April 10, 2009 / Vol. 7, No. 4 / CHINESE OPTICS LETTERS 325
Deep silicon grating as
high-extinction-ratio p olarizing beam splitter
Jijun Feng (
úúú
)
1∗
, Changhe Zhou (
)
1,2
, Bo Wang (
)
1
,
and Jiangjun Zheng (
)
1
1
Information Optics Lab, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800
2
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800
∗
E-mail: fjijun@siom.ac.cn
Received July 18, 2008
A deep binary silicon grating as high-extinction-ratio reflective polarizing beam splitter (PBS) at the
wavelength of 1550 nm is presented. The design is based on the phenomenon of total internal reflection
(TIR) by using the rigorous coupled wave analysis (RCWA). The extinction ratio of the rectangular PBS
grating can reach 2.5×10
5
with the optimum grating period of 397 nm and groove depth of 1.092 μm.
The efficiencies of TM-polarized wave in the 0th order and TE-polarized wave in the −1st order can both
reach unity at the Littrow angle. Holographic recording technology and inductively coupled plasma (ICP)
etching could be used to fabricate the silicon PBS grating.
OCIS co des: 050.1950, 230.1360, 230.5440, 060.4510.
doi: 10.3788/COL20090704.0325.
Polarizing beam splitter (PBS), which plays an impor-
tant role in optical communication, can split an incident
beam into two orthogonally polarized light beams. The
underlying principle of conventional PBSs is based on
natural birefringent effects
[1]
or refraction effect at mul-
tilayer dielectric coatings
[2]
. However, the conventional
multilayer film PBS is expensive due to the complicated
procedure of multilayer coating. Nowadays, photonic
crystal
[3]
as PBS has aroused more and more attention.
The structure of photonic crystal is so complicated that
it is difficult to be fabricated.
It seems much more interesting if a simple subwave-
length grating can function as a polarizer, which has
been reported in transmission
[4,5]
, reflection
[6]
,mold
injection
[7]
, embedded metal wire
[8]
,andmetalwire-
grids
[9]
as broadband polarizers
[10,11]
. For practical use
in optical communication, good extinction ratio and high
efficiency with a wide angular bandwidth and a broad
wavelength range are needed. Among different types
of diffraction gratings, metallic reflection grating has
the disadvantage of power-absorption and low damage
threshold. Low-contrast dielectric grating, such as fused-
silica grating, is suitable to function as transmission PBS
for its low refractive index. High-contrast dielectric grat-
ing, such as silicon grating, is better to be used as total-
internal-reflection (TIR) PBS grating for its high refrac-
tive index. TIR dielectric gratings can achieve diffrac-
tion efficiency as high as 100%. It was previously re-
ported that dielectric TIR gratings with only two diffrac-
tion orders can achieve unity diffraction efficiency under
the Littrow condition to generate high-efficiency reflec-
tive diffraction
[12−15]
. Marciante et al.
[14]
designed and
fabricated a high-dispersion, high-efficiency, surface-relief
diffraction grating with extremely low polarization sen-
sitivity for L-band (1560–1610 nm) telecommunication
applications.
Figure 1 depicts such a TIR grating with period Λ and
groove depth h between two materials with refractive in-
dices n
1
and n
2
(n
1
>n
2
). In the Littrow mount, the
diffracted −1st order light retraces the same path as the
incident light. There will exist only two diffracted or-
ders: the 0th and −1st orders, if the grating period Λ
satisfies
[13]
n
1
>
λ
2Λ
>n
2
, (1)
where λ is the vacuum wavelength of the light incident
from material n
1
.
It is known that silicon is the most common material in
semiconductors. Since Tsang et al. fabricated the silicon
grating
[16]
, the fabrication technology and the applica-
tion of silicon gratings developed rapidly. Because of the
high refractive index of silicon
[17]
, it is appropriate to
utilize the TIR phenomenon of the material. Based on
the phenomenon of TIR, the high-extinction-ratio reflec-
tive silicon PBS grating can be designed regardless of
grating tooth shape
[13−15]
. Without loss of generality,
a rectangular grating is considered. As shown in Fig.
1, n
1
and n
2
are refractive indices of silicon and air,
respectively, and b is the ridge width. The duty cycle f
Fig. 1. Schematic illustration of a silicon TIR PBS grating.
n
1
and n
2
are refractive indices of silicon and air, respectively,
Λ is grating period, b is ridge width, h is depth, θ
i
is incident
angle (under Littrow mounting), θ
0
and θ
−1
denote diffraction
angles of the 0th and −1st orders in silicon, respectively.
1671-7694/2009/040325-04
c
2009 Chinese Optics Letters
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