The phenomenon of multiple beats (MBs) arising from nondipole-type excitons weakly confined in a thin film is theoretically elucidated using a nonlocal transient-response theory. Kojima et al. previously demonstrated for a GaAs thin film that the degenerate four-wave mixing signals from the quantized levels of the center-of-mass motion of excitons exhibit complex interference between beats under femtosecond-order pulse incidence [Kojima et al., J. Phys. Soc. Jpn. 77, 044701 (2008)]. This leads to an ultrafast optical response on the order of femtoseconds. This effect occurs in a size region beyond the long-wavelength approximation regime due to the resonant enhancement of the internal field, wherein the usual dipole selection rule is violated. Our analysis of MBs employs a model of the nonlocal multilevel system that considers the spatial interplay between excitonic waves and the radiation field to elucidate the mechanism behind the observed ultrafast response.

• Received 16 February 2009

DOI:https://doi.org/10.1103/PhysRevB.79.193308