Holographic Recording of Surface Relief Gratings on
As
40
S
60-x
Se
x
Thin Films
L. Loghina
1
, J. Teteris
2
and M. Vlcek
1
1
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice,
Studentska 95, Pardubice, Czech Republic
2
Institute of Solid State Physics, University of Latvia, Riga, Latvia
Keywords: Amorphous Chalcogenide Films, Surface Relief Gratings, Direct Holographic Recording.
Abstract: The studies of direct holographic recording of the surface relief gratings on amorphous As
40
S
60-x
Se
x
thin
films are presented. These gratings were created upon exposure to polarized laser beams of various
wavelengths (λ = 473 - 650 nm). The orthogonally ±45° linearly polarized light beams were used for
recording. The surface structure of the relief gratings was investigated by atomic force microscopy. The
influence of laser beam wavelength and spatial frequency of the gratings recorded (grating period Ʌ) on the
surface relief gratings formation for every composition was examined.
1 INTRODUCTION
Chalcogenide glasses are widely used as materials
for fabrication of optics and optoelectronics
elements i.e. grids, waveguides, microlenses,
photonics crystals etc. (Wang, 2013). During the
past 20 years, research in the field of optical
materials based on amorphous chalcogenides (ACh)
has made significant advances (Kovalskyi, 2006).
The photoinduced fabrication of surface relief
gratings (SRGs) on As
2
S
3
films has been reported
for the first time (Galstyan, 1997). This phenomenon
has attracted much attention due to the possibility of
direct formation of SRGs (Vlcek, 2009; Gertners,
2010). The photoinduced softening of the matrix,
formation of defects with enhanced polarizability,
and their lateral drift under the optical field gradient
force is believed to be the origin of the mass
transport (Saliminia, 2000). The photoinduced
formation of SRGs in ACh mainly has been studied
in As
2
S
3
films by green light (Teteris, 2013). SRG
recording in As-Se system by 650 nm laser was
studied by Trunov et al (Trunov, 2010). The
composition As
40
S
15
Se
45
was optimized for
recording with 632,8 nm laser light (Reinfelde,
2011).
In this work we present the studies of SRG
formation efficiency dependence in amorphous
three-component As
40
S
60-x
Se
x
films on concentration
relationship of S and Se by recording with
orthogonally ±45° linearly polarized light beams in
spectral range of various wavelengths (λ = 473 - 650
nm).
2 EXPERIMENTAL
The bulk samples of different compositions in
systems As
40
S
60-x
Se
x
(x = 0, 10, 20, 30, 40, 50, 60
at.%) were prepared by direct thermal synthesis
from high purity elements (99.999%) in evacuated
quartz ampoules. Thin films of glassy alloy were
prepared using thermal evaporation technique upon
cleaned glass substrates at room temperature (Tesla
Corporation, model UP-858 ) at a pressure of
∼2×10
−4
Pa, and with evaporation rate 1 - 2 nm.s
-1
.
The thickness of thin films was measured directly
during their deposition by dynamic weighing
method and was about 1000 nm.
The surface relief formation experiments were
performed using a holographic recording system
(Reinfelde, 2011). All experiments were realized at
room temperatures. Recording of SRGs was
performed with different lasers λ = 473 – 650 nm.
The orthogonally ±45° linearly polarized light
beams with equal intensities (I
1
= I
2
~ 0,6 W/cm
2
)
were used for recording, thus providing the optimal
conditions for surface relief grating formation. The
grating period Ʌ was changed by varying the angle
between the recording beams. The readout of the
121
Loghina L., Teteris J. and Vlcek M..
Holographic Recording of Surface Relief Gratings on As40S60-xSex Thin Films.
DOI: 10.5220/0005403201210124
In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 121-124
ISBN: 978-989-758-092-5
Copyright
c
 2015 SCITEPRESS (Science and Technology Publications, Lda.)

diffraction efficiency η) was performed by p-
polarized light at the Bragg angle using diode lasers
(653 nm or 672 nm). The intensity of the first - order
diffracted, I
d
, and transmitted I
t
, beams were
measured using two photodiodes. The kinetics of the
recording process was monitored by the time
behavior of the gratings diffraction efficiency,
determined as η(t) = I
d
/(I
t
+I
d
). Such determination
of diffraction efficiency characterizes the
holographic properties of material taking into
consideration the losses associated with absorption,
Fresnel reflection and a light-scattering. In all
experiments time of the holographic exposure was
9000 sec. The SRG profile height ∆h was examined
by atomic force microscopy (AFM).
3 RESULTS AND DISCUSSION
This part of the work is devoted to the study of the
dependence of the first – order diffraction efficiency
for each of the compositions in the surface relief
formation on the wavelength of the recording laser
beams.
Figure 1 shows the dependence of diffraction
efficiency of surface relief gratings on the Se-
content (x, at.%) in amorphous As
40
S
60-x
Se
x
thin
films at holographic recording with different
wavelength lasers.
Figure 1: The first - order diffraction efficiency (η, %)
dependence on selenium content (x) for As
40
S
60-x
Se
x
thin
films for different recording laser wavelengths: film
thickness d ≈ 1,0 µm, exposure dose E ≈ 13,4 kJ/cm
2
,
period of gratings Ʌ ≈ 1 μm.
Figure 2 illustrates the spectral dependence of
diffractive efficiency η, % on wavelength of
recording laser beams for studied As
40
S
60-x
Se
x
thin
films.
