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RESEARCH ARTICLE
Flat Retroreflector Based on a Metasurface Doublet Enabling
Reliable and Angle-Tolerant Free-Space Optical Link
Hongliang Li, Woo-Bin Lee, Changyi Zhou, Duk-Yong Choi,* and Sang-Shin Lee*
DOI: 10.1002/adom.202100796
networks,[] augmented/mixed reality,[]
optical sensors,[] and laser tracking.[]
For free-space optical communications in
the case of wireless data transfer between
mobile (e.g., aircrafts, ships, and satel-
lites) and stationary terminals, both pas-
sive and active modulating retroreflectors
have been extensively investigated.[–]
The wavefront pertaining to geometrical
optical devices is mediated by the accu-
mulated phase shift over a relatively long
propagation distance.[,] Therefore, con-
ventional retroreflectors such as cat’s eye
and corner-cube types are particularly
subject to shortcomings in terms of their
bulky size, heavy weight, and nonplanar
shape.
Metasurfaces, incorporating specifically
engineered subwavelength nanostruc-
tures, have been prevalently leveraged
to tailor the phase, amplitude, polariza-
tion, and spectrum of light beams.[–]
Metasurface-based devices based on nano-
resonators allow for dispersion charac-
teristics that are dictated mainly by the
geometrical parameters and structural
arrangement, and they facilitate facile
wavefront manipulations unlike conven-
tional geometrical optics-based components.[–] Recently, flat
retroreflectors (FRRs) based on a singlet metasurface were pro-
posed, providing merits with respect to their miniaturization
and integration with other optic/electronic devices.[,] How-
ever, their limited angles of operation may constitute a concern
to be solved.[] To mitigate this, metasurface doublets (MDs),
which are geared to render successive beam manipulations,
have been actively studied.[–] For example, an MD-based
retroreflector that render wide-angle retroreflections at a wave-
length of nm was proposed.[] Compared with an -nm
wavelength, which is mostly used for imaging and biometrics,[]
a -nm wavelength in the C-band is unequivocally desirable
for long-distance optical communications from the perspective
of its minimal attenuation in optical fibers, good eye safety, and
compatibility with an erbium-doped fiber amplifier (EDFA).[–]
To the best of our knowledge, there has been no report on the
demonstration of a metasurface-based retroreflector for imple-
menting free-space optical communications.
In this paper, a reliable and highly angle tolerant optical link
capitalizing on an MD-based FRR is proposed and demonstrated
to operate in the telecommunications regime centered at λ=
nm. The proposed MD incorporates arrayed hydrogenated
Facilitated by the ability to reflect radiation along its incident direction,
retroreflectors have been perceived as a pivotal component for establishing
reliable free-space optical links. However, conventional retroreflectors suer
from limited integration because of their bulky size, heavy weight, and
nonplanar shape. Metasurface-based devices consisting of subwavelength
nanostructures combine semiconductor manufacturing methods with nano-
photonics, regarded as a new platform that outperforms geometrical optics.
In this paper, a free-space optical link exploiting a flat retroreflector (FRR)
based on metasurface doublet is proposed and realized at a telecommunica-
tions wavelength of 1550nm. The top- and bottom-layer metasurfaces, com-
prising hydrogenated amorphous silicon nanopillars based on a meticulously
tailored dielectric spacer of silica, achieve the functions of a transmissive
Fourier lens and a concave mirror, respectively. The top transmissive metas-
urface performs a spatial Fourier transform and its inverse, while the bottom
reflective metasurface imposes a spatially varying momentum for reflecting
beams along their incident direction. As a proof of concept, the designed
FRR, precisely created via lithographical nanofabrication, has been readily
applied to forge a substantially reliable free-space optical link, featuring an
enhanced angular tolerance of ±25°.This work will initiate a positive prospect
for the cooperation between metasurface-based devices and wireless optical
communications.
H. Li, W.-B. Lee, C. Zhou, S.-S. Lee
Department of Electronic Engineering
Kwangwoon University
Seoul 01897, South Korea
E-mail: slee@kw.ac.kr
H. Li, W.-B. Lee, C. Zhou, S.-S. Lee
Nano Device Application Center
Kwangwoon University
Seoul 01897, South Korea
D.-Y. Choi
Laser Physics Centre, Research School of Physics
Australian National University
Canberra, ACT 2601, Australia
E-mail: duk.choi@anu.edu.au
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adom.202100796.
