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Journal of Physics Condensed Matter

Self-absorption and phonon-assisted radiative energy transfer processes in a ruby laser rod

X X Zhang, P. Hong, Baldassare Di Bartolo, Charles W. Struck, Michael A. Bass

Published Jun 20, 1994• 3 citations
Publisher

Abstract

Systematic investigations of the effect of self-absorption on the dynamic and steady-state spectroscopic properties in ruby have been carried out at different temperatures (from approximately 130 to 500 K) using a long, pink laser rod. At about 140 K,the lengthening of decay times of the R lines reached a maximum and then decreased with increasing geometric trapping. Spectral diffusion as large as 10 cm -1 was observed in the complete-trapping regime. These observations suggest reexamination of the theoretical model for phonon-assisted radiative energy transfer in ruby.

DOI
10.1088/0953-8984/6/25/005

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International Laser Science Conference•Jan 1, 1986•0 citations
Systematic investigations of the effect of self-absorption on the dynamic and steady-state spectroscopic properties in ruby have been carried out at different temperatures (from approximately 130 to 500 K) using a long, pink laser rod. At about 140 K,the lengthening of decay times of the R lines reached a maximum and then decreased with increasing geometric trapping. Spectral diffusion as large as 10 cm -1 was observed in the complete-trapping regime. These observations suggest reexamination of the theoretical model for phonon-assisted radiative energy transfer in ruby.
The first experimental evidence for distinct spectral and spatial energy transfer in ruby via nonradiative coupling is presented. Monochromatic laser excitation of the R1{R}_{1}R1​ line results in a rise of the inhomogeneous background on a millisecond time scale, consistent with a one-phonon---assisted process and contrary to recent theory. However, time-resolved studies of trap fluorescence suggest a rapid, coherent, spatial transfer within the R1{R}_{1}R1​ line which does not shift frequency.
The fluorescent decay of the 2E level of Cr3+ over the range of 77 to 300 K has been studied in experimental arrangements providing strong trapping of the O-phonon lines. At 300 K, rubies as large as 2.5-cm diameter by 33-cm length were used and were packed in highly reflective BaSO4 powder to provide adequate trapping of the O-phonon fluorescence. The measurements resulted in the determination explicitly of the lifetime vibronic transitions and the O-phonon decay rate. These studies of lifetime enhancement have potential application in the design of efficient solid-state lasers.
Fluorescence studies of the R-line emission of ruby laser rods over the temperature range of 77 to 300 K have disclosed several new features. In particular, for most of the ruby rods, the fluorescent decay time, τ, was not strictly exponential. Effects of this kind have been observed in the fluorescent decay of the 4F3/2 level of Nd3+ in YAG (yttrium aluminum garnet) and in Nd-doped glasses. In both cases, the nonexponential decay was interpreted as evidence for inhomogeneous line broadening effects, among which are strains, ion clustering, and other defects that modify the crystal field at the ion site. Our observations of nonexponential fluorescent decay in ruby are indicative of inhomogeneous line broadening effects in lightly doped (0/05%) ruby. In fact, inhomogeneous line broadening was observed in ruby at liquid helium temperatures, in the spectroscopic studies of A. L. Schawlow.
The radiative-transfer rate for inhomogeneously broadened optical lines is calculated. Photon emission and absorption is supplemented by one- and two-phonon processes to achieve energy conservation. These photon-assisted radiative transfer processes are shown to be effective at low concentrations, where electrostatic and exchange processes are weak, in the intermediate or strong radiative trapping regime. Specific application is made to the case of dilute ruby, and explicit transfer rates are calculated. The coherence factors, which undermine nonradiative single-phonon-assisted energy transfer for a small energy mismatch, are not important for radiative transfer, if the photon mean free path is longer than the wavelength of the energy matching phonon. The one-phonon process is found to vary linearly with temperature, and to be independent of the energy mismatch. However, the calculated rate for the case of ruby is found to be very slow. The two-phonon-assisted radiative-transfer process rates are calculated. They dominate the one-phonon-assisted rate for all reasonable temperatures. The results of an earlier paper are shown to be related to phonon-assisted radiative transfer if one replaces the exchange coupling between transfer sites JJJ by $\ensuremath{\sim}\frac{{\ensuremath{\tau}}_{R}^{\ensuremath{-}1}\ensuremath{\hbar}}{(\frac{{r}_{0}}{\ensuremath{\lambda}})}$. Here ${\ensuremath{\tau}}_{R}$ is the radiative lifetime, r0{r}_{0}r0​ the distance between sites, and $\ensuremath{\lambda}$ the photon wavelength involved in the radiative transfer. This replacement results in a line center to wing transfer rate equal to 1.1 \ifmmode\times\else\texttimes\fi{} 102{10}^{2}102 ${e}^{\ensuremath{-}\frac{42}{T}}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ for ruby (independent of concentration for a photon mean free path smaller than the sample size).
Several experiments on optically excited ruby are described. First, absorption transitions initiated from the metastable states,t\min{2}\max{3}2Eandt\min{2}\max{3}2T1, are investigated in the photon energy range from 5500 to 45 000 cm-1. The absorption spectrum of these excited states is determined in both\pi-and σ-polarizations and assignments for the observed absorption bands are achieved. Obtained results are compared with other experiments reported in the literature. Next, emission properties of pink ruby are studied in the region near theRlines. The emission spectrum is determined point by point by means of intense flash excitation. The observed weak bands are ascribed to theR'and phonon assistedRtransitions. Laser actions in these bands are discussed and experimental results on phonon terminated laser amplification are presented. An experiment using a giant pulse as an excitation source reveals that the transition time fromt\min{2}\max{3}2Etot\min{2}\max{3}2T1is shorter than a fraction of 1μs at room temperature.