基于GST纳米棒的相变超表面光路切换装置

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"Deflection angle switching with a metasurface based on phase-change nanorods [Invited]" 这篇研究论文提出了一种创新的光路切换设备，利用基于相变材料Ge2Sb2Te5（GST）的活动元表面。GST具有两种独特的相态：非晶态和结晶态。通过排列多个各向异性GST纳米棒，理论上和数值上展示了具有相反相位梯度的渐变元表面。在正常入射的圆偏振光波长为1650nm的情况下，这种设计能实现交叉极化的光线偏转角度切换。元表面是一种人造结构，能够操控电磁波，如光，通过调整其尺寸和形状来实现特定的光学特性。在本研究中，GST纳米棒阵列构建的元表面在不同相态下表现出不同的相位梯度，这使得在不改变设备厚度的情况下，可以改变光的传播路径。非晶态和结晶态的GST具有不同的光学性质，这使得通过调控材料的状态可以改变光的传播方向。当入射光为1650nm波长的圆偏振光时，元表面能够转换光的极化状态，导致光线偏离原来的传播方向。这种偏转是由于元表面在不同相态下的相位差异造成的，这使得光的反射或折射路径发生变化。这种现象对于光通信、光电子学和光学信息处理等领域具有重要的应用潜力，因为它们提供了一种动态、可编程的方式来控制光的路径，而不依赖于传统的机械或光学元件。此外，由于元表面的厚度远小于光波长，因此这种技术可能引领超薄、集成光学器件的发展。GST的相变特性也意味着这种开关可以是电控的，通过施加适当的电压来改变材料的相态，从而实现对光路的快速、高效控制。这种光路切换设备的潜在应用可能包括光学数据处理、光开关、光调制器以及自适应光学系统等。这篇论文展示了如何利用相变材料GST的特性，设计出一种新型的元表面，能够在两个相态之间切换，实现光路的动态控制。这项工作为开发高性能、低能耗的光学组件提供了新的思路和方法，有可能对未来的光学技术和信息技术产生深远影响。

Deflection angle switching with a metasurface based

on phase-change nanorods [Invited]

Chulsoo Choi (崔哲洙)

, Sun-Je Kim (金宣濟)

, Jeong-Geun Yun (尹政根)

Jangwoon Sung (成長雲)

, Seung-Yeol Lee (李承烈)

, and Byoungho Lee (李竝浩)

School of Electrical and Computer Engineering and Inter-University Semiconductor Research Center,

Seoul National University, Gwanak-Gu Gwanakro 1, Seoul 08826, South Korea

School of Electronics Engineering, Kyungpook National University, Daegu 41566, South Korea

*Corresponding author: byoungho@snu.ac.kr

Received January 28, 2018; accepted February 9, 2018; posted online April 20, 2018

We propose the active metasurface using phase-change material Ge

(GST), which has two distinct

phases so called amorphous and crystalline phases, for an ultrathin light path switching device. By arranging

multiple anisotropic GST nanorods, the gradient metasurface, which has opposite directions of phase gradients

at the two distinct phases of GST, is demonstrated theoretically and numerically. As a result, in the case of

normal incidence of circularly polarized light at the wavelength of 1650 nm, the cross-polarized light deflects

to −55.6° at the amorphous phase and þ55.6° at the crystalline phase with the signal-to-noise ratio above 10 dB.

OCIS codes: 160.3918, 220.1080, 250.6715.

doi: 10.3788/COL201816.050009.

Optical elements presenting active characteristics are es-

sential components that increase throughput of optical

data processing. Light path switching devices that ac-

tively modulate the direction of light propagation, espe-

cially, are in high demand for many applications, such

as optical communication

[1]

, optical data storage

[2]

, and

optical integrated circuit

[3]

. For a macroscopic optical sys-

tem, most of these devices are operated by mechanical ac-

tuation harnessing geometrical deformation of optical

elements

[4]

or spatial light modulation of flat optic compo-

nents through an external pump signal

[5]

. However, it is

quite difficult to miniaturize light path switching devices

without losing reliable performance, though there is an in-

creasing demand for microscopic optical devices.

Recently, metasurfaces have been intensively investi-

gated and used as substitutions for bulky optical elements

in the miniaturization of the optical system. A Metasur-

face, a flat optical device with sub-wavelength thickness, is

composed of periodically arranged nanoantennas to modu-

late the optical characteristics of an input light signal,

such as spectrum, polarization, and wavefront

[6–9]

. Incor-

porating optically active ingredients in a metasurface is

required for the application of an active optical system.

In various active devices, tuning by an external signal

has been realized by using non-linear materials

[10,11]

, such

as a building block of a metasurface, or by combining

functional materials like environmental interacting

materials

[12–14]

. Harnessing the phase-change material,

(GST) is the most effective among available

approaches, as their high-index contrast characteristics

and reversible switching property guarantee reliable

switching within nanoseconds

[15,16]

. Most notably, GST

is efficient in terms of power consumption by virtue of

its non-volatile phase-change characteristic—it maintain s

its transited phase even after the external stimuli is

ceased

[17–19]

. Among the studies concerning GST

metasurface

[20–22]

, there have been some demonstrations that

achieve compact light path switching by Yin et al.

[23]

and

Cao et al.

[24]

. However, these devices only guarantee small

switching angles of 23.6° and 11° for transmitted light with

an efficiency about 5%, respectively. Including the work

done by Yin et al.

[23]

, previous metasurfaces designed for

beam switching are usually demonstrated by simply adding

two different types of meta-atoms on a single layer. There-

fore, only the half of meta-atoms are dominantly reactive,

whereas other half of the meta-atoms are less reactive.

These less reactive meta-atoms that transmit light make

unavoidable noise that eventually degrade the signal-to-

noise ratio and restrict the range of the switching angle.

In this Letter, we propose an active metasurface based

on the array of GST nanorods operating at 1650 nm. In

the proposed structure, the complex amplitude of trans-

mitted light is determined by both upper and lower

meta-atoms, which means that there is no classification

of dominantly reactive or less-reactive meta-atoms. There-

fore, it can exhibit a very large switching angle of 111.2°

without unwanted noise excited by a less-reactive meta-

atom with a signal-to-no ise ratio above 10 dB, which is

numerically verified by using COMSOL multiphysics.

By adapting the gradient metasurface concept

[25–27]

, oppo-

site directions of the phase gradient for transmitted cross-

polarized light are designed for two distinct phases of GST

in case of circularly polarized light incidence.

The refractive indices of GST in the crystalline and

amorphous phases are n

¼ 5.6 þ 0.7i, and n

¼ 3.7, re-

spectively, which are referenced from the work by

Park et al.

[17]

. Two different sizes of GST nanorods are lo-

cated in the unit cell as shown in Fig.

1. When the period

of the unit cell (with unit length of P

and P

) is in the

sub-wavelength scale, the transmitted cross-polarized

COL 16(5), 050009(2018) CHINESE OPTICS LETTERS May 10, 2018

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