1. Introduction
An optical retroreflector is used to accurately redirect incident
light back to its source and is widely anticipated to play a cru-
cial role in prominent applications, such as beyond G and G
Adv. Optical Mater. 2021, 2100796
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amorphous silicon (a-Si:H) nanopillars comprising a top trans-
missive metasurface, MS, which serves as a focusing lens,
vertically integrated with a bottom reflective metasurface, MS,
acting as a concave mirror. The designed FRR is manufactured
via electron beam lithography (EBL) with the assistance of a
dielectric spacer of a precisely controlled thickness, which is
determined by the focal length of MS. MS fulfills not only a
spatial Fourier transform but also the inverse transform, and
MS imposes a spatially varying momentum depending on
the Fourier transform of incident light. A wireless optical link
is practically established by tapping into the fabricated FRR
to provide a relaxed angular tolerance. This innovation was
particularly keenly inspected in terms of bit error rate (BER),
extinction ratio, and eye diagram. It is anticipated the proposed
MD-based FRR will play a pivotal role in enabling highly e-
cient free-space optical communications.
2. Proposed FRR and Its Design
As illustrated in Figure 1a, the proposed FRR incorporates a
pair of vertically stacked metasurfaces that mimic a cat’s eye
retroreflector in which a focusing lens and a concave mirror
are combined to reflect light back along the incident direc-
tion.[] MS, serving as a transmissive metalens having a focal
length of µm converges incident light on MS, which acts
as a reflective concave mirror. MS performs a spatial Fourier
transformation, projecting incoming light onto MS in accord-
ance with its spatial frequency components as determined by
the angle of incidence. MS is placed at the focal plane of MS
to impart double the tangential momentum of the incoming
wave to the reflected wave to achieve retroreflection.[] The
proposed FRR is designed to retroreflect input optical signals
operating at a telecommunications wavelength of nm.
MS and MS are created to exhibit dimensions of and
µm in diameter, respectively, ensuring that MS can safely
accommodate light emerging from MS. The two metasurfaces
comprising the FRR are based on a-Si:H nanopillars because of
their low cost, high refractive index, and compatibility with the
complementary metal-oxide-semiconductor process.[] Unlike
amorphous silicon which contains an abundance of dangling
bonds, in the case of a-Si:H, the hydrogen plays the role of
healing the bonds to eciently prevent the optical absorption
in the vicinity of λ= nm.[,] Besides, a-Si:H is passivated
through the hydrogen so that the occurrence of defects are sub-
stantially prohibited during the fabrication process.[] The MD
comprising an array of a-Si:H (n= .) nanopillars, which is
embedded in SU- (n= .), is created on either side of a silica
substrate. The silica substrate acts simultaneously as a dielec-
tric spacer to provide a refractive index of n= .. Its thickness
was meticulously determined to be µm, which is the same
as the focal length of both MS and MS. The MD, an integral
element for implementing the FRR, is fabricated through EBL
(see the Experimental Section and Section S, Supporting Infor-
mation). Figureb shows scanning electron microscope images
of the completed FRR alongside the nanopillar-based unit cells
of the MD.
Unit cells selected for concocting the MD are delineated
in Figure 2a,b. A group of a-Si:H nanopillars having dierent
diameters, d, are arranged in a square lattice. The period of the
lattice is p= nm in both x- and z-directions. The height of
the a-Si:H nanopillar is fixed at h= nm. The SU- layer
is -nm taller than the a-Si:H nanopillars, thus protecting
the metasurface. For the transmissive and reflective unit cells
comprising MS and MS, the calculated transmittance and
reflectance together with the corresponding phase shifts are
plotted in Figurec,d, respectively. A group of eight nanopillars
arranged along the x-axis is regarded as a supercell. The mag-
netic (H-) field distributions pertaining to the supercell of con-
cern are also revealed. Two sets of supercells for MS and MS
were individually chosen to impart an entire π phase shift,
giving rise to a high transmittance over % and a near-perfect
reflectance in response to a normally incident beam, respec-
tively. As in the case of resonance in a truncated waveguide-
like cavity, the optical fields are witnessed to be chiefly confined
to the nanopillars. Hence, the local phase shift is dominantly
governed by the nanopillar of concern but is hardly aected
by its neighboring nanopillars and the periodicity of the unit
cells. Owing to the circular symmetry of the nanopillars, the
MD is not susceptible to light polarization. The unit cells for
Adv. Optical Mater. 2021, 2100796
Figure 1. Proposed FRR based on an MD, which comprises vertically stacked metasurfaces including MS1 and MS2: a) MD-based FRR enabling free-
space beam retroreflection; b) top and perspective views of scanning electron micrographs of the fabricated FRR at dierent scales.
