The First-principles Investigation About The Birefringence And SHG Response In Phosphates

The ultraviolet nonlinear optical（NLO）materials play an important role in industrial processing,information technology,and so on.Among them,phosphate compounds have attracted much attention due to their relatively large UV/DUV cut-off edge and relatively large SHG response.On the other hand,their birefringence is relatively low.How to get the good performance UV NLO compounds with balanced cut-off edge,SHG response and birefringence has become a hotspot of NLO investigation.In this thesis,we briefly review the research status of NLO materials and the basic characteristics of some common NLO compounds.Besides,we also review briefly the essential idea of density functional theory and the using method of several common pre-and post-first principles codes.The main study content is as follows:Firstly,we use the first principles to investigate electronic structures and optical properties of Te₂P₂O₉,and the obtained results are in good agreement with the experimental values.The Born effective charges and SHG density of Te₂P₂O₉ exhibit that the contribution to the birefringence and SHG response originates mainly from the TeO₅ group.Next,we study electronic structures and optical response of Ba₂TeO（PO₄）₂ and Te₃O₃（PO₄）₂.The results show that these two tellurium phosphates also own large birefringence like Te₂P₂O₉.The real-space atom-cutting results and distortion indicate that the birefringence comes from the TeO_x polyhedron.Finally,we apply the first principles method to study the electronic structure and optical properties of LiCs₂PO₄,and the obtained results are in good agreement with the experimental values.By checking the projected density of states（PDOS）,the Born effective charges,and SHG density in detail,we conclude that the O atoms in LiCs₂PO₄ play a crucial role in birefringence and SHG response.In summary,we investigate the origination of SHG response,the band-gap,the refractive indices and birefringence of Te₂P₂O₉ and LiCs₂PO₄.It is expected that the results can provide certain reference value for the future study of novel candidate materials.