Echo peak-shift spectroscopy of non-Markovian exciton dynamics in quantum wells

Three-pulse four-wave mixing is used to observe the loss of system memory for excitons confined in quantum wells. By measuring the photon echo peak shift with pulse delay, we obtain the two-time exciton frequency fluctuation correlation function. Spectral diffusion of localized excitons gives rise to exponential decay of the correlation function, while static inhomogeneity gives an offset. Nonmonotonic behavior during the first few picoseconds indicates complex memory effects due to the interaction of excitons with acoustic phonons.