Polarization effect of antireflection coating for soi material system

International Journal of Science and Research (IJSR), India Online ISSN: 2319-7064
Volume 1 Issue 3, December 2012
www.ijsr.net
Polarization Effect of Antireflection Coating for
SOI Material System
Imran Khan1
1
Dept. of EEE, Jessore Science & Technology University (JSTU),
Jessore-7408, Bangladesh
ikhan@jstu.edu.bd
Abstract: For Silicon on Insulator (SOI) material system light transmission & polarization effect due to antireflection coating is
investigated with the aid of CAD tool Essential Macleod & MATLAB. In case of one layer coating the transmission is 93.0440 % for TM
polarized light and 92.6245 % for TE polarized light after layer thickness optimization. Multilayer coating for SOI material system
(using different thicknesses and materials) some time gave identical result and some time less than the transmission got for single layer
coating. Average transmission of light is increased about 19% due to the use of antireflection coating.
Keywords: Polarization, Silicon on Insulator, Transverse Electric, Transverse Magnetic
1. Introduction
The propagation of electromagnetic waves generated from
any light source normally influenced by the layered stack of
media. The travelling light waves will be reflected,
transmitted, refracted multiple times by interfaces between
different media. So, different waves will present to the
electromagnetic field in each layer of the stack which results
interference phenomena [1], [2]. Some of the light incident
on the stack of layers will be transmitted, part of it will be
absorbed and part of it will be reflected. Maximum
transmission and minimum reflection is desired at many
applications. One of these applications is the antireflection
(AR) coating, the design and fabrication method is discussed
in [3]. One example of this AR coating is the efficient
coupling of light from a fiber to waveguide or photonic
integrated circuit and its surrounding medium. This AR
coating is used in several related applications [4]-[6] Here an
antireflection coating is developed so that the transmission of
the guided mode of the waveguide structure through the
interface between the photonic integrated circuit and its
surrounding medium (air) is maximized [7] and to observe
the polarization effect of light on antireflection coating. This
AR coating is also used in telecommunication application
[8].
Figure 1. Waveguide facets for SOI material system
At the end facets (input and output side) of a photonic
integrated circuit light is coupled respectively from and to an
optical fiber. In order to maximize the transmission for
horizontal coupling an antireflection coating can be
deposited on the facets of the chip. Such an antireflection
coating consists of a stack of thin films with accurately
chosen thicknesses and refractive indices. For our analysis
we will be considering the system as shown in Figure 1.The
guided mode of the waveguide shown in Figure 1 is
approximated by a Gaussian beam. In Table 1 the properties
of this beam are given, σx and σy are the typical dimensions
of the Gaussian function.
In Essential Macleod all layers of a stack are homogeneous.
In this case the substrate is the cross section of a waveguide
structure, which obviously is not homogeneous.
Consequently, we modeled this cross-section by one single
number, i.e. the effective index neff of the guided mode of the
waveguide. Here λ0 is the wavelength.
Table 1: Dimensions of the Gaussian field profile
Material System λ0 [μm] σx [μm] σy [μm] n (neff)
SOI 1.55 0.39 0.46 2.7
2. Design & Simulation of AR Coating
The AR coating is used to maximize the transmission and to
analyze the polarization effect of the AR coating, to achieve
this a single-layer coating is designed, then it was optimized
and finally design of a multi-layer coating with the aid of the
CAD tool Essential Macleod was conducted.
We consider the transmission at the interface of the
waveguides (Error! Reference source not found.). In
Essential Macleod we can only calculate the transmission
through a stack of homogeneous layers, so we model the
cross section of the waveguide structure by one single
number, the effective index neff of the guided mode of the
structure.
The CAD software enables us to calculate the transmission
of plane waves incident at an arbitrary angle, but the guided
modes of the waveguide structures are approximated by
Gaussian beams (the properties of which are given in Error!
Reference source not found.). We can solve this problem
by decomposing the Gaussian beam into its plane wave
components through spatial Fourier transform, calculating
the transmission for each of these components and then
using the inverse Fourier transform to obtain the transmitted
beam.
59

www.ijsr.net
The transmission through a layer stack is calculated, which is
built up of a semi-infinite substrate layer (in this case the
waveguide), a number of thin films and another semi-infinite
medium (in this case air). The thickness of each layer can be
set by a physical thickness and by an optical thickness. The
optical thickness is equal to the number of wavelengths the
layer contains:
n
thicknessphysical
thicknessOptical


