,
2
I (t) = |V (t)| .
Dissertations in Forestry and Natural Sciences No 45
(2.4)
(2.5)
5
Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
In general the effective pulse duration △t and the spectral width
△ω of the pulse can be calculated using standard definitions
△t
2
△ω 2 =
where
R∞
2
− ∞ ( t − t0 ) I ( t) dt
R∞
,
=
− ∞ I ( t) dt
R∞
− ω0 )2 S(ω )dω
0 (ω
R∞
,
0 S ( ω ) dω
R∞
t0 = R−∞∞
tI (t)dt
(2.6)
(2.7)
,
(2.8)
R∞
ωS(ω )dω
ω0 = R0 ∞
.
0 S ( ω ) dω
(2.9)
−∞
I (t)dt
With Equations (2.2) - (2.9), it can be shown that the quantities △t
and △ω are related to each other with following inequality:
∆t∆ω ≥
1
,
2
(2.10)
where the equality applies to Gaussian pulses. This classical physical product (2.10) of the pulse temporal duration and spectral bandwidth is known as the time-bandwidth product. When the pulse
and Fourier transform of it are real, the pulse is called a transformlimited or as bandwidth-limited. Then, for a given power spectrum,
the temporal intensity defines the shortest possible pulse duration.
Then one may use Equations (2.2)-(2.10) directly, provided that all
pulses in the train are identical. If this is not the case, the pulse train
is called partially coherent, and the quantities △t and △ω should
be considered in the sense of averages over an ensemble of pulses.
There are standard techniques such as FROG (frequency-resolved
optical gating) [45, 46] and SPIDER (spectral phase interferometry
for direct electric field reconstruction) [47], to measure the amplitude and phase of ultrashort pulses in both temporal and spectral
domains.
6
Dissertations in Forestry and Natural Sciences No 45
Ultrashort pulses
Equation (2.10) can also be expressed using the FWHM (fullwidth of half maximum) values of S(ω ) and I (t). Using frequency
ν = ω/2π instead of the angular frequency ω, we may write in
general
∆ν∆t = K,
(2.11)
where K is a number, which depends on the spectral and temporal
phase and coherence properties of the pulse train in addition to
the distributions S(ω ) and I (t). Values of K for transform limited
rectangular and Gaussian pulses are 0.892 and 0.441, respectively
[43]. For example, the half-maximum spectral width of a Gaussian
pulse with central wavelength at λ0 = 800 nm and FWHM pulse
duration of ∆t = 100 fs, is about 10 nm.
The large spectral bandwidth of the ultrashort pulse influences
light behavior in transparent media such as quartz. Of course
longer laser pulses also have a finite spectral bandwidth, but in
practice it can be ignored and the pulse can be treated as if it were
monochromatic. In dispersive transparent media the phase of ultrashort pulses is distorted, because of their wide spectral bandwidths. Mathematically this can be handled by writing first terms
of the Taylor expansion of the wavelength dependent wave number:
k( ω ) = k( ω0 ) + ( ω − ω0 )
dk(ω )
dω
1
+ ( ω − ω0 )2
2
ω0
d2 k ( ω )
.
dω 2
ω0
(2.12)
Here the first term is k(ω0 ) = ω0 n(ω0 )/c, the second term represents group velocity vg
1
=
vg (ω )
dk(ω )
dω
,
(2.13)
ω0
′′
and the third term contains group velocity dispersion k (GVD)
1
k (ω ) =
2
′′
d2 k ( ω )
dω 2
ω0
1 d
=
2 dω
Dissertations in Forestry and Natural Sciences No 45
1
vg (ω )
.
(2.14)
ω0
7
Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
′′
Both of the values vg and k are obtained from the empirical Sellmeier formula [48], which expresses the relationship between the
′′
refractive index and wavelength. If k is positive, then the higher
frequencies travel faster in the medium (down-chirped or anoma′′
lous dispersion), and if k is negative, higher frequencies travel
slower (up-chirped or normal dispersion). In addition, phase distortion delays the pulse and chirps its frequency in transparent media.
8
Dissertations in Forestry and Natural Sciences No 45
3 Ultrashort laser pulse
ablation
Laser ablation has been carried out for a long time using various lasers [49]. During the past few decades the development
of lasers has been rapid and today lasers that generate ultrashort
pulses in the fs range are commercially available. Although the
energy of these pulses is low, their peak-power is high and therefore it is possible to machine virtually all materials, e.g. dielectrics,
metals, semiconductors, ceramics etc., which is not always possible with longer pulses. It is also well known that ablation using
shorter pulses facilitates fabrication of structures with smaller features. This is because with ultrashort pulses, the material is evaporated with a minimal heat-affected zone (HAZ) [6].
When energy given by the pulses to lattice overtakes certain
limit, which is called an ablation threshold, material starts to be
removed from the material. Ablation threshold for ultrashort pulse
machining varies for semi-conductors and metals from 0.1 J/cm2 to
10 J/cm2 and is even higher for the dielectrics [50]. To be able to
realize these fluence values, ultrashort pulse laser systems usually
consist both oscillator and amplifier. Laser systems used in this
thesis are presented in Section 3.2.
3.1
FUNDAMENTALS OF ULTRASHORT PULSE ABLATION
In an ablation process the energy of a pulse is transferred to free
electrons of the material. Energy transition from photon to material
(photon absorption) can be categorized into a linear process, which
follows Beer-Lambert’s law, and a non-linear process, which happens with higher energies [6, 51, 52]. For free electron generation
there are two competing mechanisms. The first is collisional impact
ionization (avalanche ionization) and the second is photo-ionization
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
(multiphoton ionization). In impact ionization, the kinetic energy
of a free-electron is increased by absorption of the photons, which
leads to production of further free electrons from bound electrons.
The same happens to these new free electrons, and this process
continues repeatedly. Such a series is called avalanche ionization.
Respectively, in multiphoton ionization the photons release a bound
electron by giving their energy to it. This is the main process during
dielectric materials ablation, while avalanche ionization dominates
in the ablation of metals and semiconductors.
If the energy of the ultrashort pulses is transferred sufficiently
fast to the free electrons of the material, there is not enough time to
transfer energy to the lattice before the material is evaporated in the
form of a hot electron gas. Before total evaporation of matter, the
free electrons create an electron plasma. Removed electrons create
strong electric field in matter, which causes Coulomb explosion to
remove material from the surface. After this, further increase of the
energy causes material removal through thermal evaporation. This
is dominant with high fluences, whereas the Coulomb explosion is
dominant with fluences near ablation threshold. [51]
Evaporation happens because, in the electron system, the excitation energy is thermalized faster than thermalization between
the electron subsystem and the lattice takes place. When electrons
pass the energy to the lattice, the lattice is heated faster than the
heat is conducted into the material. This leads to extreme pressure,
temperature and subsequent evaporation. This energy transfer between electrons and lattice can be modeled using a two-temperature
model [50]. If the temperature of the hot electrons is Te and that of
the lattice is Ti , then for ultrashort pulse ablation of metals
Ce
10
∂Te
∂Q (z)
=−
− γ ( Te − Ti ) + S,
∂t
∂z
∂Ti
Ci
= γ ( Te − Ti ) ,
∂t
(3.1)
(3.2)
Dissertations in Forestry and Natural Sciences No 45
Ultrashort laser pulse
ablation
where the heat flux Q(z) and the laser heating-source term S are
∂Te
,
∂z
S = I (t) Aα exp (−αz) .
