An exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field, which results in a polariton in a bulk crystal, brings about the rapid radiative decay of the exciton in low-dimensional systems due to breakdown of the translational symmetry. This large decay constant at the same time makes the excitons deviate from ideal bosons so that we have a large third-order optical susceptibility enhanced by the macroscopic transition dipole moment under near-resonant excitation. The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicosecond in CdS quantum wells through the short lifetime of excitons.

• Received 28 January 1988

DOI:https://doi.org/10.1103/PhysRevB.38.1228