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RESEARCH ARTICLE
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Meta-Structured Silicon Nanophotonic Polarization Beam
Splitter with an Optical Bandwidth of 415 nm
Hongnan Xu, Yue Qin, Gaolei Hu, and Hon Ki Tsang*
The polarization beam splitter (PBS) is a pivotal element in the polarization
management of free-space optical instruments and systems. Photonic
integrated circuits for sensing, imaging, communications, and
quantum-information processing also have needs for monolithically
integrated PBSs with an ultra-broad optical bandwidth. In this paper, a novel
silicon nanophotonic PBS inspired by the crystalline Glan–Thompson prism
but implemented with silicon subwavelength-grating (SWG) metamaterials is
presented. Due to the tailored artificial anisotropy of SWGs, the meta-prism
functions like a thin-film reflector or a waveguide crossing for different
polarizations. Thus, the incident light can be steered with strong polarization
selectivity and negligible wavelength dependence. Unlike conventional PBS
designs, the routing of polarized light is enabled by the wavelength-
independent total internal reflection in anisotropy-engineered effective media,
thereby breaking the bandwidth limit. The device footprint is as small as
≈15 ×7μm2. Low insertion losses of 0.6–1.7 dB and high extinction ratios of
20–30 dB are experimentally achieved spanning a record broad bandwidth of
over 415 nm, ranging from 1.26 to 1.675 μm wavelength. These results
represent, to the best of their knowledge, the most broadband integrated PBS
ever demonstrated to date.
1. Introduction
Silicon nanophotonics has been gaining significance in the
large-scale monolithic integration of functional photonic devices,
addressing a wealth of applications, e.g., programmable pho-
tonic circuits,[1] optical communications,[2] label-free sensing,[3]
miniature spectrometers,[4] and optical coherence tomography
(OCT).[5] Silicon nanophotonic devices are formed by silicon-
on-insulator (SOI) waveguides with core dimensions at the
subwavelength scale. The high index contrast of SOI ensures
strong field confinement of guided modes. However, the small
modal area also causes waveguide birefringence, resulting in
polarization-dependent losses and dispersions for almost all sili-
con nanophotonic devices. A generic solution to this problem is
H. Xu, Y. Qin, G. Hu, H. K. Tsang
Department of Electronic Engineering
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong S. A. R. China
E-mail: hktsang@ee.cuhk.edu.hk
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/lpor.202200550
DOI: 10.1002/lpor.202200550
the polarization-diversity circuit,[6]
namely separating transverse-electric
(TE) and transverse-magnetic (TM)
components into individual circuits
and processing them independently.
The polarization beam splitter (PBS),
which separates orthogonal polarization
components, plays an indispensable role
in such a scheme. Broadband PBSs are
desirable in many optical instruments
and systems. In a fiber-to-the-home
(FTTH) network, the upstream and
downstream data traffic is allocated in
three coarse wavelength channels (i.e.,
1.31, 1.49, and 1.55 μm), thereby requir-
ing an aggregate optical bandwidth of
≈300 nm for the integrated triplexer.[7,8]
Recent advances in comb-driven opti-
cal interconnects and highly paralleled
data-center communications have been
pushing the boundary of link capacities
even further.[9,10] Therefore, it is essential
to expand the optical bandwidth of PBSs
to enable the polarization management
for a substantial number of wavelength
carriers in future optical communication
systems. The polarization-sensitive OCT collects the intensity
and retardation information by probing the birefringent sam-
ple with broadband polarized light.[11] The coherent detec-
tion of orthogonal polarization channels necessitates the use
of broadband PBSs.[12] Recently, the utilization of chip-scale
frequency-comb and supercontinuum sources has expanded the
operation bandwidth of OCT to the ultra-broadband regime
(>500 nm).[5,13 ] Other applications, such as dual-polarization
sensing and quantum-information processing,[14,15 ] also require
polarization handling over a broad optical bandwidth.
A variety of mechanisms have been proposed to real-
ized integrated PBSs by utilizing Mach–Zehnder inter-
ferometers (MZI),[16,17 ] multimode-interference couplers
(MMI),[18–20 ] distributed Bragg reflectors (DBR),[21] inverse-
designed structures,[22–24 ] and asymmetric directional couplers
(ADC).[25–36 ] Among them, the ADC-based PBS is the most ubiq-
uitous type, thanks to its simple structure and small footprint.
