Ultranarrow Nonlinear Optical Resonances in Solids

Abstract

Using a method for nonlinear laser spectroscopy which eliminates the effects of laser jitter in nearly degenerate four-wave mixing, we present the first direct measurement of the ground-state relaxation rate of optically excited ${\mathrm{Cr}}^{3+}$ ions in Cr: YAl${\mathrm{O}}_3$. The Lorentzian linewidth of the emission resonance is 9.8 Hz (FWHM). The mechanism of ground-state relaxation is established to be fluorescent decay of the ${}_{}{}^2{}_{}{}^{}E$ metastable level.