ab initio calculated frequency-dependent nonlinear optical properties on CsGeBr3

摘要

A systematic first-principles calculation of the linear and second-order optical susceptibilities as functions of frequency for CsGeBr_3 is presented. Specifically, we study the relation between the structural properties and the optical responses. Three structural deformation factors, Δα, d_(Ge), d_X are used to express the degree of distortion from the ideal perovskite structure in bond angle, Ge position, and anion position, respectively. Based on our first-Principles studies, we find that Δα and d_(Ge) increase, while d_X decreases as we substitute the halogen ion from Cl to Br and then to I. The dielectric function and the second harmonic generation coefficient are also found to increase with increasing Δα and d_(Ge). Our calculation indicates that the direct bandgap, E_g, of CsGeX_3 occurs at the R-point for all three compounds, and its magnitude decreases as Δα and d_(Ge) increase (i.e. E_g(CsGeI_3) < E_g(CsGeBr_3) < E_g(CsGeCl_3) ). Our partial density of states (PDOS) analysis reveals that the valence band maximun (VBM) and conduction band minimum (CBM) are mainly associated with the p-orbitals of Germanium. Interband and intraband analysed results for χ_(ijk)~((2)) in CsGeBr_3 can be separated into two main groups of peaks. One was contributed from the magnitude electronic bandgap; the other part was recognized to be attribution from the distortional structural factors. The magnitudes of χ_(ijk)~((2))were in the same manner with some reported experiment near the band gap.