Mechanism About SHG Response And Birefringence Induced By Lone-pair Electrons In Borate Materials

Nonlinear optical materials are widely used in industrial production,optoelectronic communication,integrated circuits,medical devices and other fields,especially in the laser field,as a kind of frequency conversion material,it can broaden the range of laser wavelength,so as to open up a new laser light source,which requires nonlinear optical crystal materials to have second harmonic generation(SHG)response and appropriate birefringence.Borates materials have followed with much interest due to their high micro polarizability,and introducing lone-pair electrons into borate compounds has long been thought to be a potent procedure to obtain the excellent NLO compounds,which can capably enhance birefringence and/or SHG response.However,along with the increasing amount of new borate compounds,the different response to birefringence and SHG coefficients induced by ns2 lone-pair electrons are also available How to obtain the SHG response-birefringence balanced nonlinear optical materials?What is the theoretical mechanism of the SHG response and birefringence affected by lone pair electrons?In this paper,the electronic structures,and optical properties of a series of AB₂O₃F₂(A=Ba,Sn,Pb)have been investigated using the first-principles method.The anisotropic lone-pair electron distribution is confirmed by the projected density of states,the projected band structures,the electron localization function(ELF)and the states nearby the Fermi level.The atomic and group's contribution to birefringence and SHG response are further investigated using the real-space atom-cutting method and band-resolved SHG method.The results show that the anisotropic lone-pair electron distribution close to the Fermi level give main contribution to the birefringence,and the expanded hybrid cation sp-oxygen p states play a key role in determining the enhanced SHG responseAll the indications that we are going to receive reasonable support from inherent electrons.The electronic structures and optical properties of AB₃O₆(A=Bi,Sb)are examined conducting the first-principles method.The atomic contribution to optical response are studied using Born effective charge and bond population.We show that the so far large ignore optical anisotropy and covalency play a critical role in affecting the optical properties of BO groups.In addition,the Bi/Sb-p states near the Fermi lever have a positive effect on the optical response,the more Sb-p states,the greater birefringence it is.In conclusion,we mainly studied the influence mechanism of lone-pair electrons on SHG response and birefringence in nonlinear optical materials AB₂O₃F₂(A=Ba,Sn,Pb)and AB₃O₆(A=Bi,Sb)It is hoped that these conclusions can provide some reference value for the screening of new NLO materials in the future.