Strong amplification of sidebands in self-focused laser beams in an atomic vapor

A cw dye-laser beam tuned to the high-frequency wing of the Na D₂ line (at 2-6 GHz from the center of the D₂ line) is focused into a Na vapor, forming a self-focused laser filament. The incident laser beam, which is mainly single frequency, contains very weak sidebands at about 1 GHz from the main frequency and of intensity 10^-3 of the main frequency. Such sidebands are found to be strongly amplified by the vapor. Amplification factors in excess of 100 have been observed at suitable Na density, laser power, and detuning. Such strong sideband amplification, previously unknown, is explained as an off-resonant stimulated Raman scattering process.