Coupling of exciton transitions associated with different quantum-well islands

We show by means of time-resolved four-wave-mixing experiments that the simultaneous excitation of energetically distinct and inhomogenously broadened exciton transitions associated with spatially separated quantum-well islands leads to a beating photon echo. This shows that the exciton transitions cannot be described by uncoupled two-level systems. We discuss bleaching, screening, and dipole-dipole interaction as possible coupling mechanisms.