Q ( z) = − k e
(3.3)
(3.4)
Here the direction z is perpendicular to the target surface, Ce and
Ci are the capacities (per unit volume) of the electron and lattice
subsystems, γ is a parameter characterizing the electron-lattice coupling, I (t) is the laser temporal intensity, A and α are the surface
absorptivity and the material absorption coefficient and ke is the
electron thermal conductivity.
3.1.1
Benefits of ultrashort pulses
Ultrashort pulses have several benefits compared to longer pulses.
In the case of longer pulses, the ablation process usually takes place
through heating, which influences the material in many ways. First
of all, the heat transfers into the zone surrounding the ablated area
(see Fig. 3.1 (a)). In this area, also called the heat affected zone
(HAZ), the properties of the material, like optical properties, can
change permanently. Added to this, there is also a molten area,
which is formed by the cooled plasma. Because of the thermal
damage, micron size cracks also form inside the material around
the ablated zone. Usually with longer pulses high energies are required for ablation and therefore shockwaves are formed in the interaction with material. These waves travel in the material over long
distances, influencing the matter in many ways. One easily observable influence is that surface in the area surrounding the ablated
zone is covered with symmetric surface ripples. When the material
is heated using long pulses, the molten matter is ejected around
the ablation zone. This ejected debris is spread around the ablation
zone, and when it hits the surface of the material, it gets stuck to
the surface.
When the duration of the pulses gets short enough, the above
mentioned problems no longer exist (Fig. 3.1 (b)). This is because
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
Ultrafast pulses
Ejected molten
material
Surface Ripples
Surface Debris
Minimal debris
Minimal melt zone
Cracks
Plasma plume
Melt zone
No cracks
No shock wave
Heat Affected Zone
(a)
(b)
Figure 3.1: Principle of long (a) and short (b) laser pulse ablation.
the energy, given to free electrons or electron plasma by the ultrashort pulse, does not have time to transfer into the lattice before
the plasma is already evaporated. Therefore for ultrashort pulses
the HAZ is minimal or in practice it can be considered to be nonexistent. Because the heat is not transferred outside the ablation
zone, there are no melt zones, cracks or shockwaves. No debris is
formed either, because of the total evaporation.
3.1.2 Ultrashort pulse ablation phenomena
When material is ablated using ultrashort pulses, self-organized
structures of different shape and size are formed. With small pulse
numbers and fluences near ablation threshold, nano-structures can
be generated, Fig. 3.2 (a). The mean size of this randomly organized
nano-roughness is of the order of tens of nanometers. When pulse
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number and fluence are increased above certain values, depending
on material properties, the ablated surface starts to organize in the
form of a linear grating. These are called laser-induced periodic
surface structures (LIPSS), Fig. 3.2 (b). In case of the linearly polarized pulses, LIPSS are orientated perpendicular to the direction
of the polarization. The period of these self-organized structures
depends on the wavelength of the pulses and the angle of incidence
of the pulses. The LIPSS and their possible applications have been
studied extensively by J. Reif and his group [53, 54] and also by
other groups [55–57].
When pulse number and fluence are increased above the values where LIPSS appear, then the material surface starts to selforganize in micrometer range, Fig. 3.2 (c) and (d). Again the size
and shape of these randomly assembled micro-size structures can
be controlled with laser parameters. After these structures, further
increase of fluence and pulse number leads to the total evaporation
of the material. The generation of micro-size structures and their
applications have been studied by various groups [58, 59] and also
by the author [1,60]. It has been seen that all of these self-generated
structures appear in metals as well as in alloys and semiconductors.
3.2
ULTRASHORT PULSE LASERS
To generate high peak-power ultrashort pulses for laser ablation,
two separate systems, an oscillator and an amplifier, are required.
In the oscillator the seed pulses are generated for the amplifiers,
where some of them are amplified. Usually, after oscillator, the
energy of the ultrashort pulses is in a range of nJ or even up to
µJ [61] with MHz repetition rate (pulses per second). In amplifiers
the energy of the pulses is increased to mJ range, but at the same
time the repetition rate is decreased to Hz-kHz range.
There are several ways to generate ultrashort pulses. In this thesis three different ultrashort laser systems are used, which all have
different types of oscillators. Solid-state- and fiber-oscillators are
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400nm
(a)
1mm
(b)
30mm
30mm
(c)
(d)
Figure 3.2: Different forms of self-organized structures generated with ultrashort laser
pulse ablation in steel. In (a) nanostructures, (b) LIPSS, (c) and (d) different forms of
coral-structures are presented.
used for generation of the pulses in the infrared wavelength range
and dye/excimer-based oscillators are employed to generate pulses
in a UV-wavelength range [62, 63]. Amplification of the IR-pulses
is achieved using solid-state-amplifiers and UV-pulses with gasamplifiers. In papers I, II and V CDP’s femtosecond system with a
combination of an oscillator (TISSA-50) and an amplifier (MPA-50)
is used [64]. In papers III and IV, pulses used in ablation are generated with a dye/excimer hybrid laser system and amplified using
KrF-amplifier [65]. In paper V, added to CDP’s femtosecond laser
system also Quantronix Integra-C laser is used [66].
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3.2.1
CDP TISSA-50 and MPA-50
The CDP oscillator (TISSA-50) provides 50 fs (FWHM) long Gaussian pulses with 800 nm central wavelength at 80-90 MHz repetition
rate, so that the spectrum is Gaussian with FWHM approx. 20 nm.
A frequency doubled diode pumped solid state (DPSS) laser is used
to pump this oscillator, which uses a Titanium doped sapphire crystal (Ti:Sapphire) for ultrashort pulse generation. The oscillator uses
a self-focusing nonlinear optical effect together with an aperture effect for mode-locking operation (Kerr lens mode-locking) and generation of femtosecond optical pulses [67]. The Ti:sapphire crystal
generates pulses with distorted group velocity, which are compensated using a standard method based on a pair of prisms.
After generation of the fs pulses, they are amplified using CDP’s
MPA-50 amplifier. In this chirped pulse amplification (CPA) based
amplifier, the pulse energy is increased from few nJ to 1 mJ. At the
same time the repetition rate is decreased to 50 Hz [68]. Before the
amplification of the pulses, they are stretched temporally using a
grating pair, because otherwise the pulse intensity is too high for
the Ti:sapphire. After this, wanted pulses are selected from the
pulse train using a pulse picker, which employs an electro-optical
Pockels cell. Amplification is done using a multi-pass Ti:Sapphire
amplifier, which is pumped using frequency doubled Nd:YAG with
50 Hz repetition rate and 12 mJ pulse energy. Amplified pulses are
compressed, using a grating pair, to their original pulse length.
