Research On High Efficiency Second-harmonic Micro-nano Functional Devices Based On Au-WS₂ Hybrid Metasurface

Metasurface is a rapidly growing field of research due to its excellent performance in light processing and versatility in ultra-thin optical applications.Compared with traditional optical components,metasurface devices realize wavefront modulation by accumulating phase changes along the direction perpendicular to the light propagation.Metasurface provides new degrees of freedom to manipulate phase and amplitude.It also has a polarization response with sub-wavelength resolution and the ability to shape the wavefront over a range much smaller than the wavelength.In the latest research,the concept of hybrid metasurface was proposed,which is a kind of metasurface design that combines the traditional metasurface with other structures.Based on the metasurface,the Au-WS₂ hybrid metasurface was designed and fabricated.We skillfully attached the WS₂ monolayer to the gold nanohole arrays,using the nonlinear susceptibility of the WS₂ monolayer itself to achieve efficient nonlinear wavefront control,such as the generation of second harmonic vortex beam,and the second harmonic hologram imaging and optimization.In the design methods,the Pancharatnam-Berry phase is adopted in this paper,which greatly simplifies the design difficulty of devices,facilitates multi-functional integration,makes full use of the high nonlinear susceptibility of two-dimensional materials,and provides a new way to realize more nonlinear control functions in the future.The main content of the paper is as follows:(1)A hybrid metasurface device based on Au-WS₂ monolayer was designed and fabricated.The gold nanohole array was designed by Pancharatnam-Berry phase and fabricated by focusing ion beam etching.The hybrid metasurface device was formed by combining the array with WS₂ monolayer by wet transfer.The optical properties of hybrid metasurface devices were investigated experimentally.The enhancement of second harmonic generation on hybrid metasurface was studied by experiments and simulation.(2)We propose and experimentally demonstrate the generation of an arbitrary second-harmonic vortex beam with quasi-nonlinear spin-orbit interaction.Here,high-quality second-harmonic vortex beams with large topological charges up to 28 are realized experimentally.The effective second-harmonic nonlinearity,?(2),was measured to be ²0 nm/V,which is 3 orders of magnitude larger than that of typical plasmonic metasurfaces.This indicated that the quasi-angular-momentum of a plasmonic spiral phase plate at the excitation wavelength(topological charge,)could be imprinted on the harmonic signals from the attached WS₂ monolayer.The generated harmonic vortex beam has a topological charge of l_n=2nq(n is the harmonic order).The results may open new avenues for generating harmonic optical vortices for optical communications and enables novel multifunctional hybrid metasurface devices to manipulate harmonic beams.(3)We propose and experimentally demonstrate chirality-selected second-harmonic holography based on Au-WS₂ nonlinear optical interface.Here,we used nonlinear geometry phase control to realize a sub-wavelength resolution.Furthermore,a binary amplitude manipulation is introduced by replacing specific rectangular nanoholes with square nanoholes.It indicates that the average intensity of holograms is increased by 39%and the holograms variation coefficient is decreased by 44% in comparison with that in pure geometry phase control.In addition,the SH signal from the monolayer WS₂ with a large second-order susceptibility is further enhanced by the strong local-field in Au nanoholes,leading to a high SH conversion efficiency of 10-6.Therefore,it offers an important stepping stone towards a multi-functional SH holography,which may have potential applications in nonlinear information encoding and processing.