Tunable Metasurfaces Based on Active Materials

Abstract

Metasurfaces, planer artificial materials composed of subwavelength unit cells, have shown superior abilities to manipulate the wavefronts of electromagnetic waves. In the last few years, metasurfaces have been a burgeoning field of research, with a large variety of functional devices, including planar lenses, beam deflectors, polarization converters, and metaholograms, being demonstrated. Up to date, the majority of metasurfaces cannot be tuned postfabrication. Yet, the dynamic control of optical properties of metasurfaces is highly desirable for a plethora of applications including free space optical communications, holographic displays, and depth sensing. Recently, much effort has been made to exploit active materials, whose optical properties can be controlled under external stimuli, for the dynamic control of metasurfaces. The tunability enabled by active materials can be attributed to various mechanisms, including but not limited to thermo‐optic effects, free‐carrier effects, and phase transitions. This short review summarizes the recent progress on tunable metasurfaces based on various approaches and analyzes their respective advantages and challenges to be confronted with. A number of potential future directions are also discussed at the end.

A Dynamically Modulated All‐Dielectric Metasurface Doublet for Directional Harmonic Generation and Manipulation in Transmission

“…It is worth noting that a similar performance can be achieved for lower levels of carrier concentrations in the intrinsic region by rescaling the design for longer wavelengths with stronger electro‐optical effects at the cost of lower amplitude efficiency. The design can also potentially be implemented using phase change materials at infrared wavelengths whose larger refractive index variations relax the condition on the high Q factor of resonances allowing for lower dissipative losses, however, further efforts still need to be made in order to achieve a fast switching and modulation speed using these materials …”

“…This has motivated investigation of several tuning approaches for all‐dielectric metasurfaces, including nonlinear effects, mechanical reconfiguration, phase‐change materials, thermo‐optical effects, and carrier‐induced electro‐optical effects . Among these tuning mechanisms, electro‐optical effects based on modulation of free carriers are of particular interest as they allow for robust and continuous tuning of the scattering from individual elements with short response time and low power consumption without suffering from crosstalk . Integration of conducting oxides such as indium‐tin‐oxide (ITO) into dielectric metasurfaces only allows for sub‐linewidth resonance tuning via field‐effect carrier accumulation due to extremely small volume of active regions which yields a relatively weak interaction with large volumetric modes of dielectric resonators and hinders maintaining of the Huygens' condition across a wide tuning range .…”

Electrically Switchable Color Tags Based on Active Liquid‐Crystal Plasmonic Metasurface Platform

“…The LSPR wavelength is strongly dependent on the structural shape and size of the nanoparticle, the polarization of incoming light, and the surrounding dielectric medium. This was shown to provide a promising way to generate, as well as modulate, color in the visible region with high efficiency . Therefore, plasmonic metasurfaces are attractive for use in high‐resolution color displays, counterfeiting elements, and various imaging applications.…”

Active Anisotropic Coding Metasurface with Independent Real‐Time Reconfigurability for Dual Polarized Waves

“…To solve the problem of fixed wave-manipulations by passive metasurfaces, more efforts have been devoted to realize active control of EM behaviors of metasurfaces upon the external stimulations by incorporating with tunable materials or components, [30,31] such as diodes, [32][33][34][35] graphene, [36,37] phase-change materials, [38,39] etc. Recently, coding metasurfaces have been proposed to control the wave functionalities using unique particles with, for example, either the "0" or "1" state in 1-bit coding case.…”

“…[29] Despite these great advances achieved by anisotropic metasurfaces, most of them are composed of passive building blocks that cannot provide tunable or reconfigurable responses. Therefore, once fabricated, these passive metasurfaces only empower one or a few functions, hindering their further uses in many realworld applications.To solve the problem of fixed wave-manipulations by passive metasurfaces, more efforts have been devoted to realize active control of EM behaviors of metasurfaces upon the external stimulations by incorporating with tunable materials or components, [30,31] such as diodes, [32][33][34][35] graphene, [36,37] phase-change materials, [38,39] etc. Recently, coding metasurfaces have been proposed to control the wave functionalities using unique particles with, for example, either the "0" or "1" state in 1-bit coding case.…”