Terahertz Emission Properties And Mechanism From XSe₂（X=Re,W）

As typical members of transition metal dichalcogenides（TMDs）,rhenium diselenide（Re Se₂）and tungsten diselenide（WSe₂）exhibit excellent optical and electrical properties such as layer-dependent photoluminescence and band structure.Compared with traditional semiconductor materials,Re Se₂ and WSe₂ are widely investigated in optoelectronic devices such as detectors and photodiodes due to their tunable bandgap and broad spectral response.The surface/interface of two-dimensional（2D）layered materials provides a platform for many physicochemical processes,such as space charge transfer,chemical bond formation,etc.It is crucial to study the physical properties of the surface/interface of layered materials for the research and design of optoelectronic devices.Terahertz（THz）spectroscopy,as a measurement and characterization technology,has the characteristics of sensitivity,nondestructive and ultrafast time resolution.The nonlinear and carrier transport characteristics of 2D materials are characterized by the polarization,polarity and other characteristics of THz waves.This work mainly studies the surface physical properties of Re Se₂ and WSe₂ and polarization states of THz waves through the THz emission properties.The work and innovations are as follows:1.THz radiation characteristics from layered Re Se₂ excited by linearly polarized lightWe have studied the THz radiation properties of layered Re Se₂ films for the first time by a mode-locked Ti:sapphire laser with a central wavelength of 800 nm.Through the dependence of the THz amplitudes on the polarization angles,the incident angles of the pump light and the azimuthal angles of the sample,we demonstrate that the main mechanism of THz radiation is the shift current induced by the photogalvanic effect（PGE）.Furthermore,the Re Se₂ films shows anisotropic THz radiation,which is caused by the difference in shift current parallel to the b-axis（Re-chain）and a-axis.The contribution ratio of the shift current along the a-axis and the Re-chain to the THz radiation is 1:2.This work provides a theoretical basis for the nonlinear study of anisotropic 2D materials and the THz radiation sources.The work has been written and it is in the process of revision.2.The emission and coherent control of THz waves from layered WSe₂ films excited by linearly polarized lightCompared with Re Se₂,WSe₂ exhibits unique elliptically polarized THz radiation properties.The results show that WSe₂ can generate two coherent orthogonal THz wave components.The physical mechanism of THz radiation is dominated by the PGE and the surface depletion field,and their contribution ratios are 61.2%and 38.8%,respectively.Through further calculation,the depletion electric field of the layered WSe₂ film is approximately 1.44×10⁴ V/m.Since the amplitude and phase difference of two orthogonal THz wave components can be tuned by the polarization angles of pump light and the azimuthal angles of sample,the polarization state of THz wave can be further regulated.We have calculated and fitted the ellipticity of THz waves in the time domain and frequency domain,respectively.We find that THz waves can be adjusted from linear polarization（ellipticityχis approximately 0）to elliptical polarization（ellipticityχis approximately 0.65）in the time domain.At the center frequency（0.52 THz）,the ellipticity of THz waves can be tuned to a near-circular polarization state（ellipticityχis approximately 0.95）.The physical mechanisms are mainly the following two points.Firstly,the drift current only contributes to the horizontal THz component,which will make the horizontal THz component carry a fixed phase information.Secondly,the phase of the shift current changes when rotating the polarization angles of the pump light or the azimuth of the sample.This work provides a new idea for the tunable ellipticity and polarization state for THz emission properties in 2D materials,and the related results have been published in The Journal of Physical Chemistry Letters.3.THz radiation characteristics of layered WSe₂ excited by circularly polarized lightThe symmetry breaking of materials can cause spin-orbit coupling（SOC）,which can lead to different types of spin splitting.In this work,to further study the spin physical properties of WSe₂ thin films,the elliptical THz radiation properties of layered WSe₂ are investigated by a transmission circularly polarized THz emission spectroscopy system.Due to the broken structural symmetry of odd-numbered layers,the structure of WSe₂ film has a D_3h point group,so the valence band of WSe₂ split caused by the SOC and its valence band splitting energy is 517 me V.Although WSe₂ has strong spin-orbit coupling,the injection current is not observed on THz radiation.It is deduced by theoretical calculation that the THz radiation process excited by circularly polarized light is dominated by the shift current.Furthermore,the ellipticities of the THz waves are 0.21 and 0.40 excited by the left-handed and right-handed circularly polarized light,respectively.This work not only proposes a method for TMDs with strong SOC to generate shift current under the excitation of circularly polarized light,but also realizes all-optical regulation of elliptical THz waves.