Giant Non‐Resonant Infrared Second Order Nonlinearity in γ ‐NaAsSe2

Abstract Infrared laser systems are vital for applications in spectroscopy, communications, and biomedical devices, where infrared nonlinear optical (NLO) crystals are required for broadband frequency down‐conversion. Such crystals need to have high non‐resonant NLO coefficients, a large bandgap, low absorption coefficient, and phase‐matchability among other competing demands; for example, a larger bandgap leads to smaller NLO coefficients. Here, the successful growth of single crystals of γ ‐NaAsSe 2 that exhibit a giant second harmonic generation (SHG) susceptibility of d 11 = 590 pm V −1 at 2 µ m wavelength is reported; this is ~18 times larger than that of commercial AgGaSe 2 while retaining a similar bandgap of ~1.87 eV, making it an outstanding candidate for quasi‐phase‐matched devices utilizing d 11 . In addition, γ ‐NaAsSe 2 is both Type I and Type II phase‐matchable, and has a transparency range up to 16 µm wavelength. Thus, γ ‐NaAsSe 2 is a promising bulk NLO crystal for infrared laser applications.