Observation of the Optical Analog of the Mössbauer Effect in Ruby

Laser-induced fluorescence line narrowing of the ruby $R_1$ Zeeman spectra at 693.4 nm is reported using a continuous, stable, single-frequency ruby laser for resonant excitation. A resolution in excess of ${10}^7$ corresponding to a linewidth of ∼ 30 MHz has been achieved. The linewidths are anisotropic with respect to the relative magnetic-field-crystal-axis orientation and, at low Cr concentrations, are expected to be determined by Cr-Al superhyperfine interactions.