Nonlinear optical (NLO) crystals can produce infrared (IR) lasers based on their frequency conversion capability. An NLO crystal material that simultaneously possesses large NLO efficiency and high laser-induced damage threshold (LDT) is crucial to generate high power laser.

In a study published in Angew. Chem. Int. Ed., 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 reported a salt-inclusion chalcogenide, Li[LiCs₂Cl][Ga₃S₆], exhibiting unprecedented nanosized porous framework constructed by monotype tetrahedra, and excellent IR NLO performance. 

The researchers adopted the design idea of double functional motifs, namely, the anti-laser-damage functional motif. They built the polycation by elements with large electronegativity difference to widen band gap and to improve the laser damage threshold. NLO functional motif that was built by distorted tetrahedral units, forming super-tetrahedra to increase the NLO coefficient. The double functional motifs were assembled at the molecular level to form a non-centrosymmetric structure. As a result, they synthesized salt-inclusion chalcogenide Li[LiCs₂Cl][Ga₃S₆]. 

The GaS₄ tetrahedral units with ordered arrangement in its structure mainly contribute to its strong NLO efficiency (0.7 times AgGaS₂). The alkali metal halide guests embedded in the nanoporous channels made the compound possess the widest optical band gap (4.18 eV) among all IR NLO chalcogenides, leading to a high LDT (4.1 times AgGaS₂). 

Besides, the researchers carried out calculations and found that the covalent [Ga₃S₆] host framework contributes to the large second harmonic generation (SHG) efficiency, and ionic guests effectively broaden the band gap. 

These experimental results together with the wide IR transmission range and phase matching behavior showed that Li[LiCs₂Cl][Ga₃S₆] is a promising IR NLO material candidate. 

Schematic illustration of constructing excellent IR NLO crystal via the synergistic action of double “functional motifs” (Image by Prof. GUO’s group)