Observation of gain due to coherence effects in a potassium-helium mixture

We report steady-state gain due to coherence effects in a potassium-helium mixture, for strong pumping near the D₂ line and probing near the D₁ line. The absorption-gain spectra are in good agreement with a four-level Raman-driven, lasing with inversion model, where collisional transfer populates the probed excited state. Using parameters corresponding to the experimental conditions, the model predicts gain without bare-state population inversion by a large margin.