Research On Second-order Nonlinear Optical Properties Of Low-dimensional Materials With Broken Symmetry

Today,with Moore's Law based on silicon materials and electronic transmission information approaching to the limit,photons become an important carrier of the next generation of information transmission devices.Finding high-efficiency and easy-to-integrate nonlinear optical component materials has become an urgent problem to solve in a high-efficiency all-optical information processing system.The quantum confinement effect of two-dimensional materials endows them with unique electrical and optical properties.With the development of two-dimensional materials,such as graphene,molybdenum disulfide and black phosphorus,becoming mature and rapidly expanding in the field of optoelectronics,the development in the field of nonlinear optical devices has been paid more and more attention.Because of the strong dependence of experimental research on experimental conditions,the tight binding method based on quantum mechanics and first principles are effective means to study the optical properties of materials.In this paper,the tight-binding model,the sum-over-state method based on the first-principles calculation and the state motion equation based on the modern polarization theory are used to study the influence of asymmetry on the second-order nonlinear optical properties of two-dimensional materials.Using tight binding theory,the second-order nonlinear current response induced by applying transverse electric field on zigzag graphene nanoribbons is studied.The edge state of graphene nanoribbons is affected by electric field,and its degeneracy and symmetry are broken,thus changing the band structure and inducing optical current injection and shift current which belong to optical second-order nonlinearity.It is found that electric field has perturbation characteristics for optical current injection response.And the shift current has nonlinear relations to the external electric field.In addition,width and Fermi level also have effects on two kinds of second-order nonlinear current responses.Studying the second-order nonlinear current response induced by nanoribbon electric field has guiding value for developing the ultrafast current response of nanoribbon or bulk photovoltaic effect.For 2D group-VA binary monolayer with in-plane polarization,using sum-over-state method based on density functional theory,we studied the effect of polarization on the second-order susceptibility tensors.The significant enhancement of second-order susceptibility tensor is predicted,due to the in-plane polarization induced by structural asymmetry.We found the giant second harmonic generation component of?-PAs monolayer along the direction of polarization.It is one or two orders higher than that of MoS₂ monolayer.The peak value originates from the intraband two-photon transition process.This study released the qualitative relations between in-plane polarization and second order nonlinear polarization tensor in 2D materials with puckled structure.For materials with out-of-plane polarization,such as 2D group-VA binary monolayer materials with blue phase and Janus MoSSe monolayer and stacked bulk,the second-order nonlinear polarization of materials caused by external electromagnetic field is studied by modern polarization theory and equation of state motion,and the influence of out-of-plane polarization on nonlinear optical response is also studied.Permanent dipoles first break the inversion symmetry of blue phosphorus,resulting in obvious differences in energy band structure.In addition,the increase of the external dipole can obviously increase the in-plane nonlinear polarizability and the out-of-plane polarizability which is close to the in-plane polarizability.The out-of-plane symmetry breaking of Janus MoSSe increases the number of non-zero independent nonlinear polarizability components,thus improving the utilization efficiency of incident light in all directions.The response of out-of-plane polarizability of bulk stacking is non-zero,which improves the defects of no out-of-plane nonlinear polarizability and layer dependence of in-plane nonlinear polarizability of MoS₂ material.Finally,the influence of exciton effect on second-order nonlinear optical properties is studied.Based on the equation of motion of state and the multi-body perturbation theory,the equation of motion of state is transformed into the equation of motion of retarded Green function,and then the linear response of 2D group-VA binary monolayer and the second-order nonlinear response of Janus TMD two-dimensional material are studied by calculating the polarizability.It is shown that spontaneous polarization,exciton,and second-order nonlinear response are closely related.Spontaneous polarization has obvious influence on the radiation lifetime,radius,and anisotropy of excitons.Due to the different directions of spontaneous polarization,it has opposite effects on exciton properties of 2D group-VA binary monolayer with different phases.The research on the second-order nonlinear response of Janus TMD shows that exciton effect can enhance the in-plane polarizability below the band gap,while reduce the in-plane polarizability above the band gap and enhance the out-of-plane component of the low-energy interval polarizability.