3. Result & Analysis
3.1 Without AR coating
To have clear understanding first of all the measurement was
done without any coating so that we can use it as reference.
In this case there is no transmission above critical angle i.e.
o
angleCritical 74.21
7.2
1
sin 1






 
We also observed 100% transmission at Brewster angle i.e.
o
angleBrewster 32.20
7.2
1
tan 1






 
For perpendicular incidence (incident angle=00
) transmission
can be calculated [9] as-
%79²11
46.0
1
1





rRT
n
n
r
eff
eff
The phase is zero everywhere i.e. Phase = 0.
Because there is no coatings, as there is coatings as a result
no phase change and no interference.
Figure 1: Transmission for all polarizations when there is no coating: magnitude (solid lines) and phase (dashed lines).
TM: red, TE: blue, Mean: green
60

www.ijsr.net
Figure 2: Input field amplitude
Figure 3: Output field amplitude (mean)
From the Figure 2 it is clear that Transverse-Electric (TE)
polarized light transmission is slightly lower than the
Transverse-Magnetic (TM) polarized light because of the
Brewster angle but both polarizations (TE & TM) have
similar amplitude profile. As there is no coating it is desired
that there will be no interference at the output. The output
should be as input (Figure 3). But it is clear from the Figure
4 that there are interferences at the output. These interference
fringes around the edges of the Gaussian beam because the
beam width is small compared to the wavelength, which
causes diffraction. Global transmission is calculated using
the Table 1 parameters and MATLAB simulator. Thus we
got global transmission as-
TM Polarized Light = 79.6163%
TE Polarized Light = 67.3830%
Mean = 73.4983%
As expected, the global value is slightly lower than the value
for perpendicular incidence because the Gaussian beam also
has plane wave components with a more oblique incidence,
for which the transmittance is lower (Figure 2).
3.2 One layer AR coating
Theoretical values for single layer coating can be calculated
as [10]-
64.17.2 ARn
ARn
thicknessphysical


4
1
So, Layer thickness is 236.25 nm.
According to the Snell’s law-
)sin()sin()sin( airairARARSOISOI nnn  
Critical angle when θair = 90° or θAR = 90° is-






























21.74sin
sin,sinmin
1
11
,
SOI
air
SOI
AR
SOI
air
critSOI
n
n
n
n
n
n


In this case the phase change will be different for TE & TM
polarization.
Due to the fact that there is a phase change, the different
plane wave components of the Gaussian beam will interfere
between each other. This will cause a lower transmission
than we see in Figure 6.
61

www.ijsr.net
Figure 5: Output field amplitude (Mean)
Figure 6: Transmission for all polarisations for a single layer coating: magnitude (solid lines) and phase (dashed lines). TM:
red, TE: blue, Mean: green
Mean transmission shows stronger interference fringes, TM
also slightly stronger interference than TE.
TM-transmission slightly higher than TE-transmission:
smaller phase shift for TM than for TE (Figure 6) results less
interference of the plane waves.
Global transmission is-
TM Polarized Light = 93.0245 %
Mean = 92.6414%
The transmission is much lower than expected, because of
(both) interference effects.
3.3 Optimized one layer AR coating
Optimization of the coating was done by changing the
thickness of the layer to increase the transmission. We set
the targets to a wavelength of 1550 nm, and 100%
transmission between 0° and different angles. We obtained
the best results for an angle range of 0°-15°. The optimized
physical thickness is 246.97 nm.
Global transmission is-
TM Polarized Light = 93.0440 %
Mean = 92.8342%
62