The idea is to translate waveguide birefringence into geometric
asymmetry by congregating different types of waveguides with
distinct birefringent characteristics. Since the phase-matching
condition can only be fulfilled for the selected polarization, the
evanescent-coupling strength can be vastly different between
TE and TM based on this scheme. Such geometric asymmetry
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can be produced by introducing nano-slots,[25,26 ] twisting core
trajectories,[27–31 ] employing bridging waveguides,[32–34 ] and
loading heterogeneous materials.[35,36 ] However, the strong
dispersion of guided modes will inevitably deteriorate the phase
matching as the working wavelength is deviated, resulting in a
narrow bandwidth, typically limited to <100 nm. In refs. [26,
29], multi-core couplers and cascaded polarizers are exploited
to filter out the residual power resulting from incomplete cou-
pling, thereby achieving a bandwidth slightly exceeding 100 nm.
Nevertheless, this strategy is not scalable due to the dispersive
nature of polarization filtering operations.
The subwavelength-grating (SWG) metamaterial, which is
created by non-resonant periodic nano-structures with a deep-
subwavelength pitch, has emerged as a new building block in sil-
icon nanophotonics.[37,38 ] The SWG can be treated as a homoge-
neous effective medium resembling a crystalline material with an
anisotropic index tensor. Remarkably, the artificial anisotropy of
SWGs is considerably stronger than that of any crystalline mate-
rials readily existing in nature. Owing to this distinctive attribute
of SWGs, the meta-structured PBS has attracted tremendous re-
search interest.[39,40 ] One straightforward approach is to build
an ADC by combining regular waveguides with SWG waveg-
uides, as demonstrated in refs. [41–48]. For most of these de-
signs, polarization selectivity is still offered by geometric asym-
metry, therefore it does not deterministically improve the opti-
cal bandwidth. Recent research has proven that the multiband
operation is only possible at the expense of degraded extinction
ratios (≈10 dB).[46] The first breakthrough on the performance
ceiling is reported in ref. [49]. The proposed design is based on
a “hetero-anisotropic” structure whose index distribution alters
with the orientation of the electric field, allowing it to behave as
either a MMI or two isolated waveguides at different polarization
states. Notably, in this design, polarization selectivity is offered
by effective-medium anisotropy as opposed to geometric asym-
metry, leading to a broad optical bandwidth (≈215 nm) beyond
the phase-matching limit. Nevertheless, the bandwidth restric-
tion still applies due to the wavelength dependence of polariza-
tion routing. To be specific, the routing of polarized light streams
relies on evanescent coupling or multimode interference in a
coupler. In essence, these processes are induced by the interfer-
ence that occurs between symmetric and anti-symmetric modes,
and it is common knowledge that such an interference is suscep-
tible to changes in wavelengths.
Anisotropic optical crystals have been extensively employed in
the realization of free-space polarization-handling instruments,
e.g., Glan-Thompson prisms, Rochon prisms, Wollaston prisms,
and wave plates.[50] The Glan–Thompson prism is one of the
most commonly used types of free-space PBSs, which consists
of two bonded calcite wedges separated by a cement-filled gap.[51]
Due to the distinct ordinary and extraordinary medium indices of
calcite, total internal reflection (TIR) only applies to the selected
polarization state, thereby separating light streams with orthog-
onal polarizations. The medium anisotropy required by a Glan–
Thompson prism can be fully emulated by using SWG metama-
terials. Based on this concept, we propose and demonstrate an
SWG-enabled meta-prism serving as an ultra-broadband PBS.
The artificial anisotropy of SWGs is engineered to improve po-
larization selectivity. Moreover, the wavelength dependence of
polarization routing is eliminated since the dispersion of the
TIR effect is inherently weak. The device footprint is as small as
≈15 ×7μm2. In the experiment, we observe insertion losses of
0.6–1.7 dB and high extinction ratios of 20–30 dB throughout a
record broad bandwidth of >415 nm. These results represent, to
the best of our knowledge, the most broadband integrated PBS
ever demonstrated to date.
