Effect Of Microscopic Anionic Groups On The Properties Of Infrared Nonlinear Optical Materials

Mid/far infrared?IR?nonlinear optical?NLO?materials are the key materials for acquiring mid/far IR lasers through frequency conversion technology,therefore they have important applications in laser based IR countermeasures,laser communications,and space antimissile.Currently,commercial NLO materials in this region include AgGaS₂,AgGaSe₂,and ZnGeP₂.However,due to their own weaknesses?too low laser damage threshold and other defects?,their wide applications are limited in the development of laser technology.One of the main reasons is that there is a contradiction between the laser damage threshold?corresponding to the bandgap?and the NLO effect in general.Therefore,designing a new material that can simultaneously have a high laser damage threshold and a large NLO effect is the hotspot and open question in this area.Therefore,balancing the contradictory relationship between laser damage threshold and NLO effects is the key to explore new materials.In view of this,based on the first-principles calculations,this work systematically studies the influence mechanism of microscopic units on NLO effects in two types of typical mid/far IR systems,including structures-electronic structures-NLO effects,and screening high laser damage threshold and large NLO effects of active units.This study provides a direction for the rational design and effective synthesis of NLO materials that achieve excellent performances.The main research content of this work is as follows:1)We prove and propose that[SiP₄],[GeP₄],[SnP₄]and[GeAs₄]tetrahedra are the dominant groups for obtaining large NLO effects.The intrinsic mechanism of the macroscopic properties of the five kinds of ?-?-?₂ chalcopyrite semiconductor materials,ZnGeP₂,ZnSiP₂,ZnSnP₂,CdGeP₂,and CdGeAs₂,was investigated by the first-principles calculations.The band gap determining factor was shown by the density of state analysis and the effective active groups of NLO effects were shown by second harmonic generation?SHG?density analysis.The NLO effects were calculated by the sum-over-states method:d₁₄=50.47 pm/V,36.53 pm/V,55.51 pm/V,79.38 pm/V,207.61 pm/V,respectively.In addition,the volume effect is the main reason that causes different NLO effects of this system.In order to prove the contribution of the units,we calculated the contribution of microscopic groups to NLO effects by means of the real-space atom cutting tool.The results show that the contributions of[SiP₄],[GeP₄],[SnP₄]and[GeAs₄]tetrahedra are much larger than others,so they can be used as the dominant structrual units of NLO materials,and the[GeAs₄]tetrahedra are the best.2)In addition to the contribution mechanism of the tetrahedra,what is the influence of other configuration units on the band gap and NLO effects?To address this issue,we systematically studied and compared series of typical infrared NLO materials:Pb₂B₂S₅?BS₄?and Pb₃P₂S₈?PS₄?containing tetrahedral units,Pb₉As₄S₁₅?AsS₃?containing trigonal pyramid units,and Pb₂P₂S₆?P₂S₆?containing ethane-like units,Y₂PbS₄?YS₆?containing octahedral units.Studies have shown that the NLO effects of Pb₂P₂S₆ and Y₂PbS₄ are 4.73 pm/V and 7.32 pm/V,respectively,indicating that these two systems do not have obvious advantages.The NLO effect of Pb₉As₄S₁₅ is as large as 25.14 pm/V,which is twice that of commercial AgGaS₂ crystal.Through analysis of SHG density,it is found that Pb²⁺cations contribute less to NLO effects,and[AsS₃]trigonal pyramids play a decisive contribution to NLO effects.The main reason is that[AsS₃]has lone pair electrons and generates large structural distortions.Therefore,the[AsS₃]trigonal pyramids are the dominant groups of NLO effects.