  Many physical systems display quantized energy states. In optics, interacting
resonant cavities show a transmission spectrum with split eigenfrequencies,
similar to the split energy levels that result from interacting states in
bonded multi-atomic, i.e. molecular, systems. Here, we study the nonlinear
dynamics of photonic diatomic molecules in linearly coupled microresonators and
demonstrate that the system supports the formation of self-enforcing solitary
waves when a laser is tuned across a split energy level. The output corresponds
to a frequency comb (microcomb) whose characteristics in terms of power
spectral distribution are unattainable in single-mode (atomic) systems.
Photonic molecule microcombs are coherent, reproducible, and reach high
conversion efficiency and spectral flatness whilst operated with a laser power
of a few milliwatts. These properties can favor the heterogeneous integration
of microcombs with semiconductor laser technology and facilitate applications
in optical communications, spectroscopy and astronomy.