2. Design and Analysis
Figure 1a–c exhibits the 3D view of the proposed integrated
PBS. The detailed description about structural parameters can be
found in Section S1 (Supporting Information). The structure con-
sists of a Glan–Thompson-like meta-prism connected to four in-
put/output sections, as illustrated in Figure 1d. The input/output
sections are aligned to the central point at a specific incident
angle (𝜃in). Regular waveguides and SWG waveguides are inter-
faced through an adiabatic taper.[49] In order to generate a quasi-
plane wave with weak divergence, the width of input/output sec-
tions is expanded to the multimode regime. The meta-prism is
constructed by assembling SWG<100>and SWG<010>with opti-
cal axes pointing in x-andy-directions, respectively. In the free-
propagation region, SWG<100>resides on two flanks, whereas
SWG<010>is a nano-stripe sandwiched in the middle. These
SWGs can be treated as effective mediums (EM<100>,EM
<010>)
depicted by index ellipsoids, as illustrated in Figure 1e. The rigor
of this equivalence will be discussed later. The duty cycles (fSWG)
of SWG<100>and SWG<010>are initially configured to be identi-
cal. Hence, the difference between the index ellipsoids of EM<100>
and EM<010>is a 90°rotation around the z-axis. The correspond-
ing index tensors can be expressed as:[52]
𝜺<100>
SWG =
n2
e00
0n2
o0
00n2
o
(1)
𝜺<010>
SWG =
n2
o00
0n2
e0
00n2
o
(2)
where nodenotes the ordinary medium index, and nedenotes the
extraordinary medium index. Here, noand necan be formulated
by applying Rytov’s theory:[53]
n2
o=fSWGn2
Si +1−fSWGn2
SiO2(3)
1
n2
e
=fSWG
n2
Si
+1−fSWG
n2
SiO2
(4)
where nSi denotes the refractive index of silicon cores, nSiO2de-
notes the refractive index of oxide claddings, and fSWG denotes
the duty cycle of SWGs. In Equations (3) and (4), the higher-order
terms are negligible in designs which have the SWG pitch much
smaller than wavelengths. The rigorous expression of noand ne
can be found in ref. [54]. In addition, it can be derived from Equa-
tions (3) and (4) that the anisotropy of SWGs is always negative
(i.e., no>ne), regardless of fSWG. We firstly discuss the behav-
ior of TE-polarized light in EM<100>and EM<010>. It is assumed
that the in-plane TE electric displacement (DTE) is neither paral-
lel nor perpendicular to the optical axis. In this case, TE medium
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Figure 1. Conceptual illustration of the meta-structured silicon nanophotonic polarization beam splitter (PBS). a) 3D-view of the PBS. Enlarged views
of b) the Glan–Thompson-like meta-prism and c) the input/output section with key parameters labeled. d) Schematic configuration of the PBS. The
structure is built by assembling subwavelength-grating (SWG<100>,SWG
<010>) metamaterials with orthogonal optical-axis orientations, which can be
treated as effective mediums (EM<100>,EM
<010>) with diverse anisotropy. e) Index ellipsoids of effective mediums. The green lines represent the optical
axes. The red and blue arrows represent the TE and TM electric displacement (DTE,DTM ). The difference between these index ellipsoids is a 90°rotation
around the z-axis. The TE medium indices (n<100>SWG,TE, n<010>SWG,TE) are distinct in EM<100>and EM<010>due to the in-plane DTE. In contrast,
the TM medium indices (n<100>SWG,TM, n<010>SWG,TM) are consistent in EM<100>and EM<010>since DTM is always perpendicular to the optical
axis. f) Principle of the Glan–Thompson-like meta-prism. The anisotropic medium index of SWGs is mapped into the polarization-selective effective
index (neff,TE,neff,TM ) of slab modes. For TE, the incident light will be routed to the BAR port by means of total internal reflection, which is induced by
the neff,TE contrast between EM<100>and EM<010>. For TM, the incident light will directly transmit into the CRO port due to the nearly uniform neff,TM
distribution.
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indices (n<100>
SWG,TE,n<010>
SWG,TE) can be obtained from the intercept of
DTE on the index ellipsoid,[50] which yields:
n<100>
SWG,TE4
=n2
ecos 𝜃<100>
D,TE 2
+n2
osin 𝜃<100>
D,TE 2
(5)
n<010>
SWG,TE4
=n2
ocos 𝜃<010>
D,TE 2
+n2
esin 𝜃<010>
D,TE 2
(6)
where 𝜃<100>
D,TE and 𝜃<010>
D,TE denote the angles between the x-axis
and DTE (see Section S2, Supporting Information). In this paper,
we use <100>and <010>to designate parameters in EM<100>
and EM<010>. For TE, a high index contrast (i.e., n<100>
SWG,TE →no,
n<010>
SWG,TE =ne) can be accomplished to support the TIR condition,
as illustrated in the first column of Figure 1e. The rigorous
derivation of n<100>
SWG,TE and n<010>
SWG,TE can be found in Section S2
(Supporting Information). In contrast, the TM electric displace-
ment (DTM) is normal to the propagation plane, resulting in the
consistent ordinary TM medium index in EM<100>and EM<010>
(i.e., n<100>SWG,TM =n<010>SWG,TM =no), as illustrated
in the second column of Figure 1e. A Glan–Thompson-like
scheme can be then established by mapping the anisotropic
medium index (nSWG) of SWGs into the polarization-selective
effective index (neff) of slab modes, as shown in Figure 1f. In
such a structure, TE-polarized light will be routed to the BAR
port by a TIR-enabled thin-film reflector composed of high-index
EM≤100≥and low-index EM≤010≥, whereas TM-polarized light will
directly transmit into the CRO port due to the nearly uniform TM
effective index over the crossing region.[55] For the conventional
Glan–Thompson prism, the calcite wedges are usually separated
by isotropic cement,[51] whereas for our modified design, the
entire meta-prism is constructed by anisotropic mediums to
improve polarization selectivity. The wavelength dependence of
TIR-based polarization routing is inherently weak, leading to
a substantial expansion of the optical bandwidth. In the strict
sense, the reflected TE wave has a small displacement at the BAR
port due to the lateral dimension of EM<010>and the penetration
depth of evanescent tunneling. Despite this, it is not necessary
to shift the location of EM<010>to compensate the misalignment,
as the walk-off distance is very short, and the influence is quite
minimal.
