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 LiNH4SO4 (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. A deep‐UV ferroelectricity nonlinear optical crystal LiNH4SO4 is explored. It exhibits short deep‐UV absorption edge, small refractive index dispersion, and moderate NLO response, which indicates that LAS is a promising NLO crystal for deep‐UV quasi‐pahse‐matching. This work highlights the potential of sulphates and provides new opportunities for deep‐UV NLO materials.