  The introduction of nonlinear nanophotonic devices to the field of optical
frequency comb metrology has enabled new opportunities for low-power and
chip-integrated clocks, high-precision frequency synthesis, and broad bandwidth
spectroscopy. However, most of these advances remain constrained to the
near-infrared region of the spectrum, which has restricted the integration of
frequency combs with numerous quantum and atomic systems in the ultraviolet and
visible. Here, we overcome this shortcoming with the introduction of
multi-segment nanophotonic thin-film lithium niobate (LN) waveguides that
combine engineered dispersion and chirped quasi-phase matching for efficient
supercontinuum generation via the combination of $\chi^{(2)}$ and $\chi^{(3)}$
nonlinearities. With only 90 pJ of pulse energy at 1550 nm, we achieve gap-free
frequency comb coverage spanning 330 to 2400 nm. The conversion efficiency from
the near-infrared pump to the UV-Visible region of 350-550 nm is nearly 20%.
Harmonic generation via the $\chi^{(2)}$ nonlinearity in the same waveguide
directly yields the carrier-envelope offset frequency and a means to verify the
comb coherence at wavelengths as short as 350 nm. Our results provide an
integrated photonics approach to create visible and UV frequency combs that
will impact precision spectroscopy, quantum information processing, and optical
clock applications in this important spectral window.
