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
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.
In this work, we study the relation between the widths of the zero-phonon lines (ZPL) of single impurity molecules (SM) in solid matrices with local effective values of the refractive index n. Since at ultralow temperatures the width of SM ZPL is determined mainly by the radiative lifetime T1, while the local-field effects produce the function T1(n), SM spectroscopy makes it possible to measure the distribution of n local values in solids. The approach proposed here for studying local fluctuations of n has been tested on the example of doped polymers and molecular crystals.