The optimization is started by analyzing three simplified mod-
els to reveal the working principle. In this paper, the design
is implemented on a SOI platform with a core thickness of
Hwg =220 nm and a buffer thickness of Hbuff =3μm. The upper
cladding is a silicon-dioxide layer with a thickness of Hclad =1μm.
The central wavelength is set as 𝜆=1.55 μm.Therefractiveindex
data used in the simulation can be found in ref. [56]. The simula-
tion setup is detailed in Section S3 (Supporting Information). The
first model (i.e., Model A) is a TIR mirror formed by EM<100>and
EM<010>slabs, as illustrated in Figure 2a. Figure 2b shows the cal-
culated noand newith varying fSWG by using Equations (3) and (4).
The maximum index contrast is as high as no−ne≈0.76, which
is about five times better than that of calcite (no−ne≈0.15).
In Figure 2c, we show the calculated neff of TE and TM modes
supported by EM≤100≥and EM≤010≥slabs, with the index tensor
of slab cores modeled by Equations (1) and (2). Here, the slab
is assumed to be semi-infinite, since in the free-propagation re-
gion (see Figure 1b), the launched light behaves like a quasi-plane
wave and is confined only in the z-direction (see Section S3, Sup-
porting Information). The simulation is carried out by using the
1D finite-different frequency-domain (FDFD) method. To sim-
plify the calculation, the ordinary and extraordinary effective in-
dices are defined as follows (no
eff,TE ,n
e
eff ,TE,n
o
eff ,TM,n
e
eff,TM ):
no
eff ,TE =n<100>
eff ,TE 𝜃<100>
k,TE =90◦
=n<010>
eff ,TE 𝜃<010>
k,TE =0◦(7)
ne
eff ,TE =n<100>
eff ,TE 𝜃<100>
k,TE =0◦
=n<010>
eff ,TE 𝜃<010>
k,TE =90◦(8)
no
eff ,TM =n<100>
eff ,TM𝜃<100>
k,TM =90◦
=n<010>
eff,TM 𝜃<010>
k,TM =0◦(9)
ne
eff ,TM =n<100>
eff ,TM𝜃<100>
k,TM =0◦
=n<010>
eff ,TM𝜃<010>
k,TM =90◦(10)
where 𝜃<100>k,TE, 𝜃<010>k,TE, 𝜃<100>k,TM, and 𝜃<010>
k,TM denote the angles between the y-axis and wave vectors
(see Section S2, Supporting Information). Notably, noeff,TM
is slightly lower than ne eff,TM, indicating that the TM effec-
tive index is also dependent with 𝜃in. This unanticipated phe-
nomenon can be explained by the in-plane electric component
of the TM modes, as detailed in Section S4 (Supporting Informa-
tion). Nevertheless, TM-polarized light will not be strongly dis-
turbed by EM<010>since the index contrast between noeff,TM
and ne eff,TM is quite small. The fitting equations are utilized
to obtain TE and TM effective indices (n<100>eff,TE, n<010>
eff,TE, n<100>eff,TM, n<010>eff,TM) in EM<100>and EM<010>
slabs under oblique incidence:
n<100>
eff ,TE 𝜃<100>
k,TE
=no
eff ,TEpsin 𝜃<100>
k,TE 2
+ne
eff ,TEpcos 𝜃<100>
k,TE 21
p
(11)
n<010>
eff ,TE 𝜃<010>
k,TE
=no
eff ,TEpcos 𝜃<010>
k,TE 2
+ne
eff ,TEpsin 𝜃<010>
k,TE 21
p
(12)
n<100>
eff ,TM𝜃<100>
k,TM
=no
eff ,TMpsin 𝜃<100>
k,TM 2
+ne
eff ,TMpcos 𝜃<100>
k,TM 21
p
(13)
n<010>
eff ,TM𝜃<010>
k,TM
=no
eff ,TMpcos 𝜃<010>
k,TM 2
+ne
eff ,TMpsin 𝜃<010>
k,TM 21
p
(14)
where pis a fitting parameter to be determined. The index
ellipsoids described by Equations (11)–(14) are not in the stan-
dard form due to the fact that waveguide birefringence also
affects neff. The fitting parameter is optimized to be p=2by
cross-referencing with FDFD simulation results. More details
about this method can be found in Section S4 (Supporting
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Figure 2. Analysis of simplified models. a) Model A: a total-internal-reflection (TIR) mirror formed by effective-medium (EM<100>,EM
<010>) slabs.
