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Dual-layered metasurfaces for asymmetric focusing
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Asymmetric transmission, defined as the difference between the forward and backward transmission, enables a plethora of applications for on-chip integration and telecommunications. However, the traditional method for asymmetric transmission is to control the propagation direction of the waves, hindering further applications. Metasurfaces, a kind of two-dimensional metamaterials, have shown an unprecedented ability to manipulate the propagation direction, phase, and polarization of electromagneti
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Dual-layered metasurfaces for asymmetric
focusing
BINGSHUANG YAO,
1
XIAOFEI ZANG,
1,2,
*ZHEN LI,
1
LIN CHEN,
1,2
JINGYA XIE,
1,2
YIMING ZHU,
1,2,3
AND SONGLIN ZHUANG
1
1
Terahertz Technology Innovation Research Institute, Terahertz Spectrum and Imaging Technology Cooperative Innovation Center,
Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, Shanghai 200093, China
2
Shanghai Institute of Intelligent Science and Technology, Tongji University, Shanghai 200092, China
3
e-mail: ymzhu@usst.edu.cn
*Corresponding author: xfzang@usst.edu.cn
Received 9 January 2020; revised 16 March 2020; accepted 17 March 2020; posted 20 March 2020 (Doc. ID 387672); published 6 May 2020
Asymmetric transmission, defined as the difference between the forward and backward transmission, enables a
plethora of applications for on-chip integration and telecommunications. However, the traditional method for
asymmetric transmission is to control the pro pagation direction of the waves, hindering further applications.
Metasurfaces, a kind of two-dimensional metamaterials, have shown an unprecedented ability to manipulate
the propagation direction, phase, and polarization of electromagnetic waves. Here we propose and experimentally
demonstrate a metasurface-based directional device consisting of a geometric metasurface with spatially rotated
microrods and metallic gratings, which can simultaneously control the phase, polarization, and propagation di-
rection of waves, resulting in asymmetric focusing in the terahertz region. These dual-layered metasurfaces for
asymmetric focusing can work in a wide bandwidth ranging from 0.6 to 1.1 THz. The flexible and robust ap-
proach for designing broadband asymmetric focusing may open a new avenue for compact devices with potential
applications in encryption, information processing, and communication.
© 2020 Chinese Laser Press
https://doi.org/10.1364/PRJ.387672
1. INTRODUCTION
Asymmetric transmission is defined as the difference in the total
transmission between the forward and backward directions, ex-
hibiting great potential applications such as directionally sen-
sitive beam splitting [1,2], multiplexing [3], communications
[4], and optical interconnection [5]. A typical method for
asymmetric transmission is to break the time-reversal symmetr y
using an external magnetic field [6], time-varying component
[7,8], or nonlinear materials [9], leading to a topological-
protected edge mode (nonreciprocal transmissi on). Although
this nonreciprocal edge mode is immune to backscattering,
it suffers from complicated materials, external biases, and bulk
configurations. Based on the conventional passive and linear
materials (chiral metamaterials [10–14], asymmetric metallic
gratings [15], traditional polarization devices [16], and pho-
tonic crystals [17]), the reciprocal asymmetric transmission
is realized by manipulating the propagation direction of electro-
magnetic (EM) waves.
Metasurfaces, two-dimensional counterparts of metamateri-
als, can tailor the propagation direction, phase, and polarization
of EM waves, providing a platform to manipulate asymmetric
transmission in multiple degrees of freedom, i.e., polarization
and phase modulation. Benefiting from the superior capability
in the local manipulation of the wavefront of EM waves, a
plethora of applications such as metalenses [18–26], wave
plates [27–30], vortex beam convertors [31–33 ], and holo-
grams [34–39] have been demonstrated. Nevertheless, devices
based on a single-layered metasurface show symmetric trans-
mission, i.e., identical transmission between the forward and
backward directions. Recently, multilayered plasmonic meta-
surfaces have been proposed to demonstrate the asymmetric
transmission. For example, Frese et al. [40] and Chen et al. [41]
designed dual/triple-layered plasmonic metasurfaces to realize
an asymmetric meta-hologram and directional Janus, respec-
tively. Sun et al. [42] reported asymmetric dual-frequency
meta-holograms based on a triple-layered transmissive metasur-
face. These metasurface-based asymmetric devices work in a
narrow band because of their resonant unit cells. In addition,
study on metasurface-based asymmetric focusing is limited.
