Light Field Modulation Based On Hybrid Nonlinear Metasurface

The realization of nonlinear optical processes,especially the frequency up-conversion process of optical second harmonic,third harmonic and other higher harmonic modes,open up a new path for the development of optics.It has good prospects in the fields of shortwave light sources,photonics and optical information.Benefiting from the advantages of ultra-thin,light and compact,optical metaurfaces have a good prospect in micro-optical devices.Due to the high degree of freedom of the structure,the metasurface exhibits many novel functions.Traditional nonlinear metasurfaces use dielectric or metal plasmons to generate and control nonlinear signals.While the hybrid metasurface realizes this function by combining multiple materials.In this article,we have proved and demonstrated that by combining metal plasmon structures and highly nonlinear materials,nonlinear optical fields can be generated and controlled efficiently and flexibly.It reveals the difference between hybrid metasurfaces and traditional metasurfaces in nonlinear optical field phase control.And through the cleverly designed phase plate,the unique phase control characteristics of the nonlinear hybrid metasurfaces are fully demonstrated.The main content of the paper is as follows:(1)we demonstrate that plasmonic metasurfaces can serve as a chiral valley-photon interface to steer the nonlinear excitonic emission from different valleys to the desired directions in free space at room temperature.The second harmonic generation with the nonlinear valley selective rule occurs exactly at the exciton resonance from K and K'valleys in monolayer tungsten disulfide.Thanks to the improved near-field confinement of light,second harmonic generation conversion efficiency is enhanced by 1 order of intensity.Besides,this metasurface,can imprint a spin-dependent gradient wavefront,known as Pancharatnam-Berry phase arising from the spin-orbit coupling,to the different chiral components of light.The light with chirality-dependent phase can,then,selectively pump monolayer WS₂ via the spin-valley locked coherent nonlinear process and steer the different valley excitonic emission in any directions as indicated by the evolutionary trajectory of light,analog to the photonic spin-Hall effect in a nonlinear version.(2)we experimentally report a recipe for full control of second harmonic photons via an integrated nonlinear optical interface composed of plasmonic nanosieves strongly coupled to transition metal dichalcogenides monolayer.Second harmonic generation,originating from the monolayer WS₂,is enhanced by the strong local fields within each Au nanoholes,and simultaneously the geometric phase determined by individual Au nanoholes can be locally engineered and imparted to the WS₂ monolayer on top of the Au nanosieves.Experimental results indicate that an effective second harmonic generation susceptibility of?25 nm/V is achieved,3 orders of magnitude larger than conventional plasmonic metasurfaces.Based on these principles,we demonstrate in the following several multifunctional nonlinear optical devices for free-space transmissive nonlinear optical interfaces for orbital angular momentum generation,beam steering,versatile polarization control and holograms.Further,the three-dimensional holographic imaging were discussed and preliminarily demonstrated.(3)we proposed and experimentally demonstrated the chiral nonlinear hybrid metasurface for third-harmonicwavefront modulation,which is composed of the Au nanohole array filled with ZnO film.The“S”-shaped pixel of Au/ZnO metasurface is able to precisely control the geometry phase of the emitted third harmonic signals,while showing strong chiral response to the left circular polarization and right circular polarization light excitation.It is demonstrated that the emitted third harmonic signal can be controlled through five different channels simultaneously.For both left circular polarization and right circular polarization light excitation,the left circular polarization and right circular polarization components of the third harmonic holographic images can be designed independently.Furthermore,the third harmonic signals from each pixel of Au/ZnO metasurface can be switched on or off,forming a specific third harmonic intensity pattern on the sample surface.In addition,and the third harmonic conversion efficiency is measured to be 10^-5,which is nearly one-order of magnitude higher than that of the ZnO film with the same thickness.