Recent Developments in Deep-Ultraviolet Flat Optics.

Deep-ultraviolet flat optics (DUVFO) is rapidly emerging as a foundational platform for next-generation photonic technologies. Operating in the 200-300-nm regime, DUVFO unlocks capabilities inaccessible to conventional refractive optics, enabling compact beam shaping, high-NA (HNA) focusing, and efficient nonlinear interactions within ultrathin form factors. This review provides a comprehensive overview of the materials, device concepts, and system-level innovations driving the shift from bulky, absorption-limited DUV optics toward scalable, chip-compatible flat-optical architectures. We highlight the critical role of wide-bandgap semiconductors and low-loss metal oxides in overcoming intrinsic transparency and stability challenges; examine the evolution of linear and nonlinear DUV devices, from HNA metalenses and integrated spectroscopic modules to frequency conversion metasurfaces and chip-scale light sources; and discuss their emerging impact in lithography, bioimaging, metrology, and nanofabrication. Finally, we outline the key bottlenecks in dispersion engineering, optical characterization, and wafer-scale manufacturing, emphasizing the interdisciplinary advances required to transition DUVFO from laboratory demonstrations to manufacturable, high-performance photonic platforms.