  Optical frequency combs enable state-of-the-art applications including
frequency metrology, optical clocks, astronomical measurements, and sensing.
Recent demonstrations of microresonator-based Kerr frequency combs or
microcombs pave the way to scalable and stable comb sources on a photonic chip.
Generating microcombs in the visible wavelength range, however, has been
limited by large material dispersion and optical loss. Here we demonstrate a
scheme for efficiently generating visible microcomb in a high Q aluminum
nitride microring resonator. Enhanced Pockels effect strongly couples infrared
and visible modes into hybrid mode pairs, which participate in the Kerr
microcomb generation process and lead to strong Cherenkov radiation in the
visible band of an octave apart. A surprisingly high conversion efficiency of
22% is achieved from the pump laser to the visible comb. We further demonstrate
a robust frequency tuning of the visible comb by more than one free spectral
range and apply it to the absorption spectroscopy of a water-based dye molecule
solution. Our work marks the first step towards high-efficiency visible
microcomb generation and its utilization, and it also provides insights on the
significance of Pockels effect and its strong coupling with Kerr nonlinearity
in a single microcavity device.
