Unraveling the Deep-Level Defects Induced Optical Losses in LilnSe2 Crystal toward Enhancement of Mid-Infrared Laser Radiation

Mid-infrared (MIR) lasers generated by nonlinear optical (NLO) crystals have aroused extensive attention in various military and civilian applications. Among the MIR NLO crystal family, LiInSe2 (LISe) is a promising candidate in the generation of MIR lasers due to its wide transmission range, large bandgap, and high laser-induced damage threshold. However, the intrinsic defects in LISe induce optical losses, which will induce laser damage and hinder the performance of MIR laser generation. In this work, the mechanism of optical absorption induced by defects is revealed in LISe through transient absorption (TA) spectroscopy. It is verified that the deep-level defects in LISe give rise to the extra optical absorption, which is owing to the excited state absorption (ESA) and photo-induced absorption (PIA). Accordingly, for improving the optical properties of LISe crystals, Li2Se vapor-mediated annealing to reduce defects concentration is rationally designed. The results confirmed that the optical properties of annealed LISe are improved due to the decrease of defects concentration. Ultimately, the performance of MIR laser generation is successfully enhanced by using the annealed LISe crystal. This work provides deep insights into the defect-induced absorption losses in LISe crystals and promotes the application of LISe crystals in MIR laser generation.