Hexagonal InSe: A Defect Wurtzite-Type Infrared Nonlinear Optical Material with Moderate Birefringence Contributed by Unique InSe Unit.

The balance between second harmonic generation (SHG) intensity and laser-induced damage threshold (LIDT), together with phase-matchable behavior, is the key point for exploration of novel nonlinear optical (NLO) materials. In this work, the NLO property of defect wurtzite-type hexagonal-InSe (γ) is extensively explored first. It exhibits a strong SHG intensity of 2.6 × AgGaS (AGS) at 2.1 μm, and a high powder LIDT of 7.3 × AGS. From wurtzite to γ-InSe, the birefringence changes from 0.003 to 0.075, resulting in the phase-matchable phenomenon of γ-InSe. This is well ascribed to the contribution of the unique InSe unit in γ-InSe from the result of birefringence calculation and analysis.