Research On The Balance Mechanism Of Band Gap-SHG Response In Ga-S Nonlinear Optical System

Infrared(IR)non-linear optical(NLO)crystals have very important applications in many advanced technological fields such as infrared remote sensing,environmental monitoring,industrial control,biological tissue imaging and minimally invasive medical surgery.At the present,commercial infrared NLO materials are Ag Ga S₂(AGS),Ag Ga Se₂(AGSe)and Zn Ge P₂(ZGP).It is well known that NLO materials with excellent performance must satisfy:strong second harmonic generation(SHG)response,wide optical band gap,high laser damage threshold(LDT),suitable birefringence,wide IR absorption edge,good stability,and easy to grow large-size single crystals.Although a lot of infrared NLO crystals with excellent performance have been developed so far,it can't meet the demand.At the mean time the band gap is inversely proportional to SHG,when the band gap becomes larger,SHG becomes weaker.It is difficult in achieving a good balance between the bandgap and SHG effect,so exploring new outstanding infrared NLO materials is still a research hotspot.In addition,the origin and the differences of the nonlinear performance of some IR NLO materials with application prospects so far,and the mechanism of the influence of the metal cations on the nonlinear performance are not clear yet.Therefore,in this work,we selected the Ga-S system of chalcopyrite structural framework,which is considered as a promising application candidate in the field of IR NLO,as the research object.We selected the typical four classical chalcopyrite materials,A^IGa S₂(A^I=Ag,Cu)with normal structures and A^IIGa₂S₄(A^II=Zn,Hg)with defect structures,studied their electronic structures,optical properties and the influence of different cations and primitives on the bandgap and SHG effect using the first-principles calculation method.(1)The results uncover that the different bandgaps are mainly caused by the effect of d orbitals of A-site cations near the Fermi level.In addition,the dp hybridizations interaction between A-site cation and p orbitals of S atoms also affects the bandgap.Besides,the more powerful covalent bonds between A^II-S and Ga-S in the A^IIGa₂S₄ lead to the stronger SHG responses of Zn Ga₂S₄ and Hg Ga₂S₄.For the birefringence,the size of the cation of the A site atom mainly has an effect,namely the larger size will lead to the higher distortion of tetrahedra,then results in the larger birefringence.Furthermore,the Ga-S system with the different cations as the research object,and the mechanism of the influence of cations on the electronic structure and optical properties of the Ga-S system was analyzed.(2)It was found that the alkali metal/alkaline earth metal has no d orbital effect on the near the Fermi surface,so the bandgap is large.In addition,the interaction between A site cation and S atoms has an influence on the electron structure and SHG response.Combined with the research results of above compounds,we conclude that the frequency doubling in the Ga-S system mainly comes from the Ga S₄ tetrahedron.Meanwhile,the d orbitals of A-site cation near the Fermi level strongly effects on the bandgap.In addition,the interaction between the d orbitals of A-site cations and p orbitals of S atoms have gained influence on the SHG effects.All the above analyses conclude that A site atoms play a regulatory role in deciding the optical properties in the Ga-S system.