Nonlinear optical (NLO) crystals have the important frequency conversion capability. A great number of Infrared (IR) NLO materials with high NLO coefficient (d_ij) have been explored, but many of them suffer from non-phase matching (PM), which dramatically decreases their ultimate NLO output efficiencies. 

In a study published in CCS Chem., a research group led by Prof. GUO Guocong from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences has developed a new strategy involving modulating normal dispersion via bandgap (Eg) widening to achieve PM.  

Three new wide Eg (～4.0 eV) IR NLO salt-inclusion sulfides [ABa₃Cl₂][Ga₅S₁₀] (A= K, Rb, and Cs) with PM behaviors were successfully synthesized. Typically, the wider the Eg, the flatter the normal dispersion curves in the mid- and far-IR regions.  

Compared with isomorphism selenides, dispersion curves of wider Eg sulfides are smoother in the IR regions, which make the PM cutoff wavelength blue-shift from 5.7 to 1.7 um, leading to the PM ability at the traditional fundamental frequency laser (2.0 um). These results provide a new design idea to obtain new PM IR NLO crystal materials.  

Moreover, [ABa₃Cl₂][Ga₅S₁₀] (A= K, Rb, and Cs) possessed large second-harmonic-generation (SHG) efficiency (～1×AgGaS₂) and hitherto the highest laser-induced damage thresholds (188–200 MW/cm², at 1.06 μm, 10 ns pulse width) for known chalcogenides with second-harmonic generation (SHG) efficiencies comparable to that of commercial AgGaS₂ crystal. These merits together with wide transparent window (0.4-12.3 μm) indicated that they are promising candidates as IR NLO materials. 

This study provides an effective way to explore new PM IR NLO materials based on the non-PM ones with good SHG efficiency and laser damage threshold (LDT). 

Schematic illustration of crystal structure and millimeter-level crystal wafer (Image by Prof. GUO’s group)