Doublet achromatic metalens for broadband optical
retroreflector
Ming Deng (邓 明), Tangxuan Ren (任唐轩), Jian Wang (王 健), and Lin Chen (陈 林)
*
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
*Corresponding author: chen.lin@mail.hust.edu.cn
Received July 24, 2020 | Accepted September 11, 2020 | Posted Online January 11, 2021
A retroreflector that reflects light along its incident direction has found numerous applications in photonics, but the avail-
able metasurface schemes suffer from the issue of narrow bandwidth and/or a single angle of incidence. Here, a retro-
reflector using double layers of achromatic gradient metasurfaces is reported, which can realize retroreflection over a
continuous range of incidence angles within a wide spectral band. The first metasurface serves as a transmissive ach-
romatic lens that performs a broadband spatial Fourier transform and its inverse, while the second metasurface works
as a reflective achromatic lens that undergoes wavelength- and position-dependent phase dispersions. Using this design
strategy, a near-infrared retroreflector comprised of silicon nanopillars with the cross sections of square pillars and square
holes is numerically demonstrated, providing a high-performance retroreflection for polarization-independent incident light
waves over a continuous range of incidence angles from 0° to 16° within an extremely broad wavelength range between 1.35
and 1.95 μm. The scheme herein can offer a design strategy of broadband retroreflectors and impact numerous photonics
applications.
Keywords: metasurfaces; retroreflectors; broadband; polarization independence.
DOI: 10.3788/COL202119.023601
1. Introduction
Metasurfaces, artifically designed ultrathin two-dimensional
(2D) materials composed of subwavelength resonators, exhibit
extraordinary electromagenetic (EM) properties that are unat-
tainable with natural materials. Metasurfaces have shown great
potentials to tailor the wavefront of EM waves, which has led to
various intriguing effects, such as light bending
[1–4]
, unidirec-
tional surface plasmon coupling
[5]
, invisibility cloaks
[6]
, flat
lenses
[7–9]
, holography
[10–12]
, and generation of vortex
beams
[1,13–15]
. Optical retroreflectors capable of reflecting the
incoming light wave along its incident direction are very impor-
tant components in communications
[16,17]
, remote sensing
[18]
,
and laser tracking
[19]
. Conventional retroreflectors rely on the
utilization of macroscopic geometrical structures, such as a cor-
ner-cube retroreflector
[20]
and Eaton lens
[21,22]
, are bulky and
nonplanar and, hence, inconveniently compatible for intergra-
tion and miniaturization. Metasurfaces have recently shown
great promise for building flat ultrathin retroreflectors that dem-
onstrate its superiority over conventional schemes in terms of
compactness and intergration.
Initially, plasmonic metasurfaces/metagratings have been
used to realize retroreflection, but the resultant experiment
exhibits limited working bandwidth and/or a single incidence
angle since the provided tangential momenta are fixed after
the fabrication process
[23–26]
. To address the issue of limited
working incidence angle, reconfigurable plasmonic metasurfa-
ces/metagratings have been proposed and demonstrated, which
realizes spin-polarized retroreflection with a discrete/continu-
ous angle of incidence at a single frequency
[27,28]
. Analogous
to conventional cat’s eye retroreflectors
[19,29]
, it has been pro-
posed to employ cascaded gradient metasurfaces
[30]
. This
scheme has shown the capability of retroreflecting light over a
continuous angle at a single frequency. However, all of the afore-
mentioned schemes undergo narrow bandwidth, which limits
practical applications, as the device bandwidth is crucial for
an optical system. To date, realizing an optical metasurface ret-
roreflector with a broad operation bandwidth, continuous inci-
dence angles, and polarization independence remains largely
unexplored. Here, a broadband retroreflector with a continuous
range of incidence angles is proposed by the use of two layers
of achromatic gradient metasurfaces. The first metasurface
serves as a transmissive achromatic lens that performs a
broadband spatial Fourier transform and its inverse, while the
second metasurface works as a reflective achromatic lens
that undergoes wavelength- and position-dependent phase
dispersions. We numerically demonstrate a near-infrared retro-
reflector that provides a high-performance retroreflection for
Vol. 19, No. 2 | February 2021
© 2021 Chinese Optics Letters 023601-1 Chinese Optics Letters 19(2), 023601 (2021)