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MS and MS cover an entire π phase shift, rendering a high
transmittance and high reflectance even for obliquely incident
light, respectively (see Section S, Supporting Information).
The phase response, which is invariant to the incident angle,
indicates that the developed FRR can decently operate irrespec-
tive of the incident angle to a certain extent.
The desired hyperbolic phase distributions relating to MS
and MS, which can eectively mitigate spherical aberra-
tions,[] are described by xz nfx
zf
ψπ
λ
()
=−++(,)222 2,
where (x, z) represents the coordinates corresponding to the
center of each unit cell in the middle of the metasurface, f is the
focal length, and n is the refractive index of the dielectric spacer
between two metasurfaces. The geometry of the designed
MS and MS are sketched in Figure e,f on top of the cor-
responding achieved phase profiles. The numerical simulations
were executed to verify the operation of the proposed FRR.
Owing to limited computational resources, the central column
of the FRR, indicated by the blue dotted line, was taken into
account practically. The calculation results confirm that the pro-
posed FRR plays the role of reflecting the incoming light beam
back to its incident direction when the angle increases from
to ° (see the Experimental Section and Section S, Supporting
Information, for details). The maximum incident angle, which
is to be constrained by the eective footprint of the reflective
metalens for MS, is approximately °. For large incidence
angles, however, the performance of the FRR is predicted to
deteriorate on account of the shadow eect.[,] Nonetheless,
the allowable angle can be boosted by resorting to Fourier lens
scheme to facilitate angle-dispersion-free phase delay.[]
3. Experimental Results
The profiles of the retroreflected beams with the incident angle
were characterized using the experimental setup delineated
in Figure 3a. A light beam from a distributed feedback laser
(ALCATEL, ALMI) at λ= nm was collimated and
divided by a beam splitter (Thorlabs, BS). The transmitted
beam was focused onto the FRR by an objective lens (Thorlabs,
AC--C-ML) to render a beam diameter of ~µm. The
retroreflected beam passed through the lens and beam splitter
in the reverse direction, and was captured by a beam profiler
(CINOGY, CinCam CMOS-). As shown in Figureb, the
beam profiles were recorded for incident angles ranging from
θin= to ° in steps of °. Without moving the beam profiler,
the retroreflected beam could be detected with the help of the
unchanged light path under varying incident angles. The reflec-
tion angle θr was observed to be the same as the incident angle
as expected.
Considering that the object equipped with the retroreflector
redirects incoming light back to the transmitter, an optical link
between the former and latter could be established to identify
and track the location of the object in real time.[] As depicted
in Figurec, in order to characterize the optical link, a train of
optical pulses emanating from a light source (Fiberpia, LS-)
Adv. Optical Mater. 2021, 2100796
Figure 2. Proposed MD operating at λ= 1550nm and its design results: a) MS1 and b) MS2 unit cells made of a-Si:H nanopillars on a silica substrate;
calculated H-field distribution in conjunction with the phase and c) the transmittance and d) reflectance pertaining to the chosen supercells belonging
to MS1 and MS2, respectively; realized phase profiles for e) MS1 and f) MS2.
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was delivered to the fabricated FRR and routed with the aid of
a circulator (Thorlabs, --APC). The backward-propagating
optical signals travelled in free space over a distance of ~. m
to reach the photoreceiver (New Focus, -FC-M). Noting the
aluminum-coated area was larger than the footprint of MS,
retroreflection and specular reflection were primarily activated
depending on the position of light illumination inside and
outside of the metasurfaces, respectively. The reflected optical
pulses with and without involving the FRR led to “ON” and
“OFF” states for the optical link, respectively. As plotted in
Figured, the optical link was activated in the “ON” state for
normally and obliquely incident light via retroreflection trig-
gered by the FRR. In the absence of the FRR, however, the
optical link was deactivated to the “OFF” state for the case of
oblique incidence. The contrast between the two states, defined
as C= log PON/POFF, was measured to be ~dB, where P
and P stand for the detected peak-to-peak powers corre-
sponding to “ON” and “OFF” states, respectively.
