Study On The Nonlinear Optical Properties Of The Chalcogenides With The D¹⁰ Electron-containing Metal Cations

Infrared nonlinear optical(NLO)materials are an important part in the frequency conversion of solid-state lasers.They are widely used in civilian and military fields such as laser guidance,laser communication,laser cutting,and lidar.At present,infrared NLO materials that can be used commercially include Ag Ga S₂,Ag Ga Se₂,and Zn Ge P₂.They have excellent NLO properties,wide infrared transparent regions,and are easy to grow large single crystals,but they also have their own shortcomings,the narrow band gap will reduce the laser damage threshold,and two-photon absorption phenomenon is serious,which have greatly restricted their commercial use.Therefore,the searching for a new generation of infrared NLO materials with excellent performance is a challenge to be solved urgently.As the traditional materials Ag Ga S₂,Ag Ga Se₂ and Zn Ge P₂,they all have metal cations with d¹⁰ electron configuration(Ag⁺,Zn²⁺).Therefore,screening and analyzing compounds containing d¹⁰ electron configuration metal cations are of great positive effect and guiding significance for the discovery of new infrared NLO materials.In this paper,through high-throughput screening,we performed first-principles calculations on the optical properties of the A₈BS₆(A=Cu,Ag;B=Si,Ge,Sn)system,and the effect of A-site metal cations to strong dp orbital hybridization were analyzed.The strength of covalent bonds and the interaction of electrons on the second harmonic generation(SHG)effect were also studied.In addition,we have carried out theoretical research based on first-principles for the A₂BGe S₄(A=Cu or Ag,B=Ba,Cd or Hg)system with d¹⁰ electron configuration.Through analysis,the effects of dp hybridization on the band gap and SHG in this system are qualitatively studied.The analysis methods such as the SHG density and the hyperpolarizability of the units show that the main SHG contributor is the AS₄ units.The results are also verified by the atom replacement strategy.The distortion of units is also an important factor to contribute to the SHG response.Through a comprehensive study of distortion and alignment,we propose a geometric structure factor R_rf and find that the factor is positively related to the SHG effect.The factors R_rf can also evaluate SHG effects in polar materials quantitatively.The main source of birefringence in this system was also analyzed by response electron distribution anisotropy(REDA)method.Finally,the A₂Hg I₄(A=Na,Ag,Cu)system with a diamond-like structure was systematically studied.The results show that,due to the regulation of the cations in the system,the contribution of the anions therein to the SHG is different.Through the strategy of replacing the cations at the A site,hypothetical compounds were obtained.They have excellent overall performance and can balance the contradiction between band gap and SHG effect.The research results show that d¹⁰(Cu⁺,Ag⁺,Cd²⁺,Hg²⁺)electron configuration metal ions and sulfur will form a strong covalent bond,and they will coordinate with eachother to form a NLO active units.The dp hybridization at the top of the valence band is the difference which affects the band gap and NLO coefficients of the compound.