Time-resolving experiments on Dicke superfluorescence ofO2−centers in KCl. Two-color superfluorescence

Single crystals of KCl containing $\mathrm{O}_2^{}{}_{}{}^{-}$ centers (8×${10}^{16}$-7×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$) were excited at low temperatures ($T<\mathrm{}30\mathrm{}$ K) using frequency-quadrupled single pulses of a mode-locked Nd-doped yttrium aluminum garnet laser. At excitation intensities above 20 GW/${\mathrm{cm}}^2$ a spontaneous emission of coherent light was observed which could be identified as Dicke superfluorescence. This superfluorescence was observed on two of the zero-phonon lines in the vibrational progression of the $\mathrm{O}_2^{}{}_{}{}^{-}$ fluorescence spectrum (0-10 and 0-11 at 592.8 and 629.1 nm, respectively). We have studied the time dependence, the statistics of the delay times, and the intensities. The observed delay times vary between 0.5 and 10 ns, the observed pulse widths between 0.5 and 6 ns, and the coherence time was found to be longer than 100 ps. The observed linewidths of the superfluorescence ($0.016\mathrm{nm}\cong 14\mathrm{GHz}$) is a factor of 2.5 smaller than that of the spontaneous fluorescence (0.042 nm at 4.2 K). The superfluorescence is observed only below a critical temperature which depends on the $\mathrm{O}_2^{}{}_{}{}^{-}$ concentration (8 to 28 K for concentrations between 8×${10}^{16}$ and 7×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$).