  The on-chip creation of coherent light at visible wavelengths is crucial to
field-level deployment of spectroscopy and metrology systems. Although on-chip
lasers have been implemented in specific cases, a general solution that is not
restricted by limitations of specific gain media has not been reported. Here,
we propose creating visible light from an infrared pump by widely-separated
optical parametric oscillation (OPO) using silicon nanophotonics. The OPO
creates signal and idler light in the 700 nm and 1300 nm bands, respectively,
with a 900 nm pump. It operates at a threshold power of (0.9 +/- 0.1) mW, over
50x smaller than other widely-separated microcavity OPO works, which have only
been reported in the infrared. This low threshold enables direct pumping
without need of an intermediate optical amplifier. We further show how the
device design can be modified to generate 780 nm and 1500 nm light with a
similar power efficiency. Our nanophotonic OPO shows distinct advantages in
power efficiency, operation stability, and device scalability, and is a major
advance towards flexible on-chip generation of coherent visible light.