Calculated b) medium indices (nSWG) of SWGs and c) effective indices (neff ) of slab modes with varying duty cycles (fSWG). d) Calculated critical TIR
angles (𝜃TIR,TE) with varying fSWG . e) Model B: a Glan-Thompson prism formed by EM<100>and EM<010>slabs. f) Calculated TE and TM transmittances
(TTE,TTM ) with varying incident angles (𝜃in). Calculated g) TTE and h) TTM with varying film thicknesses (Wfilm )and𝜃in. i) Model C: a silicon nano-stripe
sandwiched by EM<100>slabs. Calculated j) TTE and k) TTM for Model B and Model C with varying 𝜃in. l) Calculated TTE and TTM with varying Wfilm.
Information). For TE, the critical TIR angle (𝜃TIR,TE) can be then
derived from Equations (11) and (12) and Snell’s law:[50]
𝜃TIR,TE =arcsin
1
2
⋅
−ne
eff ,TE2
+ne
eff ,TE4⋅no
eff ,TE2
−3⋅ne
eff ,TE2
no
eff ,TE2
−ne
eff ,TE2(15)
In Figure 2d, we show the calculated 𝜃TIR,TE with varying
fSWG by using Equation (15). The duty cycle is optimized to be
fSWG =0.65 with a maximized TIR effect and a minimum critical
TIR angle of 𝜃TIR,TE ≈53°. This optimal value of fSWG will be used
in all of the subsequent simulations.
The second model (i.e., Model B) is a Glan–Thompson-like
prism formed by EM<100>and EM<010>slabs, as illustrated in
Figure 2e. Figure 2f shows the calculated TE and TM transmit-
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tances (TTE,TTM )withvarying𝜃in . The simulation is performed
by using the 3D finite-difference time-domain (FDTD) method.
By exploiting the unitary transform, the index tensor is diagonal-
ized to make it consistent with Yee’s grids under any arbitrary
𝜃in.[57] Here, the film thickness is chosen as Wfilm =270 nm, as
an example. For TE, the calculated TTE decreases as 𝜃in increases
due to the enhanced TIR effect and depressed tunneling depth.
For TM, the near-unity TTM is maintained even at a large 𝜃in,
which confirms the near-zero index contrast between n<100>
eff,TM and n<010>eff,TM. In Figure 2g, we show the calculated
TTE with varying Wfilm and 𝜃in. It can be found that TTE can
be efficiently reduced to zero by simply increasing Wfilm when
𝜃in >𝜃
TIR,TE. If the incident angle is smaller than the critical
value (e.g., 𝜃in =45°<𝜃
TIR,TE), TTE will converge to a non-zero
level even with a large Wfilm. In this case, the non-localized trans-
mission of light will induce thin-film interferences in EM<010>,
resulting in an undulating Wfilm−TTE curve. As a comparison,
we also calculate TTM with varying Wfilm and 𝜃in,asshownin
Figure 2h. From the calculation results, TTM fluctuates little
with Wfilm when 𝜃in is relatively small. However, TTM decreases
significantly at a large incident angle (e.g., 𝜃in =75°), which
also originates in the interference effect. Therefore, we must
appropriately choose 𝜃in and Wfilm to ensure low TTE and prevent
low TTM. The effective-medium equivalence is less strict in the
EM<010>region since SWG<010>is a stand-alone nano-stripe. A
more rigorous model (i.e., Model C) is introduced by replacing
EM≤010≥with an actual SWG structure, as illustrated in Fig-
ure 2i. In Figure 2j–k, we show the calculated TTE and TTM with
varying 𝜃in. Here, the stripe width and gap width are chosen as
S<010>=130 nm and G<010>=70 nm, respectively, as an example.
Compared to the calculation results based on Model B, TTE is
lower whereas TTM is higher at any arbitrary 𝜃in, which can be ex-
plained by the so-called “skin-depth” effect.[58] Low TTE >−23 dB
and high TTM >−0.25 dB can be attained at 𝜃in =75°. We then
calculate TTE and TTM with a fixed incident angle of 𝜃in =65°and
varying Wfilm, as shown in Figure 2l. Here, S<010>and G<010>are
simultaneously modified to ensure fSWG =0.65. The calculated
TTE is diminished to <−20 dB with Wfilm =250 nm, whereas the
TTM level is unaffected by the increasing of Wfilm.
