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, alpha-Li2ZnGeS4, crystallizing in the polar, noncentrosymmetric orthorhombic space group Pna2(1). The wide optical bandgap of 4.07 eV prohibits multiphoton absorption, concurrently yielding an impressive LIDT around 61.5x 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 alpha-Li2ZnGeS4 do not hinder its second harmonic generation (SHG) response. The second-order nonlinear optical coefficient, chi((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, alpha-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.