First of all, from Figures 1 - 2 we can see that the
surface relief formation efficiency changes a lot by
changing the wavelength of recording laser beam.
All these curves somewhere reaches theirs
maximum, i.e. for every wavelength light what is
used for the holographic recording exists optimal
values of recording conditions for the best
performance. This can be explained by the fact that
for every composition is optimal wavelength closer
to absorption edge of the sample (Sangera, 1996).
Figure 2: Spectral dependence of the first - order
diffractive efficiency η, % for As
40
S
60-x
Se
x
thin films on
wavelength of recording laser beams: film thickness d ≈
1,0 µm, exposure dose E ≈ 13,4 kJ/cm
2
, x – selenium
content, at%.
As we can see from Figure 2, by changing
wavelength laser beam from 473 nm to 561 nm for
As
40
S
60
(x = 0) the best diffractive efficiency is
11,4% at 473 nm, which is different from 561 nm
case (η = 0,58%).
The maximum value of the diffraction efficiency of
15,2% is achieved by irradiation of the sample
As
40
S
30
Se
30
by wavelength laser beam 561 nm under
identical conditions (film thickness d ≈ 1μm, period
of gratings Ʌ ≈ 1 µm). AFM scan of this sample is
presented in Figure 3.
The results of detailed studies for grating period
influence on diffractive efficiency and SRG
formation by 594 nm laser recording are shown in
Figures 4 and 5, respectively. The relief depth up to
467 nm was obtained for a period of Λ=3,59 μm (see
AFM scan in Figure 6). The curves of dependencies
show the existence of an optimal grating period Ʌ
opt
m, where, at a stated illumination dose E, the
profile height ∆h and diffraction efficiency have a
maximum values while falling down versus smaller
and greater periods for the same value of E. For
PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology
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As
40
S
60
(x = 0) as well As
40
S
50
Se
10
(x = 10) changes
of diffractive efficiency and height profile are not
very significant.
Figure 3: AFM scan of the As
40
S
30
Se
30
SRG formed by the
exposure of a 561 nm wavelength laser beam.
At the same time maximum values of the diffraction
efficiency – 63% and 66% are achieved for
As
40
S
30
Se
30
and As
40
S
20
Se
40
thin films, respectively.
Figure 4: The first - order diffraction efficiency (η, %) on
grating period (Ʌ, µm) for As
40
S
60-x
Se
x
thin films: films
thickness d ≈ 1,0 µm, exposure dose E ≈ 13,5 kJ/cm
2
, x –
selenium content (at.%), recording laser wavelength λ =
594 nm.
According to (Reinfelde, 2014) the grating
period affects the SR formation efficiency. The
value of Ʌ
opt
for holographic recording of SRGs
with orthogonally ±45° linearly polarized light
beams depends on the film thickness and exposure
dose. The reason for the existence of Ʌ
opt
could be
related to some equilibrium state between the
surface tension and SR grating formation forces
created by the light intensity gradient perpendicular
to the film plane.
It is known that the first - order diffraction efficiency
for sinusoidal surface relief transmission gratings
depends on a relief height is given by
η ~  h/ 
(Yokomori, 1984). Therefore some similarity in the
curves of diffraction efficiency in Figure 4 and
curves of relief height in Figure 5 can be observed.
Figure 5: Relief profile height ∆h, with grating period Ʌ,
μm for As
40
S
60-x
Se
x
thin films: film thickness d ≈ 1,0 µm,
exposure dose E ≈ 13,5 kJ/cm
2
, x – selenium content, at%,
recording laser wavelength λ = 594 nm.
Figure 6: AFM scan of the As
40
S
30
Se
30
SRG formed by the
exposure of a 594 nm wavelength laser beam: film
thickness d ≈ 1,0 µm, exposure dose E = 13,5 kJ/cm
2
.
Figure 7 illustrates the compositional
dependence of the optical bandgap E
g
of As
40
S
60-x
Se
x
thin films and light quantum energy E
opt
for optimal
holographic recording. It is seen that the values of
E
opt
can be expressed as E
g
+(0.1-0.2)eV. According
to (Tanaka, 2011) the optical absorption coefficient
at such light quantum energy for chalcogenide
materials is about = 10
4
cm
-1
. The light penetration
depth in the film at such optical absorption
coefficient is d
p
= 1/  µm. This value coincides
to the thickness of studied films. It is known that the
whole illuminated depth of the film, where the laser
HolographicRecordingofSurfaceReliefGratingsonAs40S60-xSexThinFilms
123

light intensity is sufficiently strong, takes part in the
formation process of photoinduced SRGs
(Reinfelde, 2013).
Figure 7: Compositional dependence of the optical
bandgap E
g
[data from (Gonzalez-Leal, 2003)] and
optimal quantum energy E
opt
of light for recording in 1 μm
As
40
S
60-x
Se
x
thin films with a grating period Ʌ = 1 μm.
Consequently, the optical properties of recording
material and thickness of the films must be
considered for optimization of SRG recording.
4 CONCLUSIONS
The optimal wavelengths for holographic recording
of surface relief gratings with orthogonally ±45°
linearly polarized light beams in As40S60-xSex (x =
0, 10, 20 … 60 at.%) thin films have been
established. An influence of grating period on
surface relief formation efficiency is shown.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge funding through
the grant CZ.1.07/2.3.00/30.0058 from the Czech
Ministry of Education, Youth and Sports.
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