La3Ga5M0.5Sn0.5O14, (M = Ge, Si): Design and Synthesis of Two Langasite Nonlinear Optical Materials with Large Second Harmonic Generation and Birefringence Induced by Distorted (Sn/M)O6 Octahedra.

Nonlinear optical (NLO) coherent light sources are widely applied in many areas of science and technology. As the core medium, the NLO material is required to have a wide transparent range, a large NLO response, and a high laser damaged threshold (LDT). It is common knowledge that langasite (La₃Ga₅SiO₁₄, LGS) crystal has an underdeveloped second-harmonic generation (SHG) coefficient and a small birefringence, which seriously restrict its application in the NLO field, despite that it has a broad transmittance spectrum and a moderate LDT. Herein, we have successfully obtained novel langasite NLO crystals LGSS (La₃Ga₅Si_0.5Sn_0.5O₁₄) and LGGS (La₃Ga₅Ge_0.5Sn_0.5O₁₄), with short UV absorption edges of 209 and 212 nm, respectively. Incorporating heavy ions Sn⁴⁺ into the structure, a distorted BO₆ octahedron was adjusted by the radius difference between Sn⁴⁺ and Si⁴⁺/Ge⁴⁺, which caused the strong SHG responses in LGSS (∼10.77 × KDP) and LGGS (∼9.23 × KDP) and increased birefringences of 0.034 and 0.025, respectively. Besides, they also had large energy band gaps (4.95 eV for LGSS, and 4.93 eV for LGGS), which allowed high LDTs with LGSS of 1.3 GW/cm² and LGGS of 813 MW/cm². This work demonstrates a new strategy to enhance SHG responses and birefringence for existing NLO materials and enriches langasite family crystals.