Three-pulse four-wave-mixing is used to observe the loss of optical phase memory in an inhomogeneous quantum well. The system's ability to form a photon echo is almost entirely lost due to spectral diffusion of excitons.
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.
Using a perturbation theory based on the density-matrix formulation, we study the nonlinear optical responses of a noninteracting three-level model system to consecutive coherent pulsed excitations, as realized in several recent experiments involving excitonic transitions in quantum wells. The terahertz emission, which is a second-order response, and the four-wave mixing signal, a third-order response, are calculated within the rotating-wave approximation, in the presence of detuning, dephasing, and inhomogeneous broadening. We study the quantum beats in the photon echo from transient four-wave mixing experiments of a three-level system. We find that the temporal profile of the photon echo signal in the four-wave mixing experiments depends very strongly on the amount of inhomogeneous broadening of the energy levels involved. Both the position and the intensity of the peaks exhibit a smooth evolution from strong quantum beats to a conventional photon echo as the inhomogeneous broadening increases. These features from our noninteracting model system are compared with a recent experiment, and found to account for a number of experimental observations.