Terahertz Emission Properties And Mechanism From Molybdenum Selenide

Molybdenum Selenide(MoSe₂),as one of the representatives of transition metal dichalcogenides(TMDs),has good mechanical strength and flexibility,adjustable band-gap with thickness,strong exciton effect,spin splitting,and large spin-orbit coupling(SOC).Therefore,MoSe₂ has attracted extensive research interest in new optical,electrical,and optoelectronic devices.As semiconductor devices,many physical and chemical processes occur on the surface or interface of materials,such as chemical bonding,charge transfer,carrier transport,and so on.Terahertz(THz)emission spectroscopy has a high temporal and spatial resolution as a new detection method.It can detect the physical effects of materials in ultrafast time scale without contact and damage,further characterize the structural,electronic,and photonic properties of material surface/interface.The surface and interface characteristics of MoSe₂ are investigated by THz emission spectroscopy in this paper,which provides a theoretical and experimental basis for the application and development of MoSe₂ in ultrafast optoelectronic devices.The main work and innovation of this paper are as follows:1.THz radiation characteristics of layered MoSe₂ crystalThe THz radiation characteristics and the mechanism of the layered MoSe₂ crystal are researched for the first time via the THz emission spectrum spectroscopy.According to the dependence of THz signal with the azimuthal angle of sample,the pump fluence,and the polarization angle of the incident beam,the results show that the THz radiation is mainly caused by the strong surface depletion field effect and the weak resonant optical rectification.By calculation,the width of the depletion layer is about 37.4 nm,and the electric field of the depletion layer is about2.54?10⁷ V/m.This work shows the surface physical effects of layered MoSe₂ crystal excited by an ultrafast laser.Moreover,we research the surface characteristics of layered MoSe₂ crystals,which provide the theoretical basis for developing and applying new photoelectric devices based on layered MoSe₂.2.THz radiation characteristics of monolayer MoSe₂ thin filmThe band-gap structure,surface symmetry,carrier mobility,and other physical properties will change when MoSe₂ transits from layered to monolayer.Based on the results of the previous work,the THz radiation characteristics of monolayer MoSe₂ thin film excited by femtosecond pulses are investigated.The results show that the THz radiation mechanism of monolayer MoSe₂ thin film is the same as layered MoSe₂ crystal.It is the first time to prove the existence of surface depletion field in monolayer MoSe₂,but the radiation intensity of monolayer MoSe₂ film and layered MoSe₂ crystal is quite different under the same power excitation.Further experiments show that the contribution of resonant optical rectification effect in THz radiation of the two materials is the same.The difference is mainly caused by the different surface depletion field effect.Compared with layered MoSe₂ crystals,the contribution ratio of surface depletion field effect in monolayer MoSe₂ thin film decreased from 94.2%to 74.5%.Due to the influence of the thickness of the monolayer MoSe₂ film,the surface charge region is narrow,and the depletion layer is only limited within the range of the monolayer.As a result,the depletion layer electric field is greatly weakened,leading to the weak THz radiation of monolayer MoSe₂ thin film.Combined with the previous work,the results have been published in ACS Applied Materials&Interfaces(IF=8.758).3.THz emission characteristics of Co/MoSe₂ heterostructureIn addition to the excellent optoelectronic performance and unique application in new optoelectronic devices,the broken symmetry and strong SOC characteristics of monolayer MoSe₂ provide theoretical possibilities for the application in spintronics.In this work,the monolayer MoSe₂ and Co are used to construct the ferromagnetic material(FM)/nonferromagnetic material(NM)heterostructure.The THz radiation characteristics of the heterostructure are researched by THz emission spectroscopy.The analysis shows that THz radiation is independent of the ultrafast demagnetization in Co and the nonlinear effect in MoSe₂,mainly due to the spin to charge transition caused by the inverse spin-Hall effect(ISHE)in MoSe₂.Compared with the THz signals of monolayer MoSe₂ and Co,the THz emission intensity of the heterostructure is enhanced,which proves the strong SOC in monolayer MoSe₂.In this work,a new non-contact method is used to investigate the spintronic characteristics in monolayer MoSe₂,which is expected to be a candidate material for developing spintronic devices.Moreover,this work provides a new idea and method for improving the terahertz radiation efficiency of monolayer TMDs.