Development of waveguide-based far-ultraviolet wavelength conversion devices made of nitride semiconductor

This review systematically maps the progress of waveguide-based wavelength converters, focusing on the critical push toward the far-ultraviolet band. To overcome material limitations, we propose utilizing the strong optical nonlinearity in nitride semiconductors via precise polarization control. This approach enables the novel transverse quasi-phase-matching structure, superior for short wavelengths. We present the world-first 229 nm emission from a polarity-inverted AlN waveguide, as well as several ways of wavelength conversion free from polarity inversion. These breakthroughs pave the way for compact, high-efficiency light sources and photonic integrated circuit platforms.