Inverse Raman scattering in silicon: A free-carrier enhanced effect

Stimulated Raman scattering has been harnessed to produce the first silicon lasers and amplifiers. The Raman effect can also produce intensity-dependent nonlinear loss through a corollary process, inverse Raman scattering (IRS). This process has never been observed in a semiconductor. We demonstrate IRS in silicon---a process that is substantially modified by optically generated free carriers---achieving attenuation levels $g15\text{ }\text{dB}$ with a pump intensity of $4\text{ }\text{GW}/{\text{cm}}^{2}$. Surprisingly, free-carrier absorption, the detrimental effect that generally suppresses nonlinear effects in silicon, actually facilitates IRS by delaying the onset of contamination from coherent anti-Stokes Raman scattering. Silicon-based IRS could be a valuable tool for chip-scale signal processing.