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

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 -NaAsSe₂ that exhibit a giant second harmonic generation (SHG) susceptibility of d₁₁ = 590 pm V⁻¹ at 2 µm wavelength is reported; this is ~18 times larger than that of commercial AgGaSe₂ while retaining a similar bandgap of ~1.87 eV, making it an outstanding candidate for quasi-phase-matched devices utilizing d₁₁. In addition, γ -NaAsSe₂ is both Type I and Type II phase-matchable, and has a transparency range up to 16 µm wavelength. Thus, γ -NaAsSe₂ is a promising bulk NLO crystal for infrared laser applications.