3.2.2
Dye/Excimer hybrid laser system with KrF-amplifier
Used dye-laser system was constructed by researchers in the LaserLaboratorium Göttingen Research Group. This system, which uses
a modified commercial excimer UV-nanosecond laser (EMG 150) for
pumping, is used to generate seed pulses of a KrF-amplifier. This
oscillator system provides 500 fs long pulses with central wavelength 248 nm at tunable repetition rate from under 1 Hz to few
tens of Hz [69]. The system consists of many pulse modifying
stages, such as dye oscillators, saturable absorbers and amplifiers.
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Principally, in this oscillator seed pulses from the excimer laser are
modified to be suitable for distributed feedback dye laser (DFDL),
after which the pulses acquire a wavelength of about 497 nm and
their eventual pulse length 500 fs. Then the pulses are further amplified two times and guided twice through a saturable absorber.
Next, pulses with 0.2 mJ energy are frequency doubled using BBOcrystal, with 10 % efficiency, resulting in a wavelength 248 nm that
is suitable for a KrF-amplifier. Nowadays it is also possible to use
modified Ti:Sapphire based oscillator to generate seed pulses for a
KrF-amplifier, which enables an increase of the system’s repetition
rate [70].
After dye/excimer hybrid oscillator the pulses are amplified using a three-pass KrF-gas amplifier [65, 71]. Three off-axis passes
through the KrF-gas chamber are used, because it facilities the optimization of the gain after each pass. A second reason for this is
that it helps to decrease the amplified spontaneous emission (ASE)
background. The KrF-amplifier can increase the pulse energy up to
30 mJ.
3.2.3 Quantronix Integra-C
The third laser system used in this thesis is Quantronix Integra-C
system, which provides 130 fs long pulses at 800 nm central wavelength with 1 kHz repetition rate. Unlike in CDP TISSA-50 oscillator, in this system seed pulses for amplifier are generated using
a fiber-oscillator [72, 73]. After frequency doubling, the oscillator
provides 110 fs pulses at the central wavelength of 790 nm with
30-40 MHz repetition rate. The oscillator is passively mode-locked
by a saturable absorber and the gain medium of the laser is a high
gain Er-doped fiber.
Seed pulses in this system are amplified in two stages with two
different amplifiers. The first amplifier is a regenerative amplifier [74, 75] and second multipass amplifier [76], like in Sect. 3.2.1.
Before amplification, the pulses are temporally stretched using the
grating pair based stretcher and then amplified, whereafter they
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are compressed with another grating pair. The main difference between this amplifier and the one described in Sect. 3.2.1 is that here
there are two separate Ti:sapphire crystals, which makes possible
the amplification of pulse energy up to 3.5 mJ.
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4 Diffractive optics
In this Chapter the basics of the interference of electromagnetic
waves, diffraction gratings, and more general diffractive optical elements (DOEs) are presented [77, 78]. All of these are used in the
experimental part of this thesis. Their applications in the case of
ultrashort pulses are discussed in Chapter 5.
4.1
INTERFERENCE
Interference of waves concerns phenomena that take place when
two or more waves overlap each other. These waves may be mutually coherent meaning that their phase difference is constant in
time. Additionally, the waves may have almost the same frequency
and state of polarization. If all these conditions are true, the beams
interfere strongly with each; in general, the interference pattern
must be determined using the coherence theory of electromagnetic
fields [79]. For example, if the polarization vectors of two linearly
polarized plane waves are orthogonal, the waves do not interfere
with each other at all.
The most classical example of interference is Thomas Young’s
double slit experiment or simply Young’s experiment. In this experiment a wave passes through two slits near each other and generates two separate waves behind the slits, which interfere with
each other after propagation. The interference field oscillates quasiperiodically, varying regularly from minima to maxima. The period of the interference pattern depends on the distance between
the Young pinholes and on the distance between the Young screen
and observation plane. The visibility of the pattern depends on the
light intensities at the two pinholes and on the degree of spatial
coherence of the incident light, and can be used to determine the
latter.
In this work we consider light fields originating from a single
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laser, and therefore we may consider it as spatially fully coherent at
each frequency. Since mode-locked lasers are employed, the light is
also nearly coherent in spectral and temporal sense. Hence the only
significant factors affecting the visibility of interference fringes are
polarization and temporal overlap of the pulses.
Monochromatic two-beam interference fields always have sinusoidal intensity distributions, while interference of three or four
plane waves produces pattern consisting of elliptic or round spikes
etc.. Let us first consider a plane electromagnetic wave Ei (r, ω ) oscillating at frequency ω:
h
i
Ei (r, ω ) = E0i (ω ) exp i ki · r − ωt ,
(4.1)
where
E0i (ω ) = E0xi x̂ + E0yi ŷ + E0zi ẑ
(4.2)
is the complex amplitude of the electric field vector and
ki = k xi x̂ + kyi ŷ + kzi ẑ
(4.3)
is the wave vector. Considering the interference of N plane waves,
the time-averaged energy density hwe (r̄ )i can be expressed as
2
ε 0 n2 N
(4.4)
hwe (r̄)i =
∑ E i (r ) .
4 i =1
Figure 4.1 illustrates the case of four-wave interference, where all
waves propagate at the same angle θ with respect to the optical
axis.
For example, let us take four waves that propagate in following
directions:
k1 (ω ) = k xy x̂ + kz ẑ,
(4.5)
k2 (ω ) = k xy ŷ + kz ẑ,
(4.6)
k3 (ω ) = −k xy x̂ + kz ẑ,
(4.7)
k4 (ω ) = −k xy ŷ + kz ẑ
20
(4.8)
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Diffractive optics
ky
Ey1
Ex1
Ez1
Ey4
θ
Ex4
θ
Ey2
Ez4
kz
Ez2
Ex2
kx
Ey3
Ex3
Ez3
Figure 4.1: Interference of four waves propagating so that the angle between the optical
axis and the wave vector of each wave is always angle θ.
where k xy = k0 sin θ, kz = k0 cos θ and k0 = ω/c. This case is the
same as in Fig. 4.1, but let us now assume that all waves are linearly
polarized and in same phase. Then the polarization and vectors can
be represented as
E01 (ω ) = ( Ex x̂ − Ez ẑ) /4,
(4.9)
E02 (ω ) = ( E0 x̂) /4,
(4.10)
E03 (ω ) = ( Ex x̂ + Ez ẑ) /4,
(4.11)
E04 (ω ) = ( E0 x̂) /4,
(4.12)
where Ex = E0 cos θ and Ez = E0 sin θ. Now we can calculate the
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5
[µm]
4
3
2
1
1
2
3
[µm]
4
5
Figure 4.2: Interference pattern of four linearly polarized waves with interference angle
θ = 30o and λ = 800 nm.
sum field of these four waves E(r, ω ):
4
E(r, ω ) =
∑ E i (r )
i =1
E = 0 cos θ cos k xy x + cos k xy y x̂ + i sin θ sin k xy x ẑ
2
× exp (ikz z − iωt) .
(4.13)
Eventually the time-averaged energy density hwe (r̄)i of this fourwave interference pattern can be calculated:
2
ε 0 n2 E(r, ω )
4
2
E02 ε 0 n2 =
cos θ cos k xy x + cos k xy y
+ sin2 θ sin2 k xy x .
