  Frequency microcombs, successors to mode-locked laser and fiber combs, enable
miniature rulers of light for applications including precision metrology,
molecular fingerprinting, and exoplanet discoveries. To enable the frequency
ruling function, microcombs must be stabilized by locking their carrier-envelop
offset frequency. So far, the microcomb stabilization remains compounded by the
elaborate optics external to the chip, thus evading its scaling benefit. To
address this challenge, here we demonstrate a nanophotonic chip solution based
on aluminum nitride thin films, which simultaneously offer optical Kerr
nonlinearity for generating octave soliton combs and Pockels nonlinearity for
enabling heterodyne detection of the offset frequency. The agile dispersion
control of crystalline III-Nitride photonics permits high-fidelity generation
of solitons with features including 1.5-octave comb span, dual dispersive
waves, and sub-terahertz repetition rates down to 220 gigahertz. These
attractive characteristics, aided by on-chip phase-matched aluminum nitride
waveguides, allow the full determination of the offset frequency. Our
proof-of-principle demonstration represents an important milestone towards
fully-integrated self-locked microcombs for portable optical atomic clocks and
frequency synthesizers.
