Second-order nonlinear optical (NLO) crystals are essential for frequency conversion. However, commercially available infrared NLO crystals often struggle to simultaneously deliver strong second-harmonic generation (SHG), high laser-induced damage threshold (LIDT), and sufficient birefringence to meet the requirements of phase-matching (PM).

In a study published in Angewandte Chemie International Edition, Prof. GUO Guocong and Prof. LIU Binwen from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences proposed a strategy which achieves PM in infrared NLO materials by amplifying birefringence through anisotropic lattice distortion.

Researchers synthesized a family of salt-inclusion chalcogenides, [ABa₃Br₂][Ga₅Se₁₀] (A = Cs 1, Rb 2, K 3, Na 4). They demonstrated that substituting Cl for Br induced a pronounced directional lattice distortion, enhancing optical anisotropy and enabling PM capability.

The Br-containing compounds crystallized in a tetragonal system and exhibited a smaller c/a ratio than the Cl-analogues, a sign of anisotropic lattice distortion. The distortion drove a measurable divergence between refractive indices along the a- and c-axes, consistent with axis-dependent redistribution of electron density. Anisotropic behavior significantly enhanced the birefringence of compounds 1-4, realizing the transition from non-PM to PM characteristics at near 2.0 um laser.

In powder SHG measurements at 1910 nm, the Br-based materials exhibited SHG signals of 1.04~1.58 times those of infrared material AgGaS₂, while maintaining band gaps greater than 3.0 eV, which supported both frequency conversion performance and reduced absorption risk under high-power irradiation. Besides, the LIDT values were roughly 4.30~4.88 times those of AgGaS₂, highlighting the robustness of these crystals under intense laser conditions.

The study achieves the critical transition from non-PM to PM behavior through significant anisotropic lattice distortion, offering a new avenue for developing NLO materials.