  Expanding far beyond traditional applications in optical interconnects at
telecommunications wavelengths, the silicon nanophotonic integrated circuit
platform has recently proven its merits for working with mid-infrared (mid-IR)
optical signals in the 2-8 {\mu}m range. Mid-IR integrated optical systems are
capable of addressing applications including industrial process and
environmental monitoring, threat detection, medical diagnostics, and free-space
communication. Rapid progress has led to the demonstration of various silicon
components designed for the on-chip processing of mid-IR signals, including
waveguides, vertical grating couplers, microcavities, and electrooptic
modulators. Even so, a notable obstacle to the continued advancement of
chip-scale systems is imposed by the narrow-bandgap semiconductors, such as
InSb and HgCdTe, traditionally used to convert mid-IR photons to electrical
currents. The cryogenic or multi-stage thermo-electric cooling required to
suppress dark current noise, exponentially dependent upon the ratio Eg/kT, can
limit the development of small, low-power, and low-cost integrated optical
systems for the mid-IR. However, if the mid-IR optical signal could be
spectrally translated to shorter wavelengths, for example within the
near-infrared telecom band, photodetectors using wider bandgap semiconductors
such as InGaAs or Ge could be used to eliminate prohibitive cooling
requirements. Moreover, telecom band detectors typically perform with higher
detectivity and faster response times when compared with their mid-IR
counterparts. Here we address these challenges with a silicon-integrated
approach to spectral translation, by employing efficient four-wave mixing (FWM)
and large optical parametric gain in silicon nanophotonic wires.
