α-Li2ZnGeS4: A Wide-Bandgap Diamond-like Semiconductor with Excellent Balance between Laser-Induced Damage Threshold and Second Harmonic Generation Response

Exploring new nonlinear optical (NLO) materials with high laser-induced damage threshold (LIDT) in the infrared (IR) region is vital for the development of technologies relying on tunable laser systems. Herein, we report on a quaternary diamond-like semiconductor, α-Li2ZnGeS4, crystallizing in the polar, noncentrosymmetric orthorhombic space group Pna21. The wide optical bandgap of 4.07 eV prohibits multiphoton absorption, concurrently yielding an impressive LIDT around 61.5× that of the benchmark NLO material AgGaSe2 at 1064 nm. It also features phase-matchability for three-wave mixing. Notably, the large bandgap and the outstanding LIDT of α-Li2ZnGeS4 do not hinder its second harmonic generation (SHG) response. The second-order nonlinear optical coefficient, χ(2), was estimated to be 26 pm/V, which exceeds that of several commercially available IR-NLO crystals. In general, there is usually a trade-off between the LIDT and the NLO coefficient; however, α-Li2ZnGeS4 features an excellent balance between an outstanding LIDT and a strong SHG response, making the compound a promising candidate for next-generation IR-NLO devices.