# Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms

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## Abstract

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## 1. Introduction

## 2. Multifunctional Meta-Devices Based on the “Merging” Concept

## 3. Multifunctional Metasurfaces Based on Anisotropic Single-Structure Meta-Atoms for Two Polarizations

_{xx}, r

_{yy}, t

_{xx}, t

_{yy}denote the reflection/transmission coefficients of the meat-atom (periodically repeated to form a periodic metasurface), respectively. In order to make the devices exhibit working efficiencies as high as possible, ideally one requires the designed meta-atoms to be either perfectly reflective:

#### 3.1. Multifunctional Metasurfaces Exhibiting Similar Functionalities

#### 3.2. Multifunctional Reflective Metasurfaces Combining Distinct Functionalities

#### 3.3. Multifunctional Transmissive Metasurfaces Combining Distinct Functionalities

_{xx}and $\phi $

_{yy}) satisfying Equations (5) and (6). Experimental results shown in Figure 6e,f indicate that the device can focus y-polarized PW to a point and can refract x-polarized PW anomalously, illustrating the desired bi-functionality. The achieved working efficiency is 72%, much higher than those of other transmissive metasurfaces [25,26,27,49].

#### 3.4. Multifunctional Metasurfaces for Full-Space Manipulation of EM Waves

## 4. Conclusions and Discussion

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 1.**Multifunctional devices designed with merged structures. (

**a**) Design strategy, sample picture, and experimental characterizations of a multifunctional metasurface than can generate holographic images or a vortex beam depending on the helicity of incident circularly polarized light. Reproduced from Ref. [74] with permission. (

**b**) A metasurface that can reconstruct different images to different polarization channels. Reproduced from [64] with permission. (

**c**) A metasurface that can generate multiple hologram images as shined by circularly polarized light with different helicity. Reproduced from [63] with permission. (

**d**) A metasurface that can generate optical vortices with distinct topological charges at different longitudinal focal planes. Reproduced from [58] with permission. LCP, left CP; RCP, right CP; OAM, orbital angular momentum; LHCP, left-handed circularly polarized ; RHCP, right-handed circularly polarized.

**Figure 2.**Schematics of a bi-functional metasurface (blue) with metal ground plane (yellow) which can achieve (

**a**) focusing functionality for y-polarized incident light and (

**b**) anomalous-reflection functionality for x-polarized incident light. The phase distributions of the metasurface are depicted in (

**c**) for y-polarization excitation and in (

**d**) for x-polarization excitation. Reproduced from [90] with permission.

**Figure 3.**(

**a**) Schematics of an MIM meta-atom consisting of a metallic patch resonator and a metal ground plane (yellow) separated by a dielectric spacer (blue). (

**b**) Tuning the reflection phase ${\phi}_{xx}$ of a microwave MIM meta-atom by varying a and b. Reproduced from [93] with permission. (

**c**) A polarization beam splitter made by bifunctional gradient metasurface constructed with MIM meta-atoms consisting of metal-cross planar resonators and metal ground plane (yellow) separated by a dielectric spacer (blue). Reproduced from [79] with permission. (

**d**) Experimental characterizations on a unidirectional polarization-controlled SPP (red) coupler at telecom wavelength. Reproduced from [72] with permission. (

**e**) A bi-functional meta-hologram device consisting of metal pattern and metal ground plane (yellow) separated by a dielectric spacer (blue) that can generate different hologram image depending on the incident linear polarization. Reproduced from [59] with permission.

**Figure 4.**A reflective bifunctional metasurfaces that behaves as (

**a**) a focusing lens and (

**b**) a PW-to-SW (propagating wave to surface wave) convertor when excited by incident waves with polarizations E//y (red light) and E//x (blue light), respectively. (

**c**) Measured $\mathrm{Re}[\overrightarrow{E}]$ distributions on both xoz and yoz planes as the metasurface is illuminated by a normally incident y-polarized plane wave. (

**d**) Measured $\mathrm{Re}[\overrightarrow{E}]$ pattern on the xy-plane using a monopole antenna placed vertically and 8 mm above the metasurface and the mushroom structure, when the metasurface is illuminated by a normally incident x-polarized plane wave. Reproduced from [93] with permission. (

**e**) The working principle of similar type of bi-functional metasurface operating for visible light realized in [94]. The top panel shows the schematic of the unit cell consisting of an Ag nanobrick (grey) on top of a spacer (green) and Ag substrate. Reproduced from [94] with permission. SPPs, surface plasmon polaritons.