16
(4.14)
hwe (r̄)i =
This is periodic at ±45o directions in the xy coordinate system. In
Fig. 4.2 a few periods of time-averaged energy density (4.14) are
plotted, when θ = 30o and λ = 800 nm. From Equation (4.14) the
spatial period of the four-wave time-average energy density can be
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Diffractive optics
seen
π=
k0 sin θ
2πn sin θ
λ
√ d=
√
d ⇐⇒ d = √
,
2
λ 2
2n sin θ
(4.15)
where n is the refractive index of the medium.
The visibility of the interference pattern can be defined as follows:
V=
Imax − Imin
,
Imax + Imin
(4.16)
where Imax is maximum and respectively Imin minimum intensity
value of the interference fields oscillation. As Equation (4.16) shows,
the visibility of the interference pattern can vary between the values
0 ≤ V ≤ 1. In (4.14) the visibility is perfect V = 1. The spatial
period (4.15) is valid for four-wave interference intensity distribution. Respectively, it can be shown, that in case of the two-wave
interference, interfering with angle of 2θ, the spatial period of the
time-average energy density is
d=
4.2
λ
.
2n sin θ
(4.17)
DIFFRACTION GRATINGS
Any periodic micro- or nano-size surface structure, which varies
regularly in one or more spatial directions, can be called a diffraction grating. These gratings can be divided into groups in various
ways like according to their dimensions (two-, three-dimensional
and volume gratings) or depth variation (e.g. binary, multi-level or
continuous gratings). In this thesis only linear and crossed gratings are considered, with binary depth variation. The simplest case
is a linear binary grating, which is formed from grooves or ridges
next to each other as shown in Fig. 4.3 (a). Respectively, two basic crossed-gratings are formed, with either circular or square holes
or pillars. An example of the latter case is shown in Fig. 4.3 (b).
Naturally more complex gratings can and have been studied and
fabricated, but they are not used in this thesis.
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(a)
(b)
Figure 4.3: Schematics of periodic (a) linear and (b) crossed binary surface relief gratings.
When a grating is illuminated with a plane wave, it divides
the wave into several outgoing (reflected and/or transmitted) plane
waves, so called diffraction orders. In the case of the linear-grating,
light incident in a plane perpendicular to the grating grooves is divided into diffraction orders in one dimension (dots in line) and
respectively in two dimensions in the case of crossed-gratings. The
number of propagating diffraction orders depends on the wavelength and direction of the illuminating light, the grating periods
dx and dy in x- and y-directions and the refractive indices of the media before and after the grating ni and nt , respectively. The transverse wave vector components of the transmitted (m, n)-diffraction
orders can be solved using equations
2π
,
dx
2π
= ky,i + nKy = ky,i + n ,
dy
k xm = k x,i + mK x = k x,i + m
kyn
(4.18)
(4.19)
where k x,i and ky,i are wave-vector components of the incoming
plane wave. The allowed wave-vectors of these transmitted diffraction orders are clarified in Fig. 4.4 (a), where they are presented in
(k x , ky )-space. The red circle has a radius of κt = k0 nt . Wave vectors
inside it represent propagating orders and the ones outside it are
non-propagating or evanescent orders. The same is also valid for
the reflected orders if we consider a circle of radius κi = k0 ni . Using
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spherical polar coordinates (ϑ,ϕ) defined by (see Fig. 4.4 (b))
k x = k0 n sin ϑ cos ϕ,
(4.20)
ky = k0 n sin ϑ sin ϕ,
(4.21)
kz = k0 n cos ϑ,
(4.22)
Equations (4.18) and (4.19) can be expressed in the form
λ
,
dx
(4.23)
λ
,
dy
(4.24)
nt sin ϑmn cos ϕmn = ni sin ϑin cos ϕin + m
nt sin ϑmn sin ϕmn = ni sin ϑin sin ϕin + n
where ϕin and ϑin are spherical polar angles at the incoming wave
and ϕmn and ϑmn are those of the diffracted wave (m, n). By squaring and adding Equations (4.23) and (4.24) we can also get
n2t sin2 ϑmn =
λ 2
λ 2
+ ni sin ϑin sin ϕin + n
ni sin ϑin cos ϕin + m
.
dx
dy
(4.25)
Using Equation (4.25), the diffraction angle ϑmn can be solved and
by placing this value into Equation (4.23) or (4.24) the angle ϕmn
can be solved for an arbitrary wave-vector kmn .
From Equation (4.25) we can get the classical one-dimensional
grating equation by placing conical angle ϕin = 0, moving back to
cartesian coordinates, assuming dy to go infinity and placing dx = d:
λ
nt sin ϑm = ni sin ϑin + m .
d
(4.26)
Equations (4.26) and (4.25) are for transmitted diffraction orders
and just by replacing nt with ni , these equations are valid for reflected diffraction orders. The grating equations only tell the propagation angles of the diffracted orders. To calculate efficiencies of
the diffraction orders in the domain where the grating period and
the wavelength are at the same order of magnitude, rigorous electromagnetic grating analysis techniques such as the Fourier modal
method (FMM) need to be used [80, 81].
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ky
ky
(0,1) (1,1)
(-4,0)
(k xm , k yn )
(0,0) (1,0)
kx
(-4,-3)
ϕmn
ϑmn
Ky
(0,-3)
Kx
(a)
kx
kmn
kz
(b)
Figure 4.4: (a) Grid of the allowed values of the transverse wave vector components in
case of a two-dimensionally periodic structure. Only some of the indices (m, n ) are shown
explicitly. The propagating plane waves are inside the red circle. (b) Definition of the
diffraction angles in a spherical polar coordinate system for an arbitrary propagating plane
wave (m,n). Here K x = 2π/d x and Ky = 2π/dy .
4.3 DIFFRACTIVE OPTICAL ELEMENTS
DOEs are phase and/or amplitude modulating elements that can
be used to generate rather arbitrary fields or intensity distributions
and they have a wide range of possible applications in various areas
of science, technology and consumer products [77]. In addition to
phase or amplitude of light, DOEs can modulate also the state of
polarization of light. Amplitude elements reflect or absorb some
parts of the wave and therefore their efficiency is worse than that of
the phase elements. This is the reason why nowadays mainly phase
(or polarization) modulating elements are used.
As with gratings, DOEs can be divided into groups according
to the type of their refractive index variation. Another way to classify them is according to the spatial region where the desired signal distribution is formed. The elements can be designed so that
they generate the wanted intensity distribution around the optical
axis of the system (defined by the zeroth order), or at a certain
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distance away from the axis. The first class of elements are called
on-axis and the latter off-axis elements. The benefit of the on-axis
configuration is that only a single pattern is generated and even
two phase modulating levels can be enough for generating a highquality pattern efficiently. In this case, if there are only two phase
levels, the generated intensity distribution has to be inversion symmetric. Examples of such patterns are periodic dot-arrays in one or
two dimensions. If the wanted intensity distribution is not inversion symmetric, then more phase levels are needed or an off-axis
design has to be applied. The problem of on-axis elements is that
if there are some fabrication errors, the efficiency into the zeroth
order increases, which results in a bright spot in the center of the
pattern. The off-axis elements do not have this problem, because
the zeroth order is not part of the generated intensity distribution.
