Theoretical Study On Field Dependent Optical Absorption And Refraction Of Monolayer Transition Metal Dichalcogenides

As a new two-dimensional material,transition metal dichalcogenides shows two remarkable characteristics: first,the indirect band gap of multiple layers can be changed into direct band gap by reducing the number of layers;Second,it shows stronger spinorbit coupling than other materials.These two characteristics make transition metal dichalcogenides widely used in fields such as field effect transistors,photodetectors and photoluminescence.At present,the research on the optical properties of transition metal dichalcogenides under the control of no external field is extensive,but the theoretical research on them under the control of external magnetic field and electric field is still less.In this thesis,the optical absorption properties and refractive index changes of monolayer transition metal dichalcogenides with external magnetic field and electric field are studied theoretically.In this thesis,the Hamiltonian and wave function of monolayer transition metal dichalcogenides in the absence of external field,under the control of external magnetic field and under the control of external magnetic field and electric field are obtained by using k p model.The electronic energy states in the above three systems are solved respectively,and the energy bands in the corresponding systems are obtained.Based on the perturbation theory of photon-electron interaction,the two-photon transition rates of monolayer transition metal dichalcogenides in the absence of external field,under the control of external magnetic field,and under the control of external magnetic field and electric field,respectively,are derived,and the absorption coefficients of twophoton in three systems are further obtained.Using nonlinear polarization theory,the expressions of linear refractive index changes,nonlinear refractive index changes and total refractive index changes of monolayer transition metal dichalcogenides under the control of external magnetic field and external magnetic field and electric field are derived respectively.The corresponding two-photon absorption coefficient and refractive index changes are numerically solved by Matlab software,and the corresponding absorption spectrum and refractive index changes curve are simulated.The influence of external field control on the two-photon absorption coefficient and refractive index changes of monolayer transition metal dichalcogenides is analyzed.The results show that:(1)in the absence of external field,the weight of transition metal atoms affects the two-photon absorption coefficient,and the two-photon absorption coefficient of tungsten is greater than that of molybdenum.(2)Under the applied magnetic field,the two-photon absorption coefficient and refractive index changes of the in band transition have only one peak,while the inter band transition has multiple peaks.The peak of the in band transition is in the terahertz range and the peak of the inter band transition is in the visible band.In addition,the peak of the in band transition is larger than that of the inter band transition.(3)With the increase of magnetic field intensity,the two-photon absorption peak and the summit of refractive index changes appear blue shift.(4)Under the joint regulation of external magnetic field and external electric field,the two-photon absorption peak and the peak of refractive index changes blue shift with the increase of electric field intensity.In this paper,we explain the above conclusions accordingly.The research results of this paper provide a theoretical reference for the experimental work of transition metal dichalcogenides,and also provide guidance for the application of nonlinear optics and optoelectronic devices.