Vectorial four-wave mixing field dynamics from individual excitonic transitions

Degenerate four-wave mixing (FWM) from individual localized excitonic transitions is measured using a dual-polarization extension of heterodyne spectral interferometry. A fine-structure splitting (FSS) of the localized excitons of 20–80 μeV is observed, revealing a spatially anisotropic localization in the quantum well plane, preferentially oriented along the [110] direction. The ratio of the transition dipole moments of the FSS transitions is in the range 0.7–1.4, with no significant correlation to the FSS. Retrieving both polarization components of the FWM in amplitude and phase we measure the time-resolved polarization state of the exciton after pulsed excitation, showing a coherent evolution oscillating between linear and virtually circular polarization. The polarization of the created FWM state is a result of the initial polarization state created by the first excitation pulse, its free evolution up to the arrival of the second pulse, and its conversion into the FWM controlled by the polarization of the second pulse