Direct Deep-UV Second-Harmonic Generation in Disordered Χ²-Modulated Ferroelectric Crystals

The generation of coherent deep-ultraviolet (DUV) radiation via nonlinear frequency conversion remains a major scientific and technological challenge in modern optics. To date, only a very limited number of nonlinear optical (NLO) crystals-such as KBBF, ABF, and quartz-have been experimentally demonstrated to support measurable direct second-harmonic generation (SHG) at wavelengths of 177 nm or shorter. There is a pressing need to develop alternative materials or strategies that enable efficient frequency conversion in the DUV region. In this work, it is reported that LiKSO₄ (LPS), a ferroelectric sulfate crystal with z-aligned needle-like domains, forms an intrinsically disordered χ² structure that enables DUV random quasi-phase matching (RQPM) down to 163 nm across a broad spectral range. Remarkably, under nanosecond pulsed pumping at 355 nm, an SHG output at 177 nm with an energy of 3.1 µJ and a conversion efficiency of 0.11% is achieved using an x-cut LPS crystal. This study provides experimental evidence that selecting NLO crystals compatible with RQPM offers a feasible new route for DUV frequency conversion.