The Effect Of D¹⁰ Cations On The Properties Of Infrared Nonlinear Optical Materials

Infrared?IR?nonlinear optical?NLO?crystal materials play an extremely important role in the solid state laser frequency conversion technology,these materials with high quality can be widely applied in the optical devices,such as,laser communication,laser cutting,laser guidance,laser radar,laser medical treatment,laser weapon,laser cosmetic and many military,medical and civil fields.At present,the commercial IR NLO materials mainly include AgGaS₂,AgGaSe₂ and ZnGeP₂crystals.Though they have a wide infrared transmission range and a large NLO effect,their low laser damage threshold resulting from the small band gap or the unavoidable shortcoming of two-photon absorption limit their more extensive applications in some extent.Therefore,exploring new IR NLO crystal materials with excellent properties is still a hot and difficult issue in the NLO field.Throughout the commercial IR materials and many other potential IR NLO materials,almost a large portion of those crystals contain the d¹⁰(Ag⁺,Cu⁺,Zn²⁺,Cd²⁺,Hg²⁺)metal cations.However,the mechanism of the effect of d¹⁰ metal cations on the IR NLO property is still unclear.Therefore,its mechanism was systematically investigated by using the first-principles under the density functional theory.In this work,we analyzed the effect of d¹⁰(Cd²⁺,Hg²⁺)metal cations on micro-electronic structure and optical properties of IR NLO materials in two typical systems.?1?:One is for the AB₂S₄?A=Cd,Hg;B=Al,Ga?system with the typically defective chalcopyrite structure,in this system,we found that d¹⁰(Cd²⁺,Hg²⁺)metal cations affected the electronic structure,in particular,the strength of dp hybridization between d¹⁰ cations and sulfur affects the electronic structure and optical properties;In addition,the electronegativity and cell volume effect of different cations is also a negligent factor;?2?:Furthermore,in a more complicated system Li₂MGeS₄?M=Cd,Hg?with diamond-like structure,we use the quantitative dp repulsion energy to analyze the mechanism effect of dp interaction between d¹⁰(Cd²⁺,Hg²⁺)cations and sulfur on the electronic structure and optical properties,meanwhile,it is found that the dp interaction is also the main reason of NLO effect enhancement from Cd to Hg.Combined AB₂S₄ with Li₂MGeS₄ systems,our results show that the d¹⁰(Cd²⁺,Hg²⁺)metal cations are generally inclined to form the covalent bonding with sulfur,and compared with the weak dp hybridization formed by Cd-4d and S-3p electrons,the strong dp hybridization formed by Hg-5d and S-3p electrons not only enhances the NLO effect but also reduces the band gap.