On the other hand, if there are only two phase levels in case of the
off-axis design, the entire pattern is still inversion symmetric, i.e.,
a second pattern is generated symmetrically with respect the zeroth order. Hence the efficiency into a single pattern is less than
50 %. This can be avoided by using multi-level design, whereby
the intensity of the wanted pattern can be enhanced. With Q phase
levels the efficiency of the mth diffraction order can be calculated
with following equation:
2
ηm = sinc
m
Q
sin2 (π [m + (ni − nt ) h/λ0 ])
,
Q2 sin2 (π [m + (ni − nt ) h/λ0 ] /Q)
(4.27)
where h is the height of the DOE profile.
Design and optimization of the DOE can be done several ways,
but if the feature size of the element is in the range of incident
wavelength, then accurate calculations are needed and the rigorous
diffraction theory has to be applied [77, 82]. Here finding a solution
to Maxwell’s equations and the appropriate boundary conditions
for each case, has to considered. In the paraxial domain, on the
other hand, optimization of the DOE can usually be done using
iterative methods such as the iterative Fourier transform algorithm
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SiOx
SiO2
(a)
λ = 193 nm
(b)
SiO2
(c)
Figure 4.5: Principle of DOE fabrication using SiOx and UV-nanosecond laser based
method. (a) SiO2 sample is covered with SiOx and then wanted parts of the SiOx layer
are removed using UV-nanosecond laser (b). (c) Consequently the sample is heat treated
to turn SiOx into SiO2 .
(IFTA) [83–85]. Nowadays commercial programs for design of DOE
are available. One of these is VirtualLab from LightTrans GmbH
[86], which is also used to design DOEs in this thesis.
4.3.1 Fabrication of the DOE using SiOx method
Conventional methods for fabrication of phase DOEs are usually
based on optical or electron-beam lithography and subsequent etching or material deposition. These methods requires many expensive
machines, single element fabrication is time-consuming and multiple fabrication steps are needed. There are also alternative methods
to fabricate surface structures and one of them is based on special
silicon suboxide (SiOx ) and UV-nanosecond laser ablation [87–91].
In this thesis, this method is used and studied for DOE fabrication. The processing steps of this method are presented in Fig. 4.5.
First a SiO2 substrate is covered with the required thickness of SiOx ,
which depends on eventual thickness of the desired structure. Then
selected parts of the SiOx layer are removed from the top of SiO2
using nanosecond pulse laser exposure at the UV wavelength of
λ = 193 nm. For this wavelength SiO2 is transparent, but SiOx is totally absorbing and therefore SiOx can be removed without damaging SiO2 . After removing the wanted parts of the SiOx , the sample
is heat treated, which turns SiOx into SiO2 . In this way a solid SiO2
phase element is generated.
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(a)
(b)
Figure 4.6: Principle of the (a) 3- and (b) 4-level DOE separation into two complementary
2-level DOEs.
Above mentioned optical properties also facilitates the removal
of the SiOx -layer from the interface of SiO2 -SiOx (see. Fig. 4.5 (b)),
which ensures better surface quality. The backside removal of the
SiOx -layer also ensures sharper edge quality. Another advantage is
that structures have always even thickness, because the SiOx layer
is either removed or not. The main disadvantage of the method
is that it is limited to fabrication of binary-phase DOEs. A solution to this problem is given in paper IV, where the number of
available phase levels is extended to four by applying two separate 2-level masks. Principle of the separation of the three (a) and
four (b) level DOE into two complementary 2-level DOE system, is
shown in Fig. 4.6. These masks are aligned accurately on top of
each other, giving the freedom to use a total of four phase modulating levels [92]. Fig. 4.7 shows a CCD-camera image of the two
separate phase masks placed accurately on top of each other. Four
phase levels already make it possible to realize more complex intensity distributions, which are often needed in beam shaping, in
particular to overcome the twin-image problem associated with the
inversion symmetry of diffraction patterns of binary phase masks.
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Figure 4.7: CCD-camera image of the two separate phase masks laid on top of each other.
In paper III this SiOx -based method is also used. Here a single
two-phase level DOE is used to generate an inversion symmetric
dot-matrix distribution, with 25×25 intensity maxima in the farfield. This pattern is shown in Fig. 4.8 (a) and it is created with an
on-axis design, meaning that the zeroth diffraction order is located
in the middle of the intensity distribution. In this case the zeroorder intensity is only 1.5 times higher than the average intensity
of all dots. Respectively, Fig. 4.8 (b) presents a CCD-camera image of the intensity distribution produced by an off-axis four-level
DOE, made with the above described method. Here the intended
intensity pattern is also a 25×25 array, meaning that two 25×25
dot matrices would be generated symmetrically around the 0th order if a binary DOE were used. In middle of Fig. 4.8 (b), the 0th
diffraction order can still be observed, but the twin intensity pattern, barely visible in the right lower corner of the figure, can be
suppressed effectively using this method.
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(a)
(b)
Figure 4.8: Far-field images intensity distributions produced by the 2-level (a) and 4-level
(b) DOEs.
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5 Ultrashort laser pulse
ablation using diffractive
optics
In this Chapter methods based on diffractive optics, introduced in
Chapter 4, are considered in more detail in connection with femtosecond laser ablation. In Sect. 5.1, various methods to interfere femtosecond pulses are covered, including a combination of
a diffraction grating and imaging optics and a two-grating interferometer. In this thesis mainly ablation of hole arrays is considered
and for this purpose interferometric femtosecond ablation is beneficial. There are also ways to ablate hole arrays using methods
like direct focusing or mask projection [93], but they are usually
time-consuming and/or size-limited. With interferometric ablation
it is possible to fabricate simultaneously large areas of structures
with periods near the wavelength of the used fs-pulses. In Sect. 5.2
the use of DOEs in femtosecond pulse ablation is presented. Like
the interferometric fs-ablation, this technique is applicable to parallel fabrication of large-area periodic structures. One benefit of the
DOE-assisted approach is that it is not limited to periodic structures, but also arbitrary structures can be realized.
5.1
INTERFERENCE LASER ABLATION
Usually either linear or dot-matrix gratings are fabricated using interferometric femtosecond ablation [94, 95], but also more complex
patterns can be realized [96, 97]. In this thesis only the fabrication
hole-arrays has been studied. Generally, the more waves that have
to be interfered, the more difficult the realization becomes. Added
to this, interfering femtosecond pulses is not straightforward, be-
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
cause of their limited temporal length. For example, a temporally
100 f s long pulse is spatially 30 µm long, meaning that to achieve
interference path difference between the beams can not exceed this.
It is possible to separate femtosecond pulse into parts using various
beam-splitters and thereafter collect them to interfere with mirrors
and delay-lines. In this case, even with two waves it is difficult
to realize the setup, because the delays of the waves have to be
carefully adjusted to temporally combine them [98,99]. Adjustment
gets naturally more difficult as the number of waves increases [100].
Instead of separating the wave using a beam splitter, it is more reasonable to use a diffraction grating [101], because in this way no
separate delay-lines are needed.
