Cumulative two-pulse photon echoes

This paper reports an unusual photon-echo phenomenon where a train of echoes, generated by a repeating two-pulse sequence, exhibits growth rather than damping. The effect, which is observed in the impurity-ion crystal ${\mathrm{Pr}}^{3+}$: ${\mathrm{YAlO}}_3$, results from an electronic ground-state population grating that is created by the two-pulse sequence and is stored coherently as a modulation in the frequency domain. The depth of modulation increases with increasing $n$, where $n$ is the number of the two-pulse sequence—a result which is predicted in a three-level density-matrix theory. The effect is closely related to previous stimulated or three-pulse photon echoes but differs in that growth is observed directly for the first time in the simpler two-pulse sequence.