Ultra-high-Q UV microring resonators based on a single-crystalline AlN platform

Development of low-loss photonic components in the ultraviolet (UV) band will open new prospects for classical and quantum optics. Compared with other integrated platforms, aluminum nitride (AlN) is particularly attractive as it features an enormous bandgap of ∼6.2 eV and intrinsic χ² and χ³ susceptibilities. In this work, we demonstrate a record quality factor of 2.1 × 10⁵ (optical loss ∼8 dB∕cm) at 390 nm based on single-crystalline AlN microrings. The low-loss AlN waveguide represents a significant milestone toward UV photonic integrated circuits as it features full compatibility for future incorporation of AlGaN-based UV emitters and receivers. On-chip UV spectroscopy, nonlinear optics, and quantum information processing can also be envisioned.