We report measurements of direct photon-echo intensity versus Cr concentration which indicate the presence of sizable radiation-damping effects in dark ruby. The departure from $n^2$ dependence for concentrations above 0.03% Cr demonstrates that modifications of self-induced transparency analyses are necessary in dense resonant media.

• Received 7 April 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.23