Direct Deep‐UV Second‐Harmonic Generation in Disordered Χ ² ‐Modulated Ferroelectric Crystals

Abstract

Abstract 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 LiKSO 4 (LPS), a ferroelectric sulfate crystal with z ‐aligned needle‐like domains, forms an intrinsically disordered χ 2 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.