Optical beam splitting and asymmetric transmission
in bi-layer metagratings
Qiangshi Shi (施强石)
1,2
, Xia Jin (金 霞)
1
, Yangyang Fu (伏洋洋)
3
, Qiannan Wu (吴倩楠)
4
, Cheng Huang (黄 程)
1
,
Baoyin Sun (孙宝印)
2*
, Lei Gao (高 雷)
2
, and Yadong Xu (徐亚东)
2**
1
College of Energy, Soochow University, Suzhou 215006, China
2
School of Physical Science and Technology, Soochow University, Suzhou 215006, China
3
College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China
4
School of Science, North University of China, Taiyuan 030051, China
*Corresponding author: bysun@suda.edu.cn
**Corresponding author: ydxu@suda.edu.cn
Received August 26, 2020 | Accepted October 19, 2020 | Posted Online February 2, 2021
In this work, inspired by advances in twisted two-dimensional materials, we design and study a new type of optical bi-layer
metasurface system, which is based on subwavelength metal slit arrays with phase-gradient modulation, referred to as
metagratings (MGs). It is shown that due to the found reversed diffraction law, the interlayer interaction that can be simply
adjusted by the gap size can produce a transition from optical beam splitting to high-efficiency asymmetric transmission of
incident light from two opposite directions. Our results provide new physics and some advantages for designing subwave-
length optical devices to realize efficient wavefront manipulation and one-way propagation.
Keywords: asymmetric transmission; high efficiency; bi-layer metagratings; abrupt phase control.
DOI: 10.3788/COL202119.042602
1. Introduction
Optical phase-gradient metasurfaces (PGMs)
[1–4]
have attracted
much attention in the past few years due to their fundamental
interest and practical importance. Typically, PGMs are con-
structed as periodic gratings consisting of a supercell spatially
repeated along an interface, and each supercell consists of m unit
cells (i.e., meta-atoms), with m being an integer. The key idea of
PGMs is to introduce an abrupt phase shift covering the range of
0to2π discretely through m unit cells of different optical
responses to ensure complete control of the outgoing wavefront.
The phase gradient provides a new degree of freedom for the
manipulation of light propagation, allowing a number of in-
triguing optical phenomena or metadevices
[5–17]
, such as the
generalized Snell’s law (GSL)
[5]
, metalenses
[6]
, the photonic spin
Hall effect
[8]
, wavefront controlling
[10,13]
, and perfect anoma-
lous diffraction
[14–16]
.
Inspired by the concept of PGMs, recently, subwavelength
metal slit arrays with phase-gradient modulation, referred as
metagratings (MGs)
[18–25]
, have been used to effectively
manipulate wavefronts and realize new phenomena or effects
beyond those predicted by the GSL. It is found that anomalous
transmission and reflection through higher-order diffraction
can be completely reversed by changing the integer parity of
the MG design, and it obeys a new set of m-dependent diffrac-
tion equations
[21]
.
Alternatively, angularly asymmetric diffraction was observed
theoretically and experimentally in MGs
[22,23]
, stemming from
the loss-induced suppression of higher-order diffraction
[24]
.
Although great progress has been made with regard to single-
layer MGs, the study of the multilayer system composed of the
single-layer MG is still relatively rare. In recent years, in con-
densed matter physics, the interaction between layers of two-
dimensional (2D) materials
[26]
, such as graphene and MoS
2
,
has led to many unusual physical properties, such as topologic
transition
[27]
. Similarly, in the multilayer system composed of
the single-layer MG, it is highly desired to explore whether
the interlayer interactions will also bring some new phenomena
that cannot be observed in a single-layer system. Although many
efforts have been devoted to the study of few-layer metasurfa-
ces
[28]
, distinctive effect and applications triggered by the
new set of diffraction laws in MG systems require further
exploration.
2. Models and Theories
In this work, as a concrete example, we design and study a bi-
layer MG with a relatively simple structure operating at the
Vol. 19, No. 4 | April 2021
© 2021 Chinese Optics Letters 042602-1 Chinese Optics Letters 19(4), 042602 (2021)
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