Modulation Of Valley Excitons In Monolayer MoS₂ By Metasurface

In addition to charge and spin of electrons,the valley degree of freedom in solids can also serve as information carrier,which initiates the research of“Valleytronics”.Utilizing valley freedom to do information processing gives the advantages of low power consumption,high speed,a high degree of integration,and less information loss.Recent studies shows that two-dimensional transition metal dichalcogenides(TMDs)are an ideal class of valleytronic materials,which have the intrinsic valley degree of freedom and can be modulated with electric or optical methods.The key step to implement electron devices is to separate the valley electrons or valley excitons in TMDs.It has be demonstrated that valley electrons can be separated by Valley Hall Effect,however,this approach needs to be realized under a low temperature environment.Recent studies have demonstrated that valley excitons can be separated at room temperature through the coupling of valley excitons and surface plasmon polaritons(SPP)by utilizing the directional propagation of SPP in a metasurface.Nevertheless,this method is constrained by low coupling efficiency,low separation efficiency,complex metasurface structure,etc.This thesis aims to simplifies the metasurface structure and optimizes the separation efficiency through the finite-difference time-domain algorithm(FDTD)theoretical simulation.The main research contents are as follows:(1)Separation of valley excitons from symmetric metal grating structures metasurface.Previous studies have shown that it is infeasible to separate valley excitons in the symmetric structures.In this work,we proposed a method to break the plane's symmetry by changing the excitation position of the light source;as a result,the directional propagation of SPP and valley excitons is realized in the symmetric structures.Additionally,we analyzed the influences of groove width,groove depth,and excitation light incidence angle on the valley exciton separation and optimized the structural parameters.(2)Separation of valley excitons from asymmetric metal grating structures metasurface.We also simulated complex asymmetric structures and noticed that and the one-way propagation of SPP is independent of the light source's location.Meanwhile,the grating groove width,groove depth,and other parameters are optimized.Performance comparison of symmetric and asymmetric structures.The comparison between these two structures indicates that the symmetric structures not only have a simple configuration but also exhibit better performance.After 3?m propagation in symmetrical and asymmetrical structure,the valley polarization rate was 51%and 30%,respectively.In symmetric structures,the coupling efficiency of exciton-SPP-exciton is 24%for monolayer MoS₂,while 20%in asymmetrical structures.In addition,the symmetric structure has higher separation efficiency within the wavelength of 600-800 nm,which is suitable for other TMDs materials not only for MoS₂.