Compared with the wavelengths of and nm, a
wavelength of nm, belonging to the C-band has been
extensively exploitedfor optical communications in view of its
salient features, including a minimal attenuation around .dB
km− in optical fibers, good eye safety, and compatibility with
an EDFA. Free-space optical data transmission mediated by the
FRR, which overcomes limitations in size, weight, and integra-
tion with other components, has been examined as delineated
in Figure S (see Section S, Supporting Information). Light
from a laser was appropriately polarized and modulated by an
electro-optic modulator (iXblue, MX-LN-), which is driven
by a bit error rate testing (BERT) scope (Agilent, NA)
and appended to a radio frequency amplifier (iXblue, DR-DG-
-MO). An optical signal, modulated according to the -
pseudorandom bit sequence data as generated by the scope, was
fed into the FRR via port of the circulator. Then, the retrore-
flected optical signal was retrieved via port in the circulator
and tethered to the BERT. The BER was monitored by varying
the data rate from to Gbps in intervals of Gbps, as shown
in Figure 4a. The BER for data rates ranging up to Gbps was
almost negligible, translating into an error-free transmission.
For data rates between and Gbps, the BER was judged to be
below an FEC threshold of . × −. When the BER is below
the FEC threshold, commonly adopted codes can be applied to
the data signals to dramatically enhance the BER, leading to
an eciency of .%, thereby enabling a variety of potential
practical applications.[–] As shown in Figureb, the BER was
invariant to the received powers ranging from to µW at
a rate of Gbps. For data rates running from to Gbps, the
BER improved with the enhanced signal power, as anticipated.
The FRR could allow for BERs well below the FEC threshold,
thereby establishing a reliable high-speed optical link.
A large angle of acceptance is primarily required to facili-
tate ecient free-space optical communications.[] As shown
in Figure 5a, the BER was satisfactorily low for a data rate
of Gbps. The incident angle varied from θin= to °. As
the data rate rose to Gbps, the BER increased slightly but
remained below the FEC threshold. The retroreflection e-
ciency dropped with the incident angle,[,] thus degrading
the BER. The extinction ratio, defined as the contrast between
Adv. Optical Mater. 2021, 2100796
Figure 3. Transfer characteristics of the fabricated FRR: a) setup for capturing retroreflected beams at various incident angles; b) observed retroreflected
beam profiles originating from the FRR; c) setup for assessing the optical link; d) data transmission via the established optical link for dierent angles
of incidence depending on the FRR.
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the statistically averaged - and -levels for the eye diagram,
has also been explored. A digital communication analyzer (Agi-
lent, DAC-J C) and signal integrity analyzer (Tektronix,
BERTScopes B) were employed to check on the optical
eye diagrams and extinction ratio. As displayed in Figure b,
the extinction ratio decreased from . to . dB when the
data rate increased from to Gbps for normal incidence.
Simultaneously, the prominent angular tolerance of the FRR
was underpinned by the fact that the extinction ratio remains
almost constant against the incident angle. Figurec reveals a
group of clear-eye diagrams in terms of the incident angle at
Gbps, signifying a highly reliable optical link. As a result, the
plausible angular tolerance of the proposed FRR was validated
in the context of insignificant variations in the BER and extinc-
tion ratio in response to the incident angle.
4. Conclusion
In summary, an MD-based FRR adapted for a free-space optical
link was proposed and experimentally demonstrated at the tel-
ecommunications wavelength of nm. The designed MD
incorporated MS and MS, which served as a Fourier focusing
lens and a concave mirror, respectively, and was precisely fabri-
cated via EBL on a silica dielectric spacer. The FRR, comprising
a-Si:H nanopillars, was successfully devised to return light
Adv. Optical Mater. 2021, 2100796
Figure 4. Test results of the proposed FRR for building a high-speed optical link: observed BERs as a function of a) the data rate and b) received signal
power.
Figure 5. Measurement results for a wide-angle optical link: a) BER and b) extinction ratio with the incident angle for data rates encompassing 1, 5, and
9Gbps; c) observed eye diagrams corresponding to the received retroreflected beam, with the incident angle varying between 0−25° at a rate of 9Gbps.
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Adv. Optical Mater. 2021, 2100796
back to its source, owing to MS carrying out a spatial Fourier
transform and its inverse while MS imparted a spatially var-
ying momentum to the Fourier transform of the incident light.
