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Journal of Applied Physics

Studies of Ruby Superfluorescence and Population Inversion

P. V. Avizonis, William R. Willoughby

Published Feb 1, 1966• 18 citations
Publisher

Abstract

It was found that by observing the fluorescent intensity of ruby along its longitudinal axis during pumping, an appreciable amount of information may be obtained. It was observed that the time at which population inversion occurs during a pump cycle is directly observable from the time-resolved fluorescent intensity. A method for establishing the actual population in the metastable state from the fluorescent intensity is demonstrated and cross correlated with two other methods for measuring population inversion. The rate equation describing population inversion has been solved including the nonlinear stimulated emission loss term, and complete agreement with experiment is demonstrated. For the particular pumping geometry used, maximum energy available, and ruby quality and size, maximum metastable population of 1.21×1019 ions/cm3 was achieved, representing 75% inversion.

DOI
10.1063/1.1708237

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It was found that by observing the fluorescent intensity of ruby along its longitudinal axis during pumping, an appreciable amount of information may be obtained. It was observed that the time at which population inversion occurs during a pump cycle is directly observable from the time-resolved fluorescent intensity. A method for establishing the actual population in the metastable state from the fluorescent intensity is demonstrated and cross correlated with two other methods for measuring population inversion. The rate equation describing population inversion has been solved including the nonlinear stimulated emission loss term, and complete agreement with experiment is demonstrated. For the particular pumping geometry used, maximum energy available, and ruby quality and size, maximum metastable population of 1.21×1019 ions/cm3 was achieved, representing 75% inversion.
Depopulation of ground-state Cr ions in ruby (DGS) under optical pumping with a Xe flash light has been measured as a function of time. Experiments have been made by means of measuring the decrease in absorbing power for a probe light (5790 Å or 5461 Å) from a Hg lamp. DGS reaches its maximum value after some delay from the peak of Xe light, and decreases after discontinuation of optical pumping with a time constant of 3.4 ms, which is the lifetime of metastable-state 2E. During laser oscillation mean value of DGS is forced to take a constant value, which varies with the condition of laser oscillation. So long as the input energy to the Xe lamp is not altered, the shorter the duration of Xe flash light is, the higher becomes the maximum value of DGS. On the basis of the rate equation, these experimental results have been explained.
Superfluorescence and amplified spontaneous emission of 29-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonons are observed following inversion of the E\ifmmode\bar\else\textasciimacron\fi{}(2{(}^{2}(2E)-2A\ifmmode\bar\else\textasciimacron\fi{}(2{(}^{2}(2E) acoustic transition of Cr3+{\mathrm{Cr}}^{3+}Cr3+ in ruby. The conditions for both of these manifestations of phonon avalanches are selected via the Cr3+{\mathrm{Cr}}^{3+}Cr3+ concentration. Superfluorescence is described in terms of a pendulum equation of the acoustic Bloch vector. Rate equations of the level and phonon populations account for amplified spontaneous emission.
The Ē-level population of a cylindrical ruby rod is experimentally measured as a function of the pump energy at two points of the cross section of the rod, namely, the center and the periphery. The measurement is performed by measuring the fluorescent energy emitted after pumping by the points considered. The rod is pumped by a spiral flashtube. It is found that the experimental data of population vs pumping follow exponential curves as expected theoretically. From these curves the pump energy necessary for equalizing the populations between ground state and laser level is obtained in a straightforward way. The knowledge of this quantity leads to the measure of the losses (scattering plus diffraction) per pass in the rod. A value of ∼45% of losses is obtained for our rod.
Optical absorption cross sections and the fluorescent quantum efficiency in ruby have been determined. This data has been used to correlate calculations with the analysis of the preceding paper. Stimulated emission from ruby under pulsed excitation has been studied in some detail; the observations are found to depend strongly on the perfection of the particular crystal under study. A peak power output of approximately 5 kw, total output energy of near 1 joule, beam collimation of less than ${10}^{\ensuremath{-}2}$ rad, and a spectral width of individual components in the output radiation of about 6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$A at 6943 A have been measured. It is suggested that mode instabilities due to temperature shifts and a time-varying magnetic field are contributing to an oscillatory behavior of the output pulse.
Time-resolved measurements of fluorescence and level populations have been made on barium vapour illuminated by an intense pulse of laser radiation (1 MW cm -2 , 50 ns FWHM) tuned to the resonance transition of Ba I, 6s 2 1 S 0 -6s6p 1 P 1 , at 5535 AA. Barium number densities between 10 13 -10 15 cm -3 were investigated, using a buffer gas of helium at 100 Torr. Fluorescence was measured with two different geometrical arrangements, one allowing partial spatial resolution. In neither case did the observed fluorescence saturate with increasing laser power, although the maximum flux was more than sufficient to burn through the vapour column, as confirmed by transmission measurements. The power loss from the laser beam was found to be compatible with the scattered radiation from the pumped column in the absence of radiation trapping. In contrast to other similar experiments, no significant ionisation was detected (<5%). Time-resolved population measurements by anomalous dispersion on the 6s6p 3 P 2 and 6s5d 1 D 2 metastable levels yielded quenching cross sections for both Ba-Ba and Ba-He collisions, but gave anomalous result for the peak populations. A qualitative explanation of the fluorescence anomaly is proposed on the basis of the AC Stark effect, and it is shown that the same effect can inhibit the onset of stimulated emission to the metastable D levels.
It is shown that the photoionization of the inner-shell electrons can yield metastable ions. Therefore, the accumulation of excited particles becomes possible at practically realizable optical pumping powers. This method of population inversion may be effective because a large part of the emission spectrum of an optically transparent plasma in a radiation source is in the form of "hard" photons, which are emitted when electrons are captured and atoms or ions go over to their ground state.