Next, we optimize the broadband performance of the full-
structured PBS without any equivalence. The first step is to de-
termine the values of a few irrelevant structural parameters. The
input waveguide width is chosen as Wsm =400 nm to meet
the single-mode requirement. To alleviate divergence and col-
limate the incident light, the width of input/output sections is
set as Wio =1.5 μm. We choose the grating pitch of SWG<100>
as Λ<100>=200 nm to prevent Bragg reflection. Given the opti-
mal duty cycle of fSWG =0.65, the stripe width and gap width of
SWG<100>can be determined accordingly (i.e., S<100>=130 nm,
G<100>=70 nm). In the adiabatic taper, the SWG period number
is chosen as Ntp =20 to ensure the slow variation of the mode
size. A short connection waveguide with Nmm =4 is inserted be-
tween the free-propagation region and each adiabatic taper. To
lower the scattering loss, the width of the free-propagation region
is chosen as Wslab =2.4 μm, while its length (Λ<100>∙N<100>)can
be determined according to Wslab and 𝜃in. Now, there are three
underdetermined structural parameters: 𝜃in,G<010>and S<010>.
We calculate the insertion-loss and extinction-ratio spectra for
the full-structured PBS with varying 𝜃in,G<010>,andS<010>,as
shown in Figure 3. The target wavelength band is set as 𝜆=1.26–
1.675 μm (i.e., from O-band to U-band). Here, the insertion loss
and extinction ratio are defined as:
ILTE =−10log10TBAR,TE (16)
ILTM =−10log10TCRO,TM (17)
ERTE =10log10
TBAR,TE
TCRO,TE
(18)
ERTM =10log10
TCRO,TM
TBAR,TM
(19)
where ILTE and ILTM denote TE and TM insertion losses, ERTE
and ERTM denote TE and TM extinction ratios, TBAR,TE and
TBAR,TM denote TE and TM transmittances at the BAR port,
TCRO,TE and TCRO,TM denote TE and TM transmittances at the
CRO port. The initial settings for the stripe width and gap width
of SWG<010>are S<010>=100 nm and G<010>=100 nm. Here, we
let the duty cycle of SWG<010>differfromthatofSWG
<010>to pro-
vide additional degrees of freedom. In the meantime, the incident
angle is tuned from 𝜃in =40°to 75°, as shown in the first column
of Figure 3. The incident angle is chosen as 𝜃in =66°to ensure
ILTE ≈1dBandIL
TM ≈1 dB for the majority of the wavelength
range. To further reduce ILTE and ILTM , we slightly modify the gap
width as G<010>=80 nm, as shown in the second column of Fig-
ure 3. The stripe width is then optimized to be S<010>=115 nm to
achieve high extinction ratios (i.e., ERTE >20 dB, ERTM >20 dB)
throughout the whole bandwidth, as shown in the third column
of Figure 3.
Thus, all the necessary structural parameters have been judi-
ciously tailored. Figure 4a shows the light propagation profiles for
the optimized PBS when TE- and TM-polarized light is injected.
Three different wavelengths (𝜆=1.31, 1.55, and 1.675 μm) are
selected to assess the broadband performance. It can be found
that SWG<010>reflects TE-polarized light and steers it to the BAR
port. The launched TM-polarized light, on the other hand, prop-
agates through the crossing region and directly enters the CRO
port. A high level of polarization selectivity can be maintained
over an ultra-broad bandwidth. In Figure 4b,c, we show the cal-
culated transmittance spectra over the wavelength range from
1.26 to 1.675 μm. The calculated insertion losses are ILTE =0.8–
1.2 dB and ILTM =0.6–1.6 dB. At longer wavelengths, ILTE and
ILTM are somewhat higher due to the increased scattering loss of
guided modes. This issue can be resolved by adopting SOI with
a thicker core layer (e.g., HWG =300 nm). High extinction ra-
tios of ERTE =20.4–26.9 dB and ERTM =20.1–28.7 dB can be
realized over a 415-nm bandwidth. Under a lower criterion of
extinction ratios (e.g., ERTE >15 dB, ERTM >15 dB), the avail-
able optical bandwidth is calculated to be >550 nm, as shown in
Section S5 (Supporting Information). Further simulations show
that low insertion losses and high extinction ratios can still be
attained despite parameter deviations and inadequate gap filling.
The tolerance analysis can be found in Section S6 (Supporting In-
formation). In Section S7 (Supporting Information), we present
the calculated polarization cross-coupling ratio. The polarization
rotation is negligible for this design.
