Raman-assisted coherent, mid-infrared frequency combs in silicon   microresonators
  We demonstrate the first low-noise mid-IR frequency comb source using a silicon microresonator. Our observation of strong Raman scattering lines in the generated comb suggests that Raman and four-wave mixing interactions play a role in assisting the transition to the low-noise state. In addition, we characterize, the intracavity comb generation dynamics using an integrated PIN diode, which takes advantage of the inherent three-photon absorption process in silicon. 
Austin G. Griffith, Mengjie Yu, Yoshitomo Okawachi, Jaime Cardenas,   Aseema Mohanty, Alexander L. Gaeta, and Michal Lipson
physics.optics