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Applied Spectroscopy

Fluorescence of Ruby Laser Rods

Milton Birnbaum

Published Nov 1, 1987• 0 citations
Publisher

Abstract

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.

DOI
10.1366/0003702874447059

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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.
Fluorescence-line-narrowing and time-resolved studies have been conducted in ruby crystals and powders, in concentrations varying from 0.025 to 0.9 at.%. The resonance fluorescence component of the R1{R}_{1}R1​ line at low temperatures is observed to increase dramatically as the laser is tuned from line center toward the low-energy wing of the inhomogeneously broadened absorption. This is presumed to indicate the presence of weakly coupled pairs in the wings of the line. Intraline and interline (${R}_{1}\ensuremath{\rightarrow}{N}_{1,2}$) energy transfer have also been studied, and an elaboration of earlier work is given. Time-resolved selective-excitation scans have revealed numerous channels in the vicinity of the RRR lines by which the different pairs can be directly excited. These are assumed to be Cr ions near enough to a pair to be in a highly perturbed environment. Various experimental results are shown to be consistent with a model of rapid transfer between resonant Cr ions which feeds the pairs via a weak, nonresonant coupling to these perturbed ions. Macroscopic strain broadening is also shown to be favored over microscopic broadening, and the implications for Anderson localization in ruby are discussed.
A large number of small but sharp changes (increases and decreases) of the R 1 fluorescence in ruby are observed as a function of applied magnetic field between 2 K and 77 K. At liquid helium temperatures, these signals have linewidths much narrower than the inhomogeneous linewidth (∼0.1 cm -1 ) of the R 1 line. All of these signals appear at the fields where the Zeeman splittings in the excited state \bar E ( 2 E ) and the ground state 4 A 2 satisfy an energy-matching condition for simultaneous transitions of two or more ions. Two distinct mechanisms are found to be responsible for the signals: spin-spin cross-relaxation between \bar E ( 2 E ) and 4 A 2 , and resonant self-trapping of the R 1 line. From the temperature dependence of the linewidths of the signals, thermal broadening of \bar E ( 2 E ) is measured below 77 K. Similar signals are observed in the N lines due to energy transfer from isolated ions to pairs.
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.
Laser-induced fluorescence line narrowing has been observed for the first time in a nonresonantly excited rare-earth system. Excitation with the 4727-\AA{} A+{A}^{+}A+ line into the 4G112^{4}G_{\frac{11}{2}}4G211​​ level of Nd3+{\mathrm{Nd}}^{3+}Nd3+ in glass results in narrowing of transitions from the 4F32^{4}F_{\frac{3}{2}}4F23​​ to both the 4I92^{4}I_{\frac{9}{2}}4I29​​ ground manifold and the 4I112^{4}I_{\frac{11}{2}}4I211​​ excited manifold. The concentration dependence of the line narrowing provides information regarding resonant ion-ion energy transfer within the inhomogeneously broadened system.
The unique optical properties of the ruby crystal that make it effective as a lasing medium were measured using a simple optical setup.Ruby's absorption of visible light was shown to be strongest at 420 and 550 nm, corresponding to its physical appearance as transparent pink.While the ruby crystal was illuminated with a 532 nm green laser, the R-line fluorescence peaked at 693.5 ± 1.1 nm.Finally, the fluorescence lifetime of ruby was measured by pulsing the laser with a signal generator and capturing the decaying fluorescence on a digital oscilloscope using a photodiode.An exponential decay time-constant of 3.6 ± 0.1 ms was obtained, corresponding to the fluorescence lifetime.