Toward the vacuum-ultraviolet frontier in nonlinear optical frequency conversion: phase-matching mechanisms across dimensions and architectures

This review establishes a unified structure–mechanism–performance framework linking crystal dimensionality, phase-matching strategies, and VUV performance limits to guide next-generation materials beyond 150 nm. Vacuum-ultraviolet (VUV, 100–200 nm) light sources are essential for semiconductor lithography, high-resolution spectroscopy, and emerging applications such as the 229 Th nuclear clock. Their realization critically depends on nonlinear optical (NLO) materials capable of efficient frequency conversion, where phase-matching is a key requirement for VUV generation. Recent advances, including record-short birefringent-phase-matching at 158.9 nm in fluorooxoborates, full-wavelength phase-matching near 193 nm, twist-phase-matching in low-dimensional materials, and additional periodic phase engineering in conventional crystals, have significantly expanded the accessible design space for short-wavelength NLO frequency conversion. However, the relationships between crystal dimensionality, structural motifs, and phase matching mechanisms remain fragmented. In this Review, we establish a unified structure–dimension–mechanism–performance framework that connects crystal dimensionality with phase matching mechanisms and VUV performances. We systematically integrate birefringent-phase matching, quasi-phase-matching, additional periodic phase-phase-matching, and twist-phase-matching across bulk, superlattice, and low-dimensional systems. We further highlight how structural evolution and chemical modulation, particularly fluorination, enable simultaneous control of bandgap, anisotropy, and nonlinear response, thereby providing insight into current VUV phase-matching limits and pathways toward overcoming the 150 nm barrier.