A superb infrared nonlinear optical (IR–NLO) material should meet the following criteria: strong second-harmonic-generation (SHG) intensity (d_ij), high laser induced damage threshold (LIDT), wide phase-matching (PM) range, broad IR transparent window, and stable physicochemical properties. However, "all in one" is hard to realize. Especially, the large d_ij and high LIDT are usually contradictory in one compound.  

A research team led by Prof. ZHU qilong and Prof. LIN hua from the Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences proposed a facile mid-temperature fluxing method to synthesize a novel IR–NLO material Sn₂Ga₂S₅. 

In this study, Sn₂Ga₂S₅ crystallized in the polar space group Pna2₁ and showed a distinctive 3D non-centrosymmetric (NCS) network made by layers and   chains via the sharing common corners.    

The researchers found that Sn₂Ga₂S₅ exhibited an excellent comprehensive performance for IR-NLO applications that surpassed current benchmark AgGaS₂ (AGS), including strong SHG response d_ij (2.5 ×AGS), high LIDT (6.6 ×AGS), wide PM range (> 725 nm), broad transparent region (0.57–13.8 μm) and low MP (ca. 958 K).  

Theoretical calculations indicated that the dimeric [Sn₂S₆] and [Ga₂S₅] asymmetric building motifs (ABMs) contributed greatly to the strong SHG effect and optical anisotropy of the compound.

Moreover, this work provides some useful guidance for the prediction and discovery of new IR-NLO chalcogenides with mixed ABMs. 

The study entitled "Sn₂Ga₂S₅: A Polar Semiconductor with Exceptional Infrared Nonlinear Optical Properties Originating from the Combined Effect of Mixed Asymmetric Building Motifs" was published in Chem. Mater.

A superb infrared nonlinear optical material Sn₂Ga₂S₅. (Image by Prof. ZHU's group)