After splitting femtosecond pulses with a diffraction grating, it
is possible to use various methods to recombine them to interfere
with each other. One way, which is also used in articles I and II, is to
use a confocal two lens system [102–105]. Collection of fs-pulses can
also be realized in other ways like by using a Schwarzschild-type
reflective objective [106], whereby only a single element is needed
after the grating. This is a beneficial method because only two elements have to be aligned relative to each other and there is no
spectral broadening in this all-reflective objective. However, the
system has it own disadvantages such as the limitations caused by
the geometry of the objective, which limits for example the input
angle. These limitations can be avoided by using a second grating, instead of lenses, to collect the fs-pulses. This configuration is
called a two-grating interferometer and it has been studied in femtosecond ablation by P. Simon and his group [107–109]. They have
studied this method in femtosecond ablation using two-wave interference. Four-wave interference with a two-grating interferometer
has been studied theoretically by Y.-S. Cheng [110–112] and in this
thesis the author has experimentally used it in femtosecond ablation (article V).
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Ultrashort laser pulse
ablation using diffractive
optics
replacements
f1 + f2
f1
Grating
Lens 1
f2
Lens 2
Figure 5.1: Setup for interfering grating separated fs-pulses using two-lens imaging. Fspulses are separated into diffraction orders and thereafter collimated using Lens 1 with
f = f 1 . Lens 2, with f = f 2 , is used to combine waves in its back focal plane. Distance
from the Grating to Lens 1 is f 1 , from Lens 1 to Lens 2 f 1 + f 2 and from Lens 2 to
interference plane f 2 .
5.1.1
Interfering ultrashort pulses using a grating and imaging
system
Interference of the fs-pulses can be realized by using a setup with a
diffraction grating and a two-lens confocal imaging system, which
is presented in Fig. 5.1. The fs-pulses are first split into diffraction
orders by the grating. The first lens with focal length of f = f1
(Lens 1) collimates the selected diffraction orders to propagate into
the same direction. This happens when the distance from the grating to the principal plane of Lens 1 is f1 . The unwanted diffraction orders are blocked using an aperture. When the second lens
(Lens 2) is placed into the distance f1 + f2 from Lens 1, the interference pattern is generated into the back focal plane of Lens 2, i.e., in
the geometrical image plane of the grating. In this way the spatial
size of the original fs-pulses in the focus of the Lens 2 is modified
by a factor M = f2 / f1 , which is the geometrical magnification of
the imaging system. Thus the interference angle in ablation plane
is
tan θabl =
tan θm
,
M
(5.1)
where θm is diffraction angle of the ±1 diffraction order. Usually,
in fs-ablation of hard materials such as metals, M is less than one,
because the ablation threshold of the materials has to be exceeded.
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
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This kind of system is flexible in terms of the number of interfering
waves, as long as the distances of all the selected diffraction orders
from the optical axis of setup are the same. This means that the
system can be used for two-, three-, four- or even multi-beam interference pattern generation. The limitation of the system is that
it can only be used for generation of certain periodic structures.
Secondly, the size of the feasible structures is limited by the numerical aperture (NA) of the available lenses. Also high intensities can
not be used because of the white light continuum generation in the
lenses [113].
5.1.2 Two-grating interferometer
Recombination of fs-pulses separated in the grating (Grating 1) can
be done using a second grating (Grating 2), as shown in Fig. 5.2.
Prior to the article V this approach, also called two-grating interferometer, has only been used in fs-ablation based on two-wave interference [107]. Instead, two-dimensional structures have earlier
been ablated using double exposure with two-wave interference
patterns [108]. In two-wave interference pattern generation, only
two linear gratings with proper design are needed. Theoretically
it can be shown that if the period of Grating 1 is d1 and that of
Grating 2 is d2 , the periods have to be fixed so that 2d1 = d2 to
avoid spectral dispersion of the interference pattern [110]. This also
defines the distance z1 between the gratings and the distance z2 between Grating 2 and interference area so that z1 = z2 and implies
that Grating 2 operates at Bragg configuration. In two-wave interference pattern generation, the Grating 2 can be one single grating,
but in four-wave interference the second grating consist of four separate linear gratings, as shown in Fig. 5.2. In this way the distance
between gratings and interference zone can be increased and at the
same time the size of the gratings can be kept reasonable, considering their fabrication. In the two-dimensional two grating interferometer the first grating has to be a crossed grating to be able
to generate four waves with equal efficiency. Naturally, the second
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Ultrashort laser pulse
ablation using diffractive
optics
Grating 2
Grating 1
z1
z2
Figure 5.2: Schematics of two-dimensional two-grating interferometer. Fs-pulses are separated into four diffraction orders using a crossed-grating (Grating 1) and collected to
interfere using four separate linear binary gratings (Grating 2). Distance from Grating 1
to Grating 2 is z2 and from Grating 2 to interference plane z2 .
grating can also be realized by a single crossed grating, but this
would lead to some problems like increased physical size of the
grating.
5.2
ULTRASHORT LASER ABLATION USING DOE
DOEs have been and are still used in laser ablation to modify the
beam shape [114, 115] and they have also been used in ultrashortpulse laser processing [116, 117]. The DOEs used in this thesis are
based on far-field or Fraunhofer diffraction, meaning that they create the wanted intensity distribution into the far-field of the element. If a focusing lens is introduced near the DOE then this
far-field image is brought into the focus of the lens [118]. At the
same time, the far-field distribution of the DOE is also demagnified. This is reasonable if the intensity distribution of the DOE
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
Aperture
Lens 1
DOE
Lens 2
Hole Aperture
Lens 4
Lens 3
Image Planes
Figure 5.3: Schematics of the ablation setup with DOE. The aperture is used to resize
the square fs-pulses to cover exactly the whole area of the DOE. Positive lenses Lens 1
and Lens 3, added to negative Lens 2, are used to image DOE’s far-field image into Hole
Aperture. The Hole Aperture is used to filter ASE and thereby to enhance the contrast of
the image. Image on Hole Aperture is thereafter imaged into ablation plane using Lens 4.
has high enough resolution (intensity peak separation versus effective width) and there are no additional intensity maxima around
the wanted distribution. If the resolution of the intensity distribution has to be increased or one wishes to remove the noise from
the neighborhood of the desired pattern, additional spatial filtering
can be applied. Naturally if the wanted fields are complex then this
can not be done. Filtering can be realized with the setup shown
in Fig. 5.3, which was also used in papers III and IV. This is done
because the used UV-femtosecond laser has strong amplified spontaneous emission (ASE) and with conventional DOE-lens combination it disturbed DOE’s far-field image. This makes it difficult to
use high fluences and pulse numbers in fs-ablation. In this setup
the far-field image of the DOE is imaged, using a three-lens system consisting of two positive lenses (Lens 1 and Lens 3) and one
negative lens (Lens 2), into the Hole Aperture. The purpose of the
lens system is to magnify the DOE’s image so that it can be filtered
using an aperture, which transmits only the wanted parts of the
DOE’s far-field image. In addition, the lens system collimates the
pulses. After Hole Aperture the intensity distribution is imaged
using Lens 4 into the sample. To have high demagnification, the
distance from Hole Aperture to Lens 4 has to be long.
