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International Conference on Luminescence - 1984

Two-Color Superfluorescence of O 2 ` − Centers in KCl

R. Florian, L.O. Schwan, D. Schmid

Published Jan 1, 1984• 1 citation
Publisher

Abstract

If O 2 ` − centers in KCl are excited in their fundamental absorption band (at about 250 nm), they yield a fluorescnece spectrum which at low temperatures is dominated by a progression of very narrow zero-phonon lines (Δν1/2≈1cm−1) with a large Stokes shift (roughly 23000 cm−1 or 2.9 eV). At very high excitation intensities (≳ 20 GW/cm2) with single pulses of the fourth harmonic of a modelocked Nd-YAG laser (λ = 266 nm, pulse duration 30 ps, pulse energy ~ 100 μJ) and at low temepratures (T < 30 K) one observes a simultaneous coherent emission at the wavelength of two of the zero-phonon lines (0-10 and 0-11 transition at 594.77 and 629.14 nm, respectively) /1/, which can be identified as two-color superfluorescence fron a three-level system /2,3/.

DOI
10.1364/icol.1984.mb38

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If O 2 ` − centers in KCl are excited in their fundamental absorption band (at about 250 nm), they yield a fluorescnece spectrum which at low temperatures is dominated by a progression of very narrow zero-phonon lines (Δν1/2≈1cm−1) with a large Stokes shift (roughly 23000 cm−1 or 2.9 eV). At very high excitation intensities (≳ 20 GW/cm2) with single pulses of the fourth harmonic of a modelocked Nd-YAG laser (λ = 266 nm, pulse duration 30 ps, pulse energy ~ 100 μJ) and at low temepratures (T < 30 K) one observes a simultaneous coherent emission at the wavelength of two of the zero-phonon lines (0-10 and 0-11 transition at 594.77 and 629.14 nm, respectively) /1/, which can be identified as two-color superfluorescence fron a three-level system /2,3/.
Single crystals of KCl containing $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ centers (8\ifmmode\times\else\texttimes\fi{}1016{10}^{16}1016-7\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 ${\mathrm{cm}}^{\ensuremath{-}3}$) were excited at low temperatures (T<30T<30T<30 K) using frequency-quadrupled single pulses of a mode-locked Nd-doped yttrium aluminum garnet laser. At excitation intensities above 20 GW/cm2{\mathrm{cm}}^{2}cm2 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 $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ 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 ($0.016\mathrm{nm}\ensuremath{\cong}14\mathrm{GHz}$) 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 $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ concentration (8 to 28 K for concentrations between 8\ifmmode\times\else\texttimes\fi{}1016{10}^{16}1016 and 7\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 ${\mathrm{cm}}^{\ensuremath{-}3}$).
In the light of the creation of a diamond laser [1], an urgent task is to determine the characteristics of diamonds in order to determine the range in which such lasers can be created not in isolated cases. This work is aimed at studying the mechanisms of creating superluminescence in diamond under the action of optical pumping by the second harmonic of an ND:YAG laser (λ = 532 nm). It was found that, upon pulsed excitation of a diamond by an ND:YAG laser with a pump intensity above ~2.0 MW/cm2 in the spectral range 700–750 nm, a nonlinear increase in intensity appears against the background of the spontaneous photoluminescence spectrum, which, with a further increase in the pump intensity, turns into a pronounced superluminescence peak with a maximum at about 718 nm. An increase in the pump intensity from 2.7 to 46 MW/cm2leads to the broadening of this peak at half maximum from 13 to 19 nm. At high levels of pump intensity, nonlinear absorption of pump radiation and accumulation of NV centers in the excited state were found. The position of the photoluminescence band was calculated as a function of various values of the inversion density of the populations of color centers, taking into account the intrinsic absorption spectrum of diamond. The calculation results are close to the experimental data.