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 Pr3+: YAlO3, 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 sequencea 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. © 1984 The American Physical Society.