Abstract

When nonlinear four-wave mixing (FWM) is resonant with a Raman transition, it causes coupling between lightwaves separated by two Raman frequency shifts. This coupling is manifested in effects such as Raman gain suppression, higher-order Stokes generation, and coherent Raman spectroscopies (e.g., CARS). An understanding of these effects, however, has been impeded by a widespread misconception that Raman-resonant FWM does not transfer energy between the lightwaves and the medium. The equations for the photon densities at each light frequency show that, in actuality, two medium excitations are created or destroyed for each photon produced by FWM. This point may be further clarified by consideration of the driving nonlinear polarization and by comparisons with broadband Raman conversion and nonresonant FWM. The Raman gain suppression effect may be understood qualitatively in terms of destructive interference between FWM and the usual stimulated Raman process.

© 1987 Optical Society of America