  Stimulated Raman scattering is a well-known nonlinear process that can be
harnessed to produce optical gain in a wide variety of media. This effect has
been used to produce the first silicon-based lasers and high-gain amplifiers.
Interestingly, the Raman effect can also produce intensity-dependent nonlinear
loss through a corollary process known as inverse Raman scattering (IRS). Here,
we demonstrate IRS in silicon--a process that is substantially modified by the
presence of optically-generated free carriers--achieving attenuation levels >15
dB with a pump intensity of 4 GW/cm^2. Ironically, we find that free-carrier
absorption, the detrimental effect that suppresses other nonlinear effects in
silicon, actually facilitates IRS by delaying the onset of contamination from
coherent anti-Stokes Raman scattering. The carriers allow significant IRS
attenuation over a wide intensity range. Silicon-based IRS could be used to
produce chip-scale wavelength-division multiplexers, optical signal inverters,
and fast optical switches.
