Transition from superradiant free-polarization decay to subradiant photon-echo

Time-resolved degenerate four-wave mixing experiments on a multiple-quantum-well Bragg structure reveal a complex real-time pattern of the nonlinear optical response from the superradiant and the subradiant coupled exciton-photon modes. We find a free-polarization decay for the superradiant mode and a photon-echo from the subradiant modes. Rotation of the Bragg structure with respect to the incident laser beams implying a reduction of the effective quantum well separation results in a distinct change of the temporal behaviour. We estimate a value of less than 40 mu eV for the nonradiative contribution to the homogeneous linewidth of free excitons in our GaAs samples. (C) 1998 Elsevier Science Ltd. All rights reserved.