Ultrafast ellipsometry of coherent processes and exciton-exciton interactions in quantum wells at negative delays

A combination of ellipsometric and pump-probe techniques are used to completely determine the time-averaged and time-resolved polarization state of the degenerate four-wave-mixing signal emitted from a GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum well following femtosecond excitation for negative time delays between the pump and probe. The polarization state of the four-wave-mixing signal is observed to exhibit a continuously varying ellipticity and a real-time rotation of the polarization ellipse at negative time delays that are dramatically and functionally different from those observed at positive delays. These distinct polarization features place stringent constraints on the physical models and are shown to be consistent with the inclusion of both many-body interactions and biexcitonic effects. © 1996 The American Physical Society.