Here we propose and experimentally demonstrate the broad-
band asymmetric focusing in the terahertz (THz) region trig-
gered by simultaneous manipulation of the propagation
direction, phase, and polarization of THz waves. This direc-
tional device for asymmetric focusing consists of a geometric
metasurface (with a nonresonant unit cell) and metallic gra-
tings. Unlike previously demonstrated resonant metasurfaces
830
Vol. 8, No. 6 / June 2020 / Photonics Research
Research Article
2327-9125/20/060830-14 Journal © 2020 Chinese Laser Press
with different structured unit cells, the geometric metasurface
consists of identical microrods, and the manipulation of phase,
polarization, and energy flux is realized by rotating the micro-
rods (to generate geometric phase). Our designed dual-layered
metasurfaces for asymmetric focusing can work in a wide band-
width ranging from 0.6 to 1.1 THz. The approach for design-
ing directional meta-devices may provide a robust platform in
developing future high-performance devices that can manipu-
late EM waves in multiple degrees of freedom.
2. DESIGN AND METHOD
Figure 1 shows the schematic of an asymmetric focusing device.
Upon the illumination of x-polarized THz waves in the forward
direction, a focal point with orthogonal polarization is observed
after the dual-layered metasurfaces. In contrast, for backward
incidence, the focal point is not generated, leading to asymmet-
ric focusing. This directional device consists of a geometric
metasurface with spatially rotated microrods and metallic
gratings, which act as a linear polarizer that can transmit
x-polarized THz waves and reflect y-polarized THz waves.
The geometric metasurface is designed to simultaneously con-
trol the phase (for focusing) and polarization (for polarization
rotation), generating a focal point with orthogonal polarization
relative to incident THz waves. Since a linearly polarized THz
beam consists of left-handed circularly polarized (LCP) and
right-handed circularly polarized (RCP) components with the
same amplitude, two opposite phase profiles [see Eq. (1)]
should be required to focus both the LCP and RCP components:
8
<
:
φ
LCP
x, y
2π
λ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x
2
y
2
f
2
p
− f
φ
RCP
x, y−
2π
λ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x
2
y
2
f
2
p
− f
, (1)
where f is the focal length and λ is wavelength. Therefore, the
focused LCP and RCP components [induced by phase modu-
lations in Eq. (1)] with the same amplitude can be combined into
a linearly polarized focal spot. Another bulk phase α
that can be generated by rotating each microrod with an angle
of α∕2 should be introduced into the geometric met asurface to
rotate the polarization orientation of the focal spot. Therefore,
the desired phase profile of the geometric metasurface is
governed by
Φx, yargfexpiα φ
LCP
x, y
expiα φ
RCP
x, yg, (2)
where α 90° is the polarization rotation angle of the
focal spot.
To manipulate the incident THz waves based on geometric
phase, each microrod should be designed as a quasi-perfect
half-wave plate. The optimized structure parameters of a gold
microrod are as follows: the width and length of the rods are
w
1
50 μm and l 160 μm [see Fig. 2(a
1
)], and the period
is 170 μm. Under the illumination of circularly polarized THz
waves, the transmitted THz waves will contain two parts: one is
the co-polarized THz waves, and the other is the cross-polarized
THz waves. When the microrod is rotated counterclockwise/
clockwise with an angle θ, the cross-polarized THz waves will
Fig. 1. Schematic of asymmetric focusing. Under the illumination
of x-polarized THz waves in the forward direction, a y-polarized focal
spot is observed, while the focal spot is not generated for backward
x-polarized incidence.
Fig. 2. Design of dual-layered metasurfaces: (a
1
) and (a
2
) schematic and the corresponding geometric phase of the microrod; (b
1
) and (b
2
) sche-
matic and the corresponding transmission spectra of the metallic gratings; (c
1
) and (c
2
) schematic and the corresponding transmission spectra of the
metasurface combined with metallic gratings; (d
1
) and (d
2
) optical images of the metasurface and metallic gratings.
