Beyond 100 THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide

Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing the electronic transitions of atoms and molecules. Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in the ultraviolet region. Here we demonstrate a simple route to chip-scale ultraviolet comb generators, simultaneously showing a gap-free frequency span of ~128 terahertz and supercontinuum sourced by an ultrafast fiber laser. The simultaneous cubic and quadratic nonlinear processes are implemented in single-crystalline aluminum nitride thin films, where a chirp-modulated taper waveguide is patterned to ensure a broad phase matching. The heterodyne beatnote characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain a high degree of coherence. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.