Study Of Mercury-based Large Frequency Effect Second-order Nonlinear Optical Crystal Materials

Nonlinear optical（NLO）crystal materials are widely used in military and civilian fields such as information transmission,medical surgery,weapon research and development,environmental monitoring and so on because of their properties of realizing laser frequency conversion.So the exploration of nonlinear optical crystal materials is of great significance.Halides and halogenated salts have attracted attention due to their excellent physical properties,and metal ions with d¹⁰ configuration(M=Cu²⁺,Ag⁺,Zn²⁺,Cd²⁺,Hg²⁺)are prone to combine with halogens（X=F^-,Cl^-,Br^-,I^-）to form polyhedral anionic groups with structural distortion,thereby forming an asymmetric structure with NLO response.Due to the large relative atomic mass and strong polarity of mercury,it is easy to combine with halogen ions to form NLO crystals with a large second harmonic generation（SHG）.Therefore,this thesis selects metal mercury halides with d¹⁰ configuration with a wide variety of A-position cations to explore new second-order nonlinear optical crystal materials with large second harmonic generation.The main work contents are as follows:（1）A mercury based organic inorganic hybrid halide[N（CH₃）₄]Hg Br₂I₂ with NLO effect was synthesized by hydrothermal method through consulting literature.And a series of characterizations were performed on it.The results show that the second-order nonlinear optical effect of[N（CH₃）₄]Hg Br₂I₂ is 0.46 times that of KDP.Unlike the known homogeneous structure of[N（CH₃）₄]Hg I₄,it can achieve phase matching.Theoretical calculations show that it is its new[Hg Br₂I₂]structural element that increases the birefringence of the crystal by 15 times compared to the[N（CH₃）₄]Hg I₄ crystal,thereby enabling the crystal to achieve phase matching.At the same time,the crystal has high temperature phase transition and fluorescence phenomena,which provides the possibility to develop multifunctional materials for optoelectronic applications in the field of fluorescence and nonlinear optics in the future.（2）A mercury based NLO crystal Ag₂Hg I₄ with large SHG effect and phase matching was synthesized by solution method,and a series of characterization were carried out.Compared to commercial crystal AGS（Ag Ga S₂）,this crystal has a large frequency doubling effect（4×AGS）,wide light transmission range（2.5-80μm）,and a higher laser damage threshold（LDT）（>10×AGS）,indicating it is a kind of mid infrared NLO crystal with great application potential.Through theoretical calculations,the profound insights of the reduction of phonon frequency and the increase of valence band non bonding electronic states on nonlinear enhancement and transmittance broadening are elaborated.This is universally applicable to all NLO optical materials,and will provide a theoretical basis for exploring new high-performance NLO candidate materials.（3）A new nonlinear optical crystal NH₄Hg Br₃·H₂O with infinite arrangement of anions[Hg Br₄]^2-chain was designed and synthesized,which exhibits a large second harmonic generation（28×KDP）,wide band gap（Eg）（3.26 e V）,high laser damage threshold（52.7×AGS）and larger birefringence values（0.183@1064 nm）,suggesting it is a potential candidate material for mid infrared nonlinear optics.As far as we know,the SHG strength of this crystal is the strongest among halides with a band gap greater than 3.0 e V.This work showcases successful cases of halide crystal material design,and this discovery opens a new door for exploring new halide mid infrared NLO materials with good comprehensive properties.