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Figure 3. Dispersion analysis of TE insertion losses (ILTE), TM insertion losses (ILTM ), TE extinction ratios (ERTE) and TM extinction ratios (ERTM)for
the full-structured PBS with varying incident angles (𝜃in), SWG<010>gap widths (G<010>)andSWG
<010>stripe widths (S<010>). The target wavelength
band is set as 𝜆=1.26–1.675 μm (i.e., from O-band to U-band). The white solid contours represent the target IL and ER levels. The yellow dashed lines
represent the optimal structural parameters.
3. Fabrication and Characterization
The fabrication was carried out at an open-access silicon pho-
tonic foundry.[59] The designed structures were patterned on a
commercial SOI wafer through electron beam lithography (EBL)
and reactive ion etching (RIE). The oxide upper cladding was
deposited by using plasma enhanced chemical vapor deposition
(PECVD). Figure 5a exhibits the scanning electron microscope
image of the fabricated PBS. To realize highly efficient single-
polarization fiber-chip coupling, TE- and TM-type grating cou-
plers (GC) were connected to each input/output port of PBSs.[60]
The coupling losses were obtained from the fabricated straight
waveguides terminated by two GCs at each end, allowing the nor-
malization of measured transmittance spectra. It should be noted
that the required measurement bandwidth (>415 nm) surpasses
the maximum operation bandwidth of GCs (<100 nm). The solu-
tion is to fabricate an array of identical PBSs each linked to a GC
operating at a specific wavelength band, as shown in Figure 5b,c.
Additionally, the inclination angle (𝜃fib) of probed single-mode
fibers was adjusted to expand the available bandwidth of GCs. In
this way, the entire bandwidth can be covered by properly choos-
ing input/output ports and modifying 𝜃fib. The detailed descrip-
tion about this method can be found in ref. [61]. For the accurate
measurement of ILTE and ILTM, several testing structures, which
consist of ten identical PBSs arranged in a chain, were also fab-
ricated on the same chip, as shown in Figure 5d.
Figure 5e,f shows the measured transmittance spectra with the
corresponding input/output ports and 𝜃fib labeled for each wave-
length band. For instance, to acquire the TTE,BAR spectrum from
𝜆=1.26 to 1.36 μm, one needs to choose I1as the input port
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Figure 4. Calculation results for the optimized PBS. a) Calculated light propagation profiles at three different wavelengths (𝜆=1.31, 1.55, and 1.65 μm)
when TE- and TM-polarized light is launched. Calculated transmittance spectra for b) TE and c) TM over the wavelength band from 𝜆=1.26 to 1.675 μm
(i.e., from O-band to U-band).
and O1(’) 1 as the output port with 𝜃fib =15°. Over the wave-
length band from 𝜆=1.26 to 1.36 μm, transmittance spectra
were measured by using an O-band tunable laser (Santec TSL510)
and a power meter (Agilent 8153A). The wavelength band from
𝜆=1.45 to 1.57 μm was covered by a second tunable laser
(Keysight 8164B). The remainder of the spectral responses was
characterized by using a broadband calibration source and an op-
tical spectrum analyzer (Yokogawa AQ6375). From the measured
spectra, high extinction ratios (ERTE >20 dB, ERTM >20 dB)
can be clearly observed over the wavelength band from 𝜆=1.26
to 1.675 μm. At the central wavelength, the extinction ratios are
measured to be ERTE ≈21 dB and ERTM ≈28 dB. The maxi-
mum extinction ratios (ERTE ≈28 dB, ERTM ≈30 dB) are ob-
tained at the O band. Figure 5g,h shows the measured ILTE and
ILTM spectra. Here, ILTE and ILTM were characterized by averag-
ing the transmittances acquired from cascaded PBSs working at
different wavelength bands. The noise is somewhat stronger at
the U-band, as the low output power from cascaded PBSs ap-
proaches the detection limit. After denoising, insertion losses are
measured to be ILTE ≈1.0–1.6 dB and ILTM ≈0.6–1.7 dB, which
is in good agreement with the simulation results. Several extra
PBSs with intentionally introduced parameter deviations were
also fabricated on the same chip (see Section S8, Supporting In-
formation). Under fabrication defects, the measured extinction
ratios are still as high as ERTE >17 dB and ERTM >19 dB.