Research Article
Vol. 8, No. 6 / June 2020 / Photonics Research 831
be added with an additional phase delay of ≈ 2θ as shown in
Fig. 2(a
2
). Figure 2(b
1
) shows the schematic of metallic gratings
where the long side of each grating is along the y axis. The
width and period of these metallic gratings are 20 μm and
40 μm, respectively. As shown in Fig. 2(b
2
), the transmission
for the incident x-polarized THz waves is much higher than
that of the incident y-polarized THz waves. By introducing
a geometric metasurface (with gold microrods) and metallic
(gold) gratings [on the other side of the polyimide film (with
permittivity of ε 3.5 0.035i)], asymmetric transmission is
realized [see Figs. 2(c
1
) and 2(d
2
)]. For forward x-polarized in-
cidence, the transmission is more than 82%, ranging from 0.6
to 1.1 THz, while it is lower than 52% under the the illumi-
nation of x-polarized THz waves from the backward direction,
demonstrating an asymmetric transmission that can work in a
broad bandwidth.
3. RESULTS
Figure 3 shows numerical simulations of asymmetric focusing.
The size of this final metasurface composed of 80 × 80 micro-
rods is 1.36 cm × 1.36 cm. Based on the finite-difference
time-domain (FDTD) method, the electric field distributions
of asymmetric focusing are calculated and shown in Figs. 3(a
1
)–
3(f
2
). For simplicity, we calculated (and measured) electric field
distributions at 0.6, 0.85, and 1.1 THz to show the broadband
characteristic of our directional devices. Figure 3(a
1
) shows the
electric field distribution of x-polarized transmission for for-
ward THz waves at f 0.6THz. The polarization of one fo-
cal spot observed at z 2.0mmis along the y axis, which is
orthogonal to the polarization of the incident THz waves, while
the focal spot [see Fig. 3(b
1
)] is not generated for backward
incidence, leading to asymmetric focusing. Here the mecha-
nism of the asymmetric focusing can be understood as follows:
for forward incidence, the x-polarized THz waves are almost
completely transmitted through the metallic gratings and si-
multaneously interact with the metasurface (microrods with
different orientations). Therefore, the transmitted x-polarized
THz waves are partly converted into y-polarized THz waves
and simultaneously focused into a focal spot. In contrast, for
backward incidence, a portion of the waves interact with the
metasurface and are converted into y-polarized THz waves.
Then the converted waves are reflected completely after inter-
acting with metallic gratings. The other part of nonconverted
(x-polarized) THz waves can pass through metallic gratings.
However, the nonconverted THz waves are not modulated by
the metasurface and thus cannot be focused into a focal spot for
backward waves. The detailed mechanism of asymmetric focus-
ing under the illumination of x-polarized (and y-polarized) THz
waves in the forward and backward directions is given in
Appendix A. Figures 3(a
2
) and 3(b
2
) show electric field distribu-
tions in the x − y plane (z 2.0mm). For forward transmis-
sion, a circularly shaped focal spot is observed at the real focal
plane (z 2.0mm), while it cannot be generated (at z
−2.0mm) for the backward case [see Figs. 3(a
2
) and 3(b
2
)].
When f 0.85 THz (1.1 THz), a focal point located at z
3.5mm(z 5.5mm) with polarization along the y axis is ob-
served for forward incidence as shown in Figs. 3(c
1
) and 3(e
1
).
In comparison with Figs. 3(a
1
), 3(c
1
), and 3(e
1
), the distance be-
tween the focal spot and the device is getting longer and longer
due to the chromatic aberration. The focal spot cannot be gen-
erated for backward incidence [see Figs. 3(d
1
) and 3(f
1
)].
Fig. 3. (a
1
)–(f
1
) Numerical simulation of electric field distributions in the x − z plane under the illumination of x-polarized THz waves in the
forward/backward direction at 0.6, 0.85, and 1.1 THz; (a
2
)–(f
2
) the corresponding electric field distributions in the x − y plane.
832 Vol. 8, No. 6 / June 2020 / Photonics Research
Research Article
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