  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.