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Dissertations in Forestry and Natural Sciences No 45
6 Ultrashort pulse ablation of
the functional structures
In this chapter the main results of papers I-V are presented. In
these papers three different functionalities are studied. In the first
Section 6.1 the results of papers I and II are discussed, where controlled fabrication of high absorption structures in metals using interferometric femtosecond ablation are studied. Fabrication of the
super-hydrophobic surfaces using DOE and UV-femtosecond laser
are presented in Section 6.2. The DOEs used in experiments were
made with SiOx -method, already presented in Section 4.3.1. The
results are shown in papers III and IV. In Section 6.3, the results
of paper V are presented. Here fabrication of sub micron size hole
structures are studied, using a two dimensional two grating interferometer. These structures diffract white light, creating visually
colorful spectra, and therefore they are attractive in decorative applications as well as in security marking, since they are not easy to
counterfeit.
6.1
HIGH ABSORPTION STRUCTURES
The simplest way to fabricate high absorption structures with femtosecond laser is to use self-organized structures, shown in Section 3.1.2. These can be generated with directly focused fs-pulses,
and almost total optical absorption from UV to IR-wavelength range
can be achieved by coral-structures with size of few tens of microns,
shown in Fig. 3.2 (d). Similar results can be achieved by replacing
these self-organizing structures with hole arrays with few micron
period, fabricated in a controlled way and shown in Fig. 6.1 (a) and
(b). These hole structures are generated using the two-lens confocal
imaging system presented in Section 5.1.1 and with laser shown in
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
2mm
1mm
(a)
(b)
Reflectance [%]
100
80
60
Flat surface
40
After ablation
20
0
500
1000
1500
Wavelength [nm]
2000
(c)
Figure 6.1: (a)-(b) High aspect ration hole arrays with approx. 2 µm period made using
interferometric femtosecond ablation in stainless steel. (c) Reflection spectra of the polished
stainless steel (upper line) and hole structured surface (lower line).
Section 3.2.1. The benefit of the interferometrically fabricated hole
arrays, compared to self-organized micro-structures is that less fluence and pulses are needed to achieve almost total optical absorption.
The reflection spectra of these structures and polished steel surfaces are presented in Fig. 6.1 (c). These spectra are taken using a
spectrophotometer with an integrating sphere and they show that
the reflectance of stainless steel can be reduced radically, across the
measured wavelength range. In wavelength range from 200 nm to
800 nm, the reflectance of the hole patterned stainless steel can be
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Ultrashort pulse ablation of the functional structures
Reflectance [%]
100
80
Flat surface
After ablation
60
40
20
0
300
500
Wavelength [nm]
700
Figure 6.2: Reflectance spectra of the hole structures (dashed line) and polished (solid line)
copper.
reduced to only a few percent. Likewise in the wavelength range
from 800 nm to 2300 nm, giant enhancement of the absorption can
be realized. In this wavelength range the reflectance increases from
few percent to about 15 %, but still absorption is enhanced ∼60 %70 % across the entire measured wavelength range, compared to a
polished flat surface. Significant absorption enhancement was also
observed for a bulk copper sample structured with holes in wavelength range from 200 nm to 800 nm, as shown in Fig. 6.2.
In order to obtain significant enhancement in absorption, the
micron-size structures need to be deep enough. In paper II the
depths of the holes were measured. This could not be done directly, but it was realized via UV-replication of the holes. The maximum depth of the holes was about 2.5 µm, giving an aspect ratio
more than 1.5 (hole diameter divided by the depth). This value was
achieved with a relatively low number of pulses (400 pulses) and a
low average fluence (0.3 J/cm2 ), and larger values did not increase
the depth of the holes. By assuming that we have perfect interference pattern, maximum fluence can be calculated to be about
1 J/cm2 .
Experiments showed that, even with shallow nano-size ripples,
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
it is possible to enhance absorption. However, to achieve optimal
enhancement, both nano- and micro-size structures have to be applied. One probable reason for the high absorption in these structures is that light is reflected and scattered multiple times in nanoand micro-size structures and in each such event some portion of
light is absorbed. Because of the depth and shape of the microstructures, the reflections penetrates light deeper into the microstructures and the probability to be reflected back gets smaller.
Therefore only a small fraction of light is reflected and major part is
absorbed into matter. Visually this means that the surface appears
almost totally colorless or black, which is why these kinds of metal
surfaces are also referred to as black metals. It is also well-known
that optical properties of the material surface are changing during
the ablation process. Heating and therefore also laser ablation in
the air conditions oxidizes materials surface [119]. This chemical
change nearby the ablation zone might have some influence on absorption although its effect on absorbtion is negligible [25].
In future, additional studies of these functional surfaces could
be carried out. In high absorption structures further investigation
of the hole-array parameters, such as the period, could lead to even
better performance. Secondly, studying these structures in silicon
could be interesting. Both reflectance and also transmittance in the
IR wavelength region can be decreased with these structures, which
makes possible use them in silicon based detector and solar cell
technologies [120].
6.2 SUPER-HYDROPHOBIC SURFACES
Hydrophobicity of the surface can be defined by using a contact
angle, is formed between the sample surface and water droplet,
shown in Fig. 6.3. If this angle is more than ∼150◦ , then the material is said to be super-hydrophobic. Respectively, the surface
is hydrophilic, if the contact angle of the surface is less than 90◦ .
Femtosecond laser ablation has been used for fabrication of superhydrophobic surfaces and usually direct focusing is used to gen-
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Ultrashort pulse ablation of the functional structures
θ
Figure 6.3: Clarification of the contact angle θ.
erate the wanted surface topography. This is the reason why the
fabrication process is slow and therefore not suitable for mass production. In papers III and IV a fast parallel method is introduced
to fabricate super-hydrophobic surfaces in a controlled way, by applying the total energy capacity of the high power fs-pulses, which
is not possible in spot by spot ablation. Here the purpose was to
fabricate high aspect ratio pillars into plastic, since such surface relief structures are known to be super-hydrophobic [121]. To be able
to fabricate high aspect ratio pillars via mass production processes,
like injection moulding and hot-embossing, the used mould should
contain a negative image of the pillars. Hence the mould has to
be covered with high aspect ratio holes. The method used in this
thesis is based on DOE ablation described in Section 5.2, which can
be used for fabrication of such hole arrays in parallel. The DOE
was designed for the laser shown in Section 3.2.2 and to produce
25×25 intensity maxima into its far field, shown in Fig. 4.8. With
single irradiation the DOE can be used to ablate an area of about
320 µm × 320 µm area simultaneously. Fig. 6.4 (a) shows microscope images of the hole arrays in steel, made with this method.
Here the period of the holes is 13 µm and the diameter of the input
hole is about 5 µm. The depth of the holes could not be measured,
because of their high aspect ratio.
