  Laser frequency combs are enabling some of the most exciting scientific
endeavours in the 21st century, ranging from the development of optical clocks
to the calibration of the astronomical spectrographs used for searching
Earth-like exoplanets. Today, dissipative Kerr solitons generated in
microresonators offer the prospect of attaining frequency combs in miniaturized
systems by capitalizing on advances in photonic integration. Most of the
applications based on soliton microcombs rely on tuning a continuous-wave laser
into a longitudinal mode of a microresonator whose dimensions are engineered to
display anomalous dispersion at the pump laser frequency. In this
configuration, however, nonlinear physics precludes from attaining dissipative
Kerr solitons with high power conversion efficiency, with typical comb powers
amounting to ~1% of the available laser power. Here, we demonstrate that this
fundamental limitation can be overcome by inducing a controllable frequency
shift to a selected cavity resonance. Experimentally, we realize this shift
using two linearly coupled anomalous-dispersion microresonators (a photonic
molecule), resulting in a coherent dissipative Kerr soliton with a conversion
efficiency exceeding 50% and excellent line spacing stability. We describe the
physical soliton dynamics in this configuration, and discover the system
displays unusual characteristics, such as the possibility to backwards initiate
solitons and stable operation with a blue detuned pump laser. By optimizing the
microcomb power available on chip, these results facilitate the practical
implementation of a scalable integrated photonic architecture for
energy-efficient applications.
