A Ferroelectric Nonlinear Optical Crystal for Deep-UV Quasi-Phase-Matching

Phase matching is indispensable for high efficiency of second-order nonlinear optical (NLO) crystals. Thousands of these crystals rely on birefringence phase matching, whereas only a few of them are quasi-phase matching without the restriction of birefringence. Herein, using oriented seeds inch-sized LiNH 4 SO 4 (LAS) single crystals are successfully grown. LAS is NLO-active with a very short deep-UV absorption edge of 171 nm and high laser damage threshold up to 1.47 GW cm −2 at 1064 nm, 10 ns, but its birefringence is merely 0.0078@532 nm, which is too small to realize birefringence phase matching in deep-UV region. Fortunately, P − E hysteresis loop, variable-temperature NLO tests, and ferroelectric domain observations confirm that LAS is ferroelectric. Furthermore, LAS exhibits small refractive index dispersion resulting in a relatively long periodic length of 1.4 µm and its single domain structure can be easily obtained by electrical poling. These attributes make LAS a scarce NLO candidate for deep-UV quasi-phase matching. This work highlights the longly overlooked potential of sulfates and provides new opportunities for deep-UV NLO materials.