Negative copies of these structures were made into a standard
polypropylene (PP) with injection moulding. One of these copies is
shown in Fig. 6.4 (b). Here it can be seen that structures have trans-
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
100mm
20mm
(a)
(b)
(c)
(d)
Figure 6.4: (a) Hole array ablated in steel using DOE and UV femtosecond pulses. (b)
Injection moulded PP copy of the structure shown in (a). (c) Water droplet on a structured
surface, shown in (b), and (d) on a flat PP surface. Contact angle of the droplet is more
than 160◦ in (c) and about 90◦ in (d).
ferred well into PP, although some pillars are elongated. This has
happened because they were stuck into holes when separating the
replica from the mould. In Fig. 6.4 (c) a water droplet has been introduced onto the structure shown in Fig. 6.4 (b), and in Fig. 6.4 (d)
onto a flat PP surface. As it can be seen, the contact angle can
be substantially enhanced by structuring the surface. The contact
angle of the structured surface is more than 160◦ , while it is only
∼ 90◦ with flat surface, hence hydrophobic surface has therefore
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Ultrashort pulse ablation of the functional structures
successfully realized.
As a future study both ablation and injection moulding parameters could be further studied. Optimization of the injection moulding process, which was not done by the author, would require some
additional effort to achieve optimal structures and hydrophobicity.
6.3
DIFFRACTIVE STRUCTURES WITH SUB MICRON
FEATURES
In paper V the fabrication of hole arrays with femtosecond laser ablation is studied using the two dimensional two grating interferometer, described in Section 5.1.2. Our results show that the method
works in practice and can be applied to fabricate sub-micron size
holes in various materials using laser ablation with Ti:Sapphire
lasers (see Sections 3.2.1 and 3.2.3). The grating pair was designed
so that the period of Grating 1 was 1560 nm and that of Grating 2
was 790 nm. Therefore the period of the generated intensity distribution is about 1100 nm. First Grating 1 was designed so that
(±1,0) and (0,±1) are the wanted diffraction orders. Theoretically,
the efficiency of a single order was optimized with FMM to 21.8 %,
giving a total efficiency of about 87 % for the sum of all (±1,0)and (0,±1)-orders. Experimentally the single order efficiency was
17 %, yielding a total efficiency of 68 %. This discrepancy is due to
fabrication errors in Grating 1, caused by the high aspect ratio of
the crossed grating structure. When these errors were taken into an
account in calculations, the results corresponded to measured values. Respectively, the theoretical efficiency of −1st -diffraction order
of Grating 2 was 97 %, which corresponded perfectly to the experimental value. The reason for this high efficiency of the Grating 2 is
that it works like a Bragg grating.
Ablation tests were done in silicon and steel, but in practice
the dot-matrix structure can be fabricated in almost any material.
An example SEM-picture of an ablated structure made in stainless
steel is presented in Fig. 6.5 (a). As this figure shows, large hole arrays can be ablated simultaneously using this method. Larger than
Dissertations in Forestry and Natural Sciences No 45
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Jarno Kaakkunen: Fabrication of functional surfaces using ultrashort
laser pulse ablation
3 mm
(a)
(b)
Figure 6.5: (a) SEM-figure of the dot-matrix structure in stainless steel, fabricated using
two-dimensional two-grating interferometer with femtosecond laser ablation. (b) The logo
of the University of Eastern Finland in round stainless steel sample with 2 inch diameter,
realized with the same structures as in (a).
0.5 mm × 0.5 mm areas can be ablated with single irradiation using
1 mJ pulse energy. The hole diameter is under one micron and the
period is as designed. Periodic structures with features of this size
give visually colorful diffraction effects, when the depth of holes is
correct. To demonstrate possible applications of the method in decorative and also security applications, University of Eastern Finland
logo was ablated into round stainless steel sample with 2 inch diameter, and is shown in Fig. 6.5 (b). This was done by moving
the sample under the four beam interference pattern and ablating
170 µm diameter round areas at single exposure. This means that
tens of thousands of holes are ablated simultaneously.
In future, some studies could be done in optimizing the geometrical and optical configuration of the two-dimensional two-grating
interferometer. Added to this, finding the grating pair’s limitations
theoretically and by means of grating preparation could be done.
Furthermore, frequency-doubled ablation at λ = 400 nm would enable realization of hole arrays with smaller periods.
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7 Conclusions
In this thesis, fabrication methods of functional surfaces with ultrashort laser ablation using diffractive optics based methods were introduced. Principles of the ultrashort pulses and laser ablation were
discussed. Diffractive optics theory behind the optical setups was
also presented. Diffractive optics based laser ablation setups were
then applied to fabrication of high absorption, water-repellant, and
light diffracting structures.
The absorptance of the metals or semiconductors can be enhanced significantly by structuring the surface of the material with
specific microstructures. Material with such surface structures can
be made by femtosecond laser ablation and are referred as black
metal or black silicon. Usually these structures are formed by material self-organization in the laser-matter interaction. In this thesis
high absorption structures were fabricated using ultrashort laser
ablation with a four-beam interference pattern, which was realized
using a two-lens confocal imaging system with a grating. In this
way generated micron size surface structures in metals were found
to be highly absorbing from UV- to IR-wavelengths. Absorption
in steel was enhanced on average more than by a factor of 5 (60 %
enhancement in absorption), compared to the absorption of the surfaces without the structuring. These controlled-way generated high
absorption surfaces were compared with the randomly generated
self-organized structures, shown in Sect. 3.1.2. Fabrication of the
hole-arrays was found to be not only faster, but also higher absorption was achieved than in self-organized structures. These high
absorption structures could be used to enhance the efficiency of the
metal and semiconductor based detectors or in solar cell technology.
The wetting properties of the surface can also be controlled by
structuring the surface of the material with specific microstructure.
The water-repellant i.e. hydrophobic surfaces can be obtained with
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laser pulse ablation
high aspect ratio microstructures or with a combination of microand nanostructures. In this thesis, the whole production chain
from the structuring of the mould to the replication of the superhydrophobic surfaces into the plastic using injection moulding, was
demonstrated. The moulds for this process were fabricated by ultrashort laser pulse ablation of hole arrays using diffractive optical
elements (DOE). DOEs were fabricated with a special method based
on nanosecond UV- laser ablation of SiOx . The DOE was used to
realize multiple spot irradiation with single exposure, which makes
fabrication of the desired structures hundreds of times faster than
single shot ablation. With this technique ablation time of the structures to the mould metal is only a few minutes per square centimeter. Super-hydrophobic injection moulded plastic copies had over
160◦ contact angle, whereas it was only about 90◦ with a flat surface.
Copying this water-repellant property into the plastic enables fast
mass production of cheap replicas and these have applications in
many fields like microfluidics, medical instruments and disposable
applications were friction of the water has to be low.
The final example of laser ablated functional surface was colorfully diffracting structures. For visually striking diffraction pattern,
surface structures should have feature sizes below one micrometer.
To obtain this with laser ablation, a two-dimensional two-grating
interferometer was designed. This system enables the realization of
larger interference angles making it possible to fabricate structures
with smaller periods and features. In these experiments holes with
under one micron diameter were realized. These structures were
ablated in steel and silicon, but they can be made in virtually any
material. Visually, these structures diffract white light into colorful
spectra. This technique enables structuring of the materials that are
not possible with traditional grating fabrication techniques. These
structures can be used in various security and decorative applications.
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