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Physical Review

Spectral Properties of a Single-Mode Ruby Laser: Evidence of Homogeneous Broadening of the Zero-Phonon Lines in Solids

Carson L. Tang, H. Statz, G. A. deMars, D. T. Wilson

Published Oct 5, 1964• 53 citations
Publisher

Abstract

According to several recent studies, the width of the sharp zero-phonon lines in the electronic spectra of impurities in solids, such as the $R$ 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 ${R}_{1}$ 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.

DOI
10.1103/physrev.136.a1

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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 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}cm\ensuremath−1 at room temperature as compared to a width of the order of 1 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}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.
Abstract A history of the development of selective laser spectroscopy is presented, beginning with a pioneering work by Yu. V. Denisov and V. A. Kizel in 1967, who were the first to demonstrate the possibility of removing the inhomogeneous broadening of luminescence spectra of impurity ions in glasses upon monochromatic resonance excitation. Selective excitation of optical centers can be achieved due to existence of zero-phonon transitions corresponding to narrow homogeneous zero-phonon lines in the spectra of impurity centers in solids, which are hidden in broad inhomogeneous optical bands upon usual nonselective excitation. The fundamentals of zero-phonon transition spectroscopy are considered and the mechanism of removing the inhomogeneous broadening of optical spectra of ions and molecules in crystals and amorphous solids under selective laser excitation of luminescence and persistent hole burning in absorption spectra is presented in detail. Various applications of selective laser spectroscopy for fundamental and applied studies are discussed.
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.
The low temperature spectra of absorption and luminescence of a wide variety of impurity doped organic and inorganic solids comprise very narrow (io 1O cm1) and intense zero-phonon lines (zPL). They arise from the purely electronic transistions in the impurities i.e. from the changes of the electronic state not accompanied by creation or annihilation of the phonons of the host solid. (ZPL may be called the optical analog of the M5sbauer line), Because of their narrowness ZPL's are very sensitive to even very slight differences in the crystal fields at the 1oca tions of impurities and the ZPL spectruim of a body of impu.rities displays a large inhomogeneous broadening (formula available on paper).
A pure-electronic zero-phonon line, which represents an optical analog of the Mössbauer γ-resonance line, is a cornerstone of three remarkable areas of spectroscopy and optics of impurity solids, namely, ultrahigh-resolution spectroscopy, persistent hole-burning spectroscopy, and spectroscopy of single impurity molecules. In recent experiments made by the photon echo technique, a zero-phonon line width as small as 78 Hz was achieved (with the Q -factor, i.e., the ratio of the transition frequency to the line width being 2×10 14 :78≈ 10 12 ), which is several tenfold lower than the frequency shift of the vibronic spectrum caused by the momentum of the absorbed or emitted photon of optical range. We estimate the position, intensity, and width of zero-phonon lines under conditions when these characteristics, as well as properties of the phonon wing, are determined not only by Stokes losses but also by the photon recoil effect. It is shown that the latter effect, within the approximation used, does not lead to strong and easily detectable phenomena. They, however, take place, and their study is becoming more topical with improving accuracy and sophistication of the experimental technique (in the photon echo configurations, etc.).
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By using far-infrared laser spectroscopy we have studied the temperature dependence of the absorption that is due to transitions between the E ((2)E) and 2A ((2)E) levels of excited Cr(3+) ions in ruby. From the temperature dependence of the linewidth we determined the inhomogeneous and the homogeneous contributions to line broadening. From the homogeneous width the lifetime of the 2A level was obtained. We found that, for emperatures up to 70 K, the lifetime is due mainly to one-phonon relaxation, whereas at higher temperatures another relaxation process, probably a two-phonon Raman process, becomes strong.