Two-photon-excited stimulated emission from atomic oxygen in rf plasmas: detection and estimation of its threshold

The two-photon-excited stimulated emission from the atomic-oxygen 33P → 33S transition is observed in a discharge environment and its threshold is determined by means of a calibration technique for the two-photon allowed laser-induced fluorescence (TALIF) signal. For the atomic oxygen concentrations and the laser powers used in our experiments, we found a relatively constant threshold, Nthr = (2.5 ± 0.4) × 1014 atoms/cm3, for the stimulated emission. The reduction of the laser-induced fluorescence signal due to rapid depletion of the laser-excited level by stimulated emission did not exceed 30%, up to O-atom concentrations 3Nthr.