Moreover, the reliability and wide-angle tolerance of the pro-
posed FRR in high-speed optical links were demonstrated by
inspecting its BER, extinction ratio, and eye diagram. In view
of its conspicuous features, we believe that metasurfaces oer
conceptually new designs for free-space optical communica-
tions, bridging the gap between metasurface devices and future
integrated communication systems.
5. Experimental Section
Nanofabrication: The proposed MD-based FRR was produced by
successively patterning a pair of metasurfaces on both sides of a
428-µm-thick silica substrate, serving as a precisely controlled dielectric
spacer (see Figure S1, Supporting Information). The substrate was
cleaned using acetone/isopropyl alcohol/deionized water in advance to
promote its adhesion to the a-Si:H film. After using plasma-enhanced
chemical vapor deposition (Plasmalab 100 from Oxford) to deposit
two 1000-nm-thick films of a-Si:H on either side of the substrate, a
positive electron beam resist (ZEP520A from Zeon Chemicals) was
spin-coated on one side of the substrate. Espacer (300Z from Showa
Denko) was subsequently coated to prevent charging during electron-
beam exposure. Both MS1 and alignment marks were written on the
resist using EBL (Raith150) accompanied by development in ZED-N50.
An aluminum film of 60-nm thickness was deposited via electron-beam
evaporation (Temescal BJD-2000) on the substrate, and it was patterned
by lifting o the resist using a solvent (ZDMAC from Zeon Co.). The
patterned aluminum was used as a hard mask during dry etching,
thereby transferring the designed pattern to the underlying a-Si:H layer
through fluorine-based inductively coupled-plasma reactive ion etching
(Oxford Plasmalab System 100). The etching condition was optimized
by precisely mixing CHF3 and SF6 gas to establish nanopillars exhibiting
a high aspect ratio and vertical sidewall, as shown in Figure 1b. Next,
wet etching was carried out to remove the residual aluminum from the
patterned nanopillars. MS1 was covered with a layer of SU-8 to protect
the nanopillars against mechanical damage during the fabrication of
MS2 on the other side of the substrate. The sample was then flipped
over to create MS2 by referring to both alignment marks on either side
via a transmission optical microscope. MS2 was constructed by fulfilling
a series of similar processes, including spin coating, EBL, aluminum
deposition, lift-o, plasma etching, aluminum removal, and SU-8
coating. Finally, a 100-nm thick aluminum of MS2 was formed on SU-8
to serve as a reflective layer. Misalignment between MS1 and MS2 was
tightly controlled to improve the performance of the proposed FRR.
Measurement Procedure: The setup and procedure adopted to assess
the proposed FRR are depicted in Figure 3a,c. The FRR was precisely
positioned with the assistance of a shortwave infrared camera (AVAL
DATA, ABA-001IR). The angle of incidence for the input light beam was
tailored by rotating the FRR via a manually controlled rotation stage.
Numerical simulations: (i) The phase and amplitude of the selected
supercells, as shown in Figure 2b, were calculated with the aid of a
simulation tool, FDTD Solutions (Ansys/Lumerical, Canada), by
scanning the cross-sectional dimensions of a single a-Si:H nanopillar
formed on the silica spacer. For the unit cells constituting MS1,
periodic and perfectly matched layer boundary conditions were applied
along both the x- and y-axes and the z-axis, respectively. For the unit
cells constituting MS2, a metal boundary condition was imposed
instead of aluminum as a reflective layer to spare the computational
resources. (ii) To explore the operation of the proposed FRR, the
nanopillars lining in the radial direction (x-axis) for MS1 and MS2 were
supplanted with two sets of one-dimensional gratings (see Section S3,
Supporting Information). The MD exhibits a footprint of 600µm along
the x-axis. The width of diracting plane wave source is set at 100µm,
equivalent to the diameter of the incident beam as observed in the
experiment.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the National Research Foundation of
Korea (NRF) grant funded by the Ministry of Education (Grant No.
2018R1A6A1A03025242) and the Korean government (MSIT) (No.
2020R1A2C3007007) and has been conducted by a research grant of
Kwangwoon University in 2021. The work was partly performed at the
ACT node of the Australian National Fabrication Facility.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Keywords
flat retroreflectors, free-space optical links, metasurface doublets,
telecommunication wavelengths
Received: April 19, 2021
Revised: June 30, 2021
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