Titanosilicates with Strong Phase-Matched Second Harmonic Generation Responses

The
search for new and efficient nonlinear optical (NLO) materials
has been an active research because of their technological importance
in laser applications. Although a large number of frequency-doubling
oxides, phosphates, borates, and fluoride-containing borates were
found, no transition-metal silicate with useful NLO properties has
been reported. We have now synthesized and grown crystals of two new
titanosilicates, Li2K4[(TiO)­Si4O12] and Li2Rb4[(TiO)­Si4O12], by using a flux and supercritical hydrothermal method.
Their unique 3D framework structures contain highly compressed TiO5 square pyramids which are arranged one over the other to
form infinite ···Ti–O···Ti–O
straight chains with alternating short and long Ti–O distances.
These two materials meet the requirements for efficient second harmonic
generation including lack of center of inversion symmetry, large susceptibility,
phase matching, transmitting at wavelengths of interest, resistant
to laser damage, and thermally stable. These attributes make them
very attractive for frequency-doubling materials.

鉴于非线性光学（NLO）材料在激光应用中的重要技术价值，开发新型高效非线性光学材料始终是研究热点领域。尽管目前已发现大量可用于倍频的氧化物、磷酸盐、硼酸盐及含氟硼酸盐材料，但尚未见具有实用非线性光学性能的过渡金属硅酸盐相关报道。本研究通过助熔剂法与超临界水热法，成功合成并生长出两种新型钛硅酸盐晶体：Li₂K₄[(TiO)­Si₄O₁₂]与Li₂Rb₄[(TiO)­Si₄O₁₂]。该类材料拥有独特的三维骨架结构，其中包含高度压缩的TiO₅四方锥结构单元，这些结构单元层层堆叠，形成了具有交替长短Ti-O键距的无限长···Ti–O···Ti–O直链。这两种材料均满足高效倍频产生的各项性能要求：无中心反演对称性、高二阶非线性极化率、相位匹配特性、目标波长透光性、抗激光损伤性能以及热稳定性。上述优异性能使其成为极具应用潜力的倍频材料。