  Many uses of lasers place the highest importance on access to specific
wavelength bands. For example, mobilizing optical-atomic clocks for a leap in
sensing requires compact lasers at frequencies spread across the visible and
near infrared. Integrated photonics enables high-performance, scalable laser
platforms, however, customizing laser-gain media to support wholly new bands is
challenging and often prohibitively mismatched in scalability to early
quantum-based sensing and information systems. Here, we demonstrate a
microresonator optical-parametric oscillator (OPO) that converts a pump laser
to an output wave within a frequency span exceeding an octave. We achieve phase
matching for oscillation by nanopatterning the microresonator to open a
photonic-crystal bandgap on the mode of the pump laser. By adjusting the
nanophotonic pattern and hence the bandgap, the ratio of output OPO wave
frequency span to required pump laser tuning is more than 10,000. We also
demonstrate tuning the oscillator in free-spectral-range steps, more finely
with temperature, and minimal additive frequency noise of the laser-conversion
process. Our work shows that nanophotonics offers control of laser conversion
in microresonators, bridging phase-matching of nonlinear optics and application
requirements for laser designs.
