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Journal of the Physical Society of Japan

Transient Absorption and Homogeneous Line Broadening of Ruby

Takasi Endo, 孝夫 村本

Published Jan 15, 1988• 0 citations
Publisher

Abstract

Transient absorption of the ruby R 1 line was measured as a function of time t . Although the optical Bloch equations predict that the transient absorption A ( t ) is asymptotically proportional to t -1/2 , it was found that A ( t ) is asymptotically proportional to t -1 . This discrepancy is explained by a truncation of the homogenous line shape at the line wings.

DOI
10.1143/jpsj.57.302

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Transient absorption of the ruby R 1 line was measured as a function of time t . Although the optical Bloch equations predict that the transient absorption A ( t ) is asymptotically proportional to t -1/2 , it was found that A ( t ) is asymptotically proportional to t -1 . This discrepancy is explained by a truncation of the homogenous line shape at the line wings.
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.
Following a brief review of laser frequency and time domain techniques for overcoming inhomogeneous broadening of optical transitions in solids, recent studies on the failure of the optical Bloch equations (OBE) in describing saturation and coherent transient phenomena in ruby will be discussed. Experiments using free induction decay (FID), as in the original work on the system Pr3+:LaF3 by DeVoe and Brewer1, as well as extensions to rotary echos2 and hole-burning3 will be described. At low fields (~ 0.4 T), the time and frequency domain linewidth variation with Rabi frequency agree provided care is taken to avoid line broadening caused by spectral diffusion4. Recent work at high fields (~ 5 T), where Cr-Cr electron spin flipping is suppressed5, shows that spectral diffusion decreases and the dephasing time increases. Observed frequency domain hole widths as narrow as 18 kHz (FWHM) have been obtained for the 4 A2(−1/2)→E¯(−1/2) optical transition in dilute ruby using a pump-probe technique4. Work with such narrow holes emphasizes the fundamental limits imposed by the uncertainty principle and as an aside, a thought experiment where single photons are transmitted through "frequency slits" will be discussed.
Recent studies of the failure of the optical Bloch equations (OBE) in describing optical saturation in a solid are extended to test another aspect of the OBE, namely the time dependence of free-induction decay (FID) predicted by the SWB theorem on optical transients [Schenzle, Wong, and Brewer, Phys. Rev. A 22, 635 (1980)]. This theorem states that, following an arbitrarily shaped excitation pulse of length T, the FID will vanish at time T following the end of the excitation pulse. Experiments with ruby violate SWB in that FID decay at least twice as long as the excitation pulse is observed. Numerical and analytical calculations using a Gauss-Markov model for the modified OBE show an extended FID decay, however, the theoretical decay shape does not agree with experiment. Thus as with earlier studies on saturation broadening, we find that the modified OBE provides only a qualitative description of radiation-atom interaction in a solid.
According to several recent studies, the width of the sharp zero-phonon lines in the electronic spectra of impurities in solids, such as the RRR lines of ruby, is due to Raman scattering of phonons by the impurity ions. The question whether such zero-phonon lines are homogeneously broadened is an important one, both for its own interest and for optical-maser applications. From a detailed study of the stimulated-emission spectra of a ruby laser operating in the traveling-wave modes, we obtain conclusive evidence that such zero-phonon lines are homogeneously broadened. Because the R1{R}_{1}R1​ line of ruby is essentially homogeneously broadened at room temperature, the traveling-wave laser oscillates for all practical purposes only in a single mode. The spectral width of the laser emission is extremely sharp. It is less than 0.005 ${\mathrm{cm}}^{\ensuremath{-}1}$ at room temperature as compared to a width of the order of 1 ${\mathrm{cm}}^{\ensuremath{-}1}$ for conventional ruby lasers of comparable intensity. In addition, the output intensity no longer shows random spikes but shows a predictable train of regularly spaced spikes which agree well with theory.