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Dual-layered metasurfaces for asymmetric

focusing

BINGSHUANG YAO,

XIAOFEI ZANG,

1,2,

*ZHEN LI,

LIN CHEN,

1,2

JINGYA XIE,

1,2

YIMING ZHU,

1,2,3

AND SONGLIN ZHUANG

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

Shanghai Institute of Intelligent Science and Technology, Tongji University, Shanghai 200092, China

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.

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

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:

LCP

x, y

2π



ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

 y

 f

− f



RCP

x, y−

2π



ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

 y

 f

− 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

Φx, yargfexpiα φ

LCP

x, y

 expiα  φ

RCP

x, yg, (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

 50 μm and l  160 μm [see Fig. 2(a

)], 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

) and (a

) schematic and the corresponding geometric phase of the microrod; (b

) and (b

) sche-

matic and the corresponding transmission spectra of the metallic gratings; (c

) and (c

) schematic and the corresponding transmission spectra of the

metasurface combined with metallic gratings; (d

) and (d

) 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

). Figure 2(b

) 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

), 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

) and 2(d

)]. 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

)–

3(f

). 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

) 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

)] 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

) and 3(b

) 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

) and 3(b

)].

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

) and 3(e

In comparison with Figs. 3(a

), 3(c

), and 3(e

), 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

) and 3(f

)].

Fig. 3. (a

)–(f

) 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

)–(f

) 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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