Modulation of Photon Echo Intensities by Ultrasonic Waves in Ruby and Alexandrite

Modulations of the intensity of two-pulse photon echoes due to ultrasonic waves are described for alexandrite and ruby. Enhancements (up to a factor of 8) of the echo intensity are observed. The main features of the echo intensity changes are well described by a model in which the ultrasonic waves modulate the optical transition frequencies of the echo-producing ${\mathrm{Cr}}^{3+}$ ions effecting a spatially dependent phase accumulation which compensates for the phase mismatch.