OBSERVATION OF GAIN DUE TO COHERENCE EFFECTS IN A POTASSIUM-HELIUM MIXTURE

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