Theoretical Study On The Effect Of Defects On The Nonlinear Optical Properties Of Two Dimensional Transition Metal Dichalcogenides

Nonlinear optics is a branch of nonlinear physics and modern optics.It mainly studies the nonlinear effects produced by the interaction between laser and various materials.The development of nonlinear optics largely depends on the development of nonlinear materials.At present,transition metal dichalcogenides(TMDCs)have attracted much attention due to their excellent nonlinear optical properties.Similar to graphene,TMDCs have layered structures.The band gap of block TMDCs is indirect,and it will change into direct when the number of layers reduceds to single layer gradually.Most of the current researches focus on the ideal two-dimensional TMDCs.However,in the preparation of two-dimensional TMDCs,defects will inevitably be introduced,and the introduction of defects will greatly change the energy band structure of materials and affect the nonlinear optical properties of materials.In this paper,the influence of defects on the nonlinear optical properties of single-layer TMDCs is studied by theoretical calculation,and the influence of exciton effect on the nonlinear optical properties of ideal single-layer TMDCs is also studied:1.The influence of defects on the two-photon absorption of single-layer TMDCs.We have constructed a 5×5 supercell and optimized the structure by using the generalized gradient approximation(GGA)based on PBE functional and the pseudo potential plane wave method.On this basis,the energy band and optical matrix elements are calculated.Then,set up the corresponding vacuum layer to avoid the influence of interlayer interaction.The TPA coefficient is calculated by the formula based on the second-order perturbation theory,and the influence of various kinds of point defects on the two-photon absorption properties of single-layer TMDCs is analyzed.The defects introduce a new energy level and further produce a new two-photon absorption peak,and enhance the two-photon absorption of TMDCs.It makes the two-photon absorption peak red shift,and makes the ZZ direction with zero TPA intensity also have a certain TPA coefficient.2.The exciton effect of two-photon absorption(TPA)in MoS₂and WS₂monolayers.On the basis of ab initio calculation of the eigenstates of BSE,we propose a theoretical strategy to calculate TPA spectra by the first principles of many bodies.This method has been tested in ideal two-dimensional monolayer transition metal dichalcogenides(MoS₂and WS₂).The calculated TPA spectra are in good agreement with some experimental and theoretical results.By tracing the sum of states(SOS)process of TPA coefficients,we also discuss the physical sources of TPA.By tracking the SOS process,we found that the first two bse-tpa peaks at the beginning of the spectrum mainly come from the transition between the valence band and the conduction band near the K point.When the input photon energy is high,the two peaks of bse-tpa mainly come from the k-point transition near?point.3.The effect of defects on the second harmonic generation(SHG)coefficient of2d-TMDCs.Based on the first principle density functional calculation,SHG with different eigenpoint defects is studied.Combined with DOS diagram and energy band structure,the physical source of each defect peak is analyzed.Each defect energy level will reduce the influence of the electron interband motion on polarization coordination,generate new transition channel and generate new characteristic peak on SHG spectrum.The positions of defect energy levels produced by V_Xdefect,V_X2defect and V_MX3defect are similar,so the positions and sizes of defect peaks produced by them are close.X2_Mdefect can produce six defect energy levels,and there is a large difference between the defect energy levels,so it can produce multiple defect energy levels.4.The effect of defects on the third harmonic generation(THG)coefficient of2d-TMDCs.Based on first principles density functional calculations,we investigate THG with different intrinsic point defects.The results show that different from the effect on SHG,each defect can significantly increase the THG coefficient of TMDCs.Each defect energy level can significantly reduce the influence of electron interband motion on the polarization coordination,and make more and wider characteristic peaks in MoSe₂,Mo Te₂and WS₂transform into single peaks similar to that of defect free MoS₂.