The Investigation Of Ultraviolet Quasi-Phase Matching Crystals With Tetrahedral Units

Ultraviolet（UV）nonlinear optical（NLO）crystals could output UV lasers through the laser frequency conversion process,which play an indispensable role in many optical and electric functional fields.The UV NLO crystals with ferroelectricity could output UV laser based on the quasi-phase matching（QPM）technique,breaking the limitation of the birefringence.Generally,UV NLO crystals with tetrahedral units could exhibit a large band gap.Besides,the uniformly arranged tetrahedral units may bring a large second harmonic generation（SHG）response.Moreover,polar structures composed of tetrahedral units are promising to exhibit ferroelectricity.Therefore,in this paper,based on the[SiO₄],[PO₄]and[SO₄]tetrahedral units,we successfully obtained three cases of UV QPM crystals by the structure selection.The main results are as follows:The KGeOPO₄（KGP）crystals,which are isomorphic with the commercial QPM crystal KTiOPO₄,were screened among the phosphate crystals with polar structure,and the crystal growth and properties research were achieved based on the molten salt method.They exhibit a wide transmission range（0.22-9.70μm）and a strong SHG response（5×KH₂PO₄）,which is beneficial to output UV laser.By a series of thermal performance tests,it is found that the KGP crystals undergo a second-order ferroelectric-paraelectric phase transition at182℃.They exhibit relatively low conductance,allowing us to successfully confirm the existence of ferroelectric polarization reversal in KGP crystals at room temperature（coercive field～9.79 k V/cm,remanent polarization～7.59μC/cm²）.This relatively low coercive field is beneficial to the implementation of the periodic polarization process.In addition,based on the measured refractive index data,the refractive index dispersion curves of KGP crystals were also fitted.According to the calculation results,it was found that the coherence length and polarization period corresponding to the 266 nm coherent output were calculated to be 2.25 and 4.50μm,respectively.The LaBSiO₅（LBS）crystals,which are isomorphic with the commercial QPM crystal LaBGeO₅,were screened among the silicate crystals with a polar structure.The crystal growth was achieved based on the solution method.The obtained LBS crystals exhibit a short UV absorption cutoff edge（～186 nm）and a moderate SHG response（～1.1×KH₂PO₄）.By a series of thermal performance tests,it is found that the LBS crystals undergo a first-order ferroelectric-paraelectric phase transition at 140-150℃.Therefore,we successfully obtained the ferroelectric hysteresis loops of the LBS crystals at 127℃（coercive field～36.98 k V/cm,remanent polarization～1.83μC/cm²）,confirming the existence of its ferroelectricity.Besides,their striped 180°domain structures were confirmed by the chemical etching method.Moreover,by comparing the ferroelectric and paraelectric phases,it is found that the ferroelectricity of LBS crystals originates from the disappearance of the twofold symmetry along a-axis.A LiKSO₄（LKS）crystal with a KBe₂BO₃F₂（KBBF）-like structure was screened in the sulfate crystals crystallized in s polar point group.The crystal growth was achieved based on the aqueous solution evaporation method.The crystal structure is composed of[LiSO₅]_∞layers with the adjacent layers connected by-S-O-Li-covalent bonds,which completely eliminates the layered growth habit.The related tests show that LKS crystals present a short deep-UV absorption cutoff edge（～160 nm）,a large SHG response（～4×KH₂PO₄）and ferroelectricity.By the structural analysis and the calculation,it is found that the dipole moment orientation of the tetrahedral groups is completely consistent,and the microscopic NLO coefficients could be effectively superimposed.Therefore,LKS crystals could exhibit excellent optical properties.