电调超构表面研究进展(特邀)

Advances in Electrically Tunable Metasurfaces(Invited)

可调谐超构表面为纳米光子器件赋予了更多的自由度与灵活度,在过去十几年中得到了快速发展。在众多调谐机制中,电调超构表面更容易与半导体集成电路结合,是光电器件小型化发展的重要研究方向。本文综述了过去十几年来主要的电调超构表面设计路线。根据调谐机制将电调超构表面分为电控载流子激发调控、液晶调控、电光晶体调控以及微机电系统调控四种设计方案。总结了不同方案的物理机制、调谐方法、研究现状以及优缺点。最后,讨论了不同调谐方案的应用前景并展望了电调超构表面未来的发展趋势。

Metasurfaces have attracted extensive attentions due to their great flexibility in controlling the properties of electromagnetic wave in the past decade.Through engineering the geometry and configuration of building subwavelength structures,all the properties of electromagnetic wave,such as amplitude,phase,polarization,etc.,can be fully manipulated at will.However,the existing metasurfaces usually provide fixed functions after the structure has been fabricated,which is unfriendly to the trend of highintegraion and multifunctional nanophotonic devices.Therefore,tunable metasurfaces gradually become a new growth area.The optical response of metasurfaces highly relies on the resonant properties of individual subwavelength structures.In general,the optical resonance of each subwavlength structure is dependent on the geometry size,configuration and refractive index of either building material or the immemdiate environment,leading to several mechanisms to realize tunable metasurfaces,i.e.,actively tuning the geometry characteristics of structures or the refractive index of involved materials.At present,there are three design routes for tunable metasurfaces:1)changing the optical response of the structure by external excitation,such as electric/magnetic/optical excitation,chemical reaction and thermal excitation;2)using special active materials,such as liquid crystals,phase change materials and functional optical crystals;3)applying an external force to make the structure deformation,such as Microelectro-mechanical System(MEMS),flexible tensile materials,etc.These control schemes can make the metasurfaces show flexible dynamic response to incident light.Among various tuning mechanisms,the functionalities of electrically tuning metasurfaces grow as one of the most promising technical routes because it can be readily integrated with mature optoelectric devices and semiconductor manufacturing process.Based on different responses of some special materials to electric fields,the design scheme of electrically tunable metasurfaces can be classified into several groups.The refractive index of some active materials,such as Transparent Conducting Oxide(TCO),graphene,Transition Metal Dichalcogenides(TMDs)andⅢ-Ⅴcompound semiconductors,can be electrically tuned by controlling the carrier density.As a result,the amplitude and phase of metasurfaces can be effectively controlled.In particular,by combining with plasmonic resonances or other types of local resonances,the tuning range and rate of amplitude and phase can be further improved.Beam deflection,dynamic focusing,optical switch,etc.,have been demonstrated at a wide frequency range by selecting suitable materials.However,the thickness of active layer contributing to the tuning effect is usually very thin,which limits the tuning performance and increases the fabrication challenging.In contrast,tunable metasurfaces based on liquid crystals provide large refractive index range along with the advantages of low loss and low cost.By covering the dielectric subwavelength structures with liquid crystals,light propogation behaviors can be controlled with low external voltages.However,long response time and microscale molecule size are inapplicable to high speed and miniaturized optoelectric devices.In addition,it is still challenging to impove the damage threshold for high power applications.Electro-optic(EO)crystals,such as lithium niobate,have a excellent optical response to external voltage and have been widely used in commericialized optoelectric deivces.By integrating with subwavelength structures,such tuning capability can be further enhanced with the footprint of the related devices several orders of magnitude smaller.With the advent of new materials,such as PMN-PT,EO crystal based optoelectric devices will attract increasing interest.Desipite the refractive index tuning scheme,the geometry or structure configuration can also be manipulated by an external voltage.Various applications based on the combinatioin of MEMS and subwavelength structures have been demonstrated,such as varifocal lens,logical calculation,and Light Detection and Ranging(LiDAR)covering a broad frequency range,which will play a crucial role in photonic devices and nanophotonic chips.In this paper,the main design schemes of electrically tunable metasurfaces in recent years have been reviewed.According to the active materials,electrically tunable metasurfaces can be divided into four groups:electrically controlled carrier excitation,liquid crystal,electro-optic crystal and MEMS.The underlying mechanisms,the developing status,pro and con of various schemes are summarized.Finally,the application prospects of different tuning schemes are discussed and the development trend of this area is forecasted.As the development of design theory,material growth and fabrication technique,we believe that electrically tunable metasurfaces will proliferate rapidly and pave the avenue for miniaturized and integrated multifunctional optoelectric devices.