4. Conclusion and Discussion
In this work, we have proposed and experimentally demonstrated
a meta-structured silicon nanophotonic PBS with insertion losses
of IL ≈0.6–1.7 dB and high extinction ratios of ER >20 dB over
a record broad bandwidth of BW20dB >415 nm. Under a relaxed
extinction-ratio criterion (e.g., ER >15 dB), the simulated band-
width can be expanded to cover an even wider wavelength range
(BW15dB >550 nm). In Table 1, the performance of several silicon
nanophotonic PBSs is summarized and compared. Here, only ex-
perimentally demonstrated PBSs with ER >20 dB are listed. We
also review the state-of-art integrated PBSs in terms of BW20dB
and device lengths to highlight the progress made in this work,
as shown in Figure 6. In ref. [44], the best reported bandwidth
to date is limited to BW20dB <240 nm with the device length
of 33.6 μm. It can be clearly seen that the demonstrated opti-
cal bandwidth (BW20dB >415 nm) is at least 175 nm better than
any prior effort without increasing the device length (≈15 μm).
The area occupied by the meta-prism (N<100>∙Λ<100>=5.8 μm) is
actually much smaller than the entire footprint; hence, it is an-
ticipated that the device length can be reduced even further by
optimizing the mode-size conversion efficiency of input/output
sections. Moreover, the extinction ratio can be further improved
by adding more nano-stripes into SWG≤010≥, as discussed in Sec-
tion S9 (Supporting Information).
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Figure 5. Experimental results. a) Scanning electron microscope image of the fabricated PBS. The scale bar represents 3 μm. Optical microscope images
of the fabricated PBSs connected to b) TE- and c) TM-type grating couplers (GC). The scale bars represent 70 μm. d) Optical microscope image of the
fabricated PBS chains for insertion-loss measurements. The scale bar represents 50 μm. In the measurement, the whole optical bandwidth was covered
by properly choosing input/output ports and modifying the tilt angle (𝜃fib) of probed fibers. Measured transmittance spectra for e) TE and f ) TM over
the wavelength band from 𝜆=1.26 to 1.675 μm. Measured g) TE and h) TM insertion-loss (ILTE,IL
TM) spectra. The simulation results for ILTE and ILTM
are also plotted for comparison.
Tabl e 1 . Performance comparison of silicon nanophotonic polarization beam splitters.
Structures Core thickness [nm] IL [dB] ER [dB] BW20dB [nm]a) Device length [μm]b)
MMI[19] 220 <2>20 77 71.5
DBR[21] 220 1–2 30 17 27.52
ADC[28] 220 0.3–0.6 >30 >90 13
ADC[29] 220 0.35 35 135 20
ADC[32] 250 1.8–2.1 22.5–22.9 <100 7.5
SWG[42] 220 <1>20 128 92.4
SWG[44] 340 0.1–1 >25 240 33.6
SWG[47] 220 NMc) >20 150 100
SWG[49] 250 <1>20 215 12.25
This work 220 0.6–1.7 20–30 >415 ≈15
a) BW20dB denotes the optical bandwidth under ER >20 dB; b) The length of leading sections is also involved in the device footprint; c) Not mentioned in the paper.
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Figure 6. Comparison of state-of-art integrated PBSs with a focus on
optical bandwidths (BW20dB) versus device lengths. Only experimentally
demonstrated PBSs with extinction ratios higher than 20 dB are summa-
rized here. The shaded regions represent the bandwidth and footprint lim-
its of previous works. The PBS demonstrated in this work offers a substan-
tial bandwidth improvement of over 175 nm, while the device footprint is
still quite small.
The confluence of optical-crystal concepts and anisotropic
metamaterials is what enables the outstanding performance.
Under the proposed design framework, polarization selectiv-
ity is offered by medium anisotropy as opposed to waveguide
birefringence, while polarization routing is achieved through
the TIR effect beyond the dispersion limit of evanescent cou-
pling or multimode interference. This strategy can be repro-
duced on any arbitrary platforms, such as thin-film III-V mate-
rials, lithium-niobate-on-insulator (LNOI) and silicon-nitride-on-
insulator (SNOI). There is a vast family of polarization-handling
instruments based on crystalline materials, such as Rochon
prisms, Wollaston prisms and wave plates,[50] that can also be
applied in photonic integrated circuits by following the method
presented in this work, thereby triggering a new class of sili-
con nanophotonic toolkits and reshaping the landscape of re-
search on polarization management. We believe that the pro-
posed PBS can be useful in the polarization management of
large-scale optical communication, high-performance metrology
and high-resolution OCT.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
This work was supported by ITF project MRP/066/020. The authors ac-
knowledge Applied Nanotools Inc. for the fabrication of devices. H.X. is
thankful to ITF Research Talent Hub for financial support.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available on request
from the corresponding author. The data are not publicly available due to
privacy or ethical restrictions.
Keywords
metamaterials, polarization, silicon photonics, subwavelength grating
Received: July 21, 2022
Revised: November 17, 2022
Published online:
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