**Figure 5.**(

**a**) Topology of the dual-layer anisotropic meta-atoms using composite cross bar and cross loop. The meta-atom contains two identical composite metallic resonators and a continuous metal plate (yellow) separated by two dielectric spacers (blue). (

**b**) Phase distribution required on the metasurface to achieve quad-beam emissions. Schematics of a bifunctional metasurface which behaves as (

**c**) a lens or (

**d**) a beam splitter to generate quad large-angle pencil beams when excited by incident waves with polarizations E//x (yellow light) and E//y (blue light), respectively. (

**e**,

**f**) Simulated (red lines) and measured (blue dashed lines) radiation patterns on x-z plane for x polarization and y polarization, demonstrating the bi-functionality possessed by the fabricated device. Reproduced from [95] with permission.

**Figure 6.**(

**a**) Photograph of a microwave transmissive bifunctional metasurface. Inset illustrates a typical meta-atom composed by four metallic layers (yellow) separated by three F4B spacers (blue). (

**b**) Transmission amplitude (blue lines) and phase (red lines) for a periodic metasurface constructed by the 4-layer meta-atom, under the excitations of y-polarized (solid lines) and x-polarized (dotted lines) incident waves, respectively. Schematics and working principles of a transmissive bifunctional metasurfaces, which behaves as (

**c**) a focusing lens and (

**d**) a beam deflector when excited by incident waves with polarizations E//y and E//x, respectively. (

**e**) Measured $\mathrm{Re}[\overrightarrow{E}]$ distributions on both xoz and yoz planes as the metasurface is illuminated by a normally-incident y-polarized plane wave. (

**f**) Measured scattered wave intensity as function of frequency and detection angle when the metasurface is illuminated by x-polarized normally incident plane waves. Reproduced from [93] with permission.

**Figure 7.**(

**a**) Schematics of a 4-layer meta-atom composed by four metallic layers separated by three spacers. Measured and Finite-Difference Time-Domain (FDTD) simulated amplitude-phase spectra of reflection (

**b**) and transmission (

**c**) for a periodic metasurface made by the meta-atom given in (

**a**), under excitations with different polarizations. Schematics of a bi-functional metasurface which behaves as (

**d**) a reflective beam deflector and (

**e**) a transmissive lens under excitations of x- and y-polarized waves, respectively. Measured scattered field intensity versus frequency and detecting angle at reflection sides (

**f**) of the metasurface shined by x-polarized microwaves and electric field distributions on both xoz and yoz planes at transmission sides (

**g**) of the metasurface shined by y-polarized microwaves. Reproduced from [97] with permission.

**Figure 8.**(

**a**) Topology of a meta-atom consisting of three metallic layers (yellow) separated by two diecltric spacers (gray). (

**b**,

**c**) Amplitude/phase (Blue circle/red star) responses of the meta-atom rotated with certain angles when illuminated by normally incident (

**b**) RCP and (

**c**) LCP waves. Schematics of a bifunctional metasurface which behaves as a transmissive focusing lens (

**d**) and a reflective diverging lens (

**e**) under excitations of LCP and RCP waves, respectively. (

**f**) Measured $|{E}_{x}{|}^{2}$ distributions on both xoz and yoz planes at the transmission side when the meta-device is illuminated by a normally incident LCP wave. (

**g**) Measured $\mathrm{Re}({E}_{x})$ distributions on the xoz plane at the reflection side of the metasurface under excitation of a normally-incident RCP wave. Reproduced from [89] with permission.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Tang, S.; Cai, T.; Xu, H.-X.; He, Q.; Sun, S.; Zhou, L.
Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms. *Appl. Sci.* **2018**, *8*, 555.
https://doi.org/10.3390/app8040555

**AMA Style**

Tang S, Cai T, Xu H-X, He Q, Sun S, Zhou L.
Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms. *Applied Sciences*. 2018; 8(4):555.
https://doi.org/10.3390/app8040555

**Chicago/Turabian Style**

Tang, Shiwei, Tong Cai, He-Xiu Xu, Qiong He, Shulin Sun, and Lei Zhou.
2018. "Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms" *Applied Sciences* 8, no. 4: 555.
https://doi.org/10.3390/app8040555