Realizing Phase‐Matching of Nonlinear Optical [ABa 3 Br 2 ][Ga 5 Se 10 ] (A = Cs, Rb, K, Na) Through Large Anisotropic Lattice Distortion

Achieving phase-matching (PM) behavior by enhancing the birefringence of nonlinear optical (NLO) crystals has been a formidable challenge. Herein, salt-inclusion chalcogenides [ABa₃Br₂][Ga₅Se₁₀] (A = Cs, 1; Rb, 2; K, 3; Na, 4) that crystal in tetragonal system were synthesized and exhibit smaller c/a ratios (where a and c are the lattice constants) when compared with their Cl-analogues, indicative of an anisotropic lattice distortion. Along the a- and c-axes, the refractive indices show a significant divergence, corresponding to the decrease or increase in electron density. Such anisotropic behavior significantly enhances the birefringence of compounds 1-4 (0.044-0.066 @546 nm), realizing the transition from non-PM (Cl-analogues) to PM (compounds 1-4) characteristics at near 2.0 um laser. Moreover, compounds 1-4 represent a rare class of selenides that combine significant PM second-harmonic generation responses (>1.0 × AgGaS₂) with wide optical band gaps (>3.0 eV). This work achieves the critical transition from non-PM to PM behavior through significant anisotropic lattice distortion, thereby offering a new avenue for developing NLO materials.