  The ability to spectrally translate lightwave signals in a compact, low-power
platform is at the heart of the promise of nonlinear nanophotonic technologies.
For example, a device to link the telecommunications band with visible and
short near-infrared wavelengths can enable a connection between
high-performance chip-integrated lasers based on scalable nanofabrication
technology with atomic systems used for time and frequency metrology. While
second-order nonlinear (\chi^(2)) systems are the natural approach for bridging
such large spectral gaps, here we show that third-order nonlinear (chi^(3))
systems, despite their typically much weaker nonlinear response, can realize
spectral translation with unprecedented performance. By combining resonant
enhancement with nanophotonic mode engineering in a silicon nitride microring
resonator, we demonstrate efficient spectral translation of a continuous-wave
signal from the telecom band (~ 1550 nm) to the visible band (~ 650 nm) through
cavity-enhanced four-wave mixing. We achieve such translation over a wide
spectral range >250 THz with a translation efficiency of (30.1 +/- 2.8) % and
using an ultra-low pump power of (329 +/- 13) uW. The translation efficiency
projects to (274 +/- 28) % at 1 mW and is more than an order of magnitude
larger than what has been achieved in current nanophotonic devices.
