Efficient Raman sideband generation in a coherent atomic medium

We demonstrate the efficient generation of Raman sidebands in a medium coherently prepared in a dark state by continuous-wave low-intensity laser radiation. Our experiment is performed in sodium vapor excited in Λ configuration on the D₁ line by two laser fields of resonant frequencies ω₁ and ω₂, and probed by a third field ω₃. First-order sidebands for frequencies ω₁, ω₂ and up to the third-order sidebands for frequency ω₃ are observed. The generation starts at a power as low as 10 {μ}W for each input field. Dependences of the intensities of both input and generated waves on the frequency difference (ω₁-ω₂), on the frequency ω₃, and on the optical density are investigated.