  Optical parametric oscillation (OPO) is distinguished by its wavelength
access, that is, the ability to flexibly generate coherent light at wavelengths
that are dramatically different from the pump laser, and in principle bounded
solely by energy conservation between the input pump field and the output
signal/idler fields. As society adopts advanced tools in quantum information
science, metrology, and sensing, microchip OPO may provide an important path
for accessing relevant wavelengths. However, a practical source of coherent
light should additionally have high conversion efficiency and high output
power. Here, we demonstrate a silicon photonics OPO device with unprecedented
performance. Our OPO device, based on the third-order ($\chi^{(3)}$)
nonlinearity in a silicon nitride microresonator, produces output signal and
idler fields widely separated from each other in frequency ($>$150 THz), and
exhibits a pump-to-idler conversion efficiency up to 29 $\%$ with a
corresponding output idler power of $>$18 mW on-chip. This performance is
achieved by suppressing competitive processes and by strongly overcoupling the
output light. This methodology can be readily applied to existing silicon
photonics platforms with heterogeneously-integrated pump lasers, enabling
flexible coherent light generation across a broad range of wavelengths with
high output power and efficiency.
