Photon Echoes

Experiments are described in which a dilute ruby crystal is found to emit spontaneously a short pulse of light, the photon echo, at a time $\approx {\tau }_s$ after irradiation by two successive ruby-laser pulses separated by ${\tau }_s$. The phenomenon is explained in terms of a macroscopic oscillating electric dipole moment, which is momentarily reformed at the time the photon echo is observed. The analysis predicts the echo polarization as well as the propagation direction relative to the input pulses. A necessary condition for obtaining echoes in ruby is the application of a moderate magnetic field close to the optic axis of the crystal, and a simple model based on ${\mathrm{Cr}}^{3+}$-Al interactions is offered to account for this magnetic-field behavior. The relaxation time of the echo is found to exceed 250 nsec at 4.2°K but to be less than 70 nsec at 14°K, and is thought to be due to phonon-induced transitions in the excited ${}_{}{}^2{}_{}{}^{}E\left(\overline{E}\right)$ level. Multiple echo formation is also described.