www.ijsr.net
The transmission is slightly higher than before the
optimization.
3.4 Multi layer AR coating
Optimization for higher transmission with multiple layers
was conducted. Used layers were maximum 5 or less, this
will limit calculation time and is also more realistic: adding
more layers will complicate fabrication.
When we use the same targets as we did and a maximum of
5 layers, the program generates a one-layer coating identical
to the one we used in the previous section. Using different
targets did not help us find a better solution, on the contrary:
some of the multi-layer coatings we created gave a lower
transmission than the one layer coating.
4. Conclusion
It is clear from the above analysis that if we use the
antireflection coating to maximize the transmission then we
get on an average 19% more transmission than if we use no
antireflection coating. The main thing to consider is the
polarization, when there is no AR coating the difference
between TM polarized light transmission (79.6163%) and
the TE polarized light transmission (67.3830%) is high. But
when we use AR coating the light transmission difference
between the TM (93.0245 %) & TE (92.2584%) polarized
light become smaller. So by using AR coating we can reduce
the polarization effect of lights. Though AR coating reduces
the polarization effect but TM polarized light transmission is
always higher than the TE polarized light. This type of
investigation can also be conducted for Silica on Silicon
(SOS) and other material system to find out the optimized
way to use AR coatings regarding polarization and
maximum transmission of light.
Acknowledgement
Author would like to thank Sofie Lambert, C.W.Oh and J. B.
Martinez for their support.
References
[1] B.E.A. Saleh, M.C.Teich, Fundamentals of Photonics,
Second Edition, Wiley & Sons, Canada, 2007. Ch.2.
[2] H.K.Raut, V.A. Ganesh, A.S. Nair and S. Ramakrishna,
“Anti-reflective coatings: A critical, in-depth review”,
Energy Environ. Sci., 2011, 4, 3779. www.rsc.org/ees.
[3] I. Hodgkinson, Q.H. Wu, “Anisotropic antireflection
coatings: design and fabrication”, OPTICS LETTERS,
Vol. 23, No. 19, October 1998, pp. 1553-1555.
[4] L. Atternas, L. Thylen, “Single-Layer Antireflection
Coating of Semiconductor Lasers: Polarization
Properties and the Influence of the Laser Structure”,
Journal of Lightwave Tech., Vol.7, No.2, February
1989, pp.426-430.
[5] R.E.Smith, M.E.Warren, J.R. Wendt and G.A. Vawter,
“Polarization-sensitive subwavelength antireflection
surfaces on a semiconductor for 975 nm”, OPTICS
LETTERS, Vol.21, No.15, August 1996, pp. 1201-
1203.
[6] R.R.Willey, “Non-Polarizing Beamsplitter and AR
Coating Design”,In Procedings of the Society of
Vacuum Coaters on 48th
Annual Technical Conference,
2005, ISSN-0737-5921, pp.391-395.
[7] I. Yamada, K.Kintaka, J.Nishii, S.Akioka, Y.
Yamagishi and M. Saito, “Transmittance enhancement
of a wire-grid polarizer by antireflection coating”,
APPLIED OPTICS,Vol.48, No.2, January 2009, pp.
316-320.
[8] J.A.Dobrowolski, J.E.Ford, B.T.Sullivan, L.Lu and
N.R. Osborne, “Conducting antireflection coatings with
low polarization dependent loss for telecommunication
applications”, Optics Express, Vol. 12 Issue.25, 2004,
pp. 6258-6269.
[9] P. Yeh, Optical Waves in Layered Media, John Wiley
and Sons, 1988.
[10] R. Baets, Photonics Syllabus, Universiteit Gent, 2006.
Author Profile
Imran Khan received his B. Sc. Engineering
degree in Electronics and Communication
Engineering from Khulna University of
Engineering & Technology, Bangladesh in
2008. He completed photonics postgraduate
course works from Ghent University
(Belgium) and University of Ottawa
(Canada) in 2010 & 2011 respectively. He
worked as a research assistant in photonics research lab, University
of Ottawa, Canada. In 2012 he joined as a faculty member in
Jessore Science & Technology University (JSTU), Bangladesh in
the Department of Electrical and Electronic Engineering (EEE). His
areas of research are photonics, SPR sensors & optoelectronics.
63

Polarization effect of antireflection coating for soi material system

More Related Content

What's hot (20)

Similar to Polarization effect of antireflection coating for soi material system (20)

More from International Journal of Science and Research (IJSR) (20)

Recently uploaded (20)

Polarization effect of antireflection coating for soi material system