Field-inhibited optical dephasing and shape locking of photon echoes

Using an acousto-optically gated cw dye laser and working on the 555.6-nm absorption line of atomic 174Yb, we have studied the shape of two-excitation-pulse photon echoes as a function of the duration and intensity of the second excitation pulse. Surprisingly, a long and intense second excitation pulse is found to generate an echo having a shape highly correlated with that of the first excitation pulse. This occurs because the high field of pulse 2 effectively stops dephasing processes throughout its duration.