The Study Of Structures And Properties Of IR NLO Chalcogenides

Infrared nonlinear optical(NLO)crystals play an essential role in laser technologies,such as laser communication,frequency conversion and electro-optic modulation,which has been widely applied in industrial,agricultural,military,medical and other fields.At present,the commercially available IR NLO materials are mainly A^IB^IIIC₂^VI and A^IIB^IVC₂^V chalcopyrite semiconductors including the well-known Ag Ga S₂,Ag Ga Se₂ and Zn Ge P₂.These crystals possess large NLO susceptibilities but suffer from drawbacks of relatively low laser damage thresholds(LDTs)or two-photon absorption,which severely hinder their wider practical applications.Hence,the exploration of new promising IR NLO materials with high LDTs and strong second harmonic generation(SHG)efficiency is a long way to go and of remarkable significance.Motivated by Chen's anionic group theory,which states that NLO efficiency mainly originated from anionic groups,most reported IR NLO crystals were obtained by changing reactants.In our fist work,we consider that the Ba-In-Q(Q=S,Se)system is close to the well-known IR NLO(Ba,Sr)-(Ga,Al)-Q system.However,all known Ba-In-Q phases are centrosymmetric(CS),precluding them from being NLO-active.Two new chalcogenides,Ba₁₃Zn₇In₁₂S₃₈ and Ba₁₂Zn₈In₁₂Se₃₈,were obtained by the introduction of d¹⁰Zn²⁺in the Ba-In-Q(Q=S,Se)system,where mixing Zn²⁺in In³⁺sites resulted in non-centrosymmetric transformation.Both compounds show good NLO properties.The laser damage threshold of Li In S₂ is lower than that of Li Ga S₂ because the In-Q bonds are more covalent than Ga-Q bonds,which is beneficial for large SHG and not good for high LDT.Based on this,a new IR NLO compound Li Ga_0.54In_0.46S₂ was successfully synthesized by the introduction of gallium in Li In S₂,where mixing Ga³⁺in In³⁺sites result in a wider band-gap of 3.86 e V and a smaller thermal expansion anisotropy value(0.26)compared with Li In S₂,leading to the large LDT of 12 times that of commercial Ag Ga S₂ under 1064 nm at room temperature.These works prove that nonlinear optical crystals can be designed and synthesized by doping.