Biexciton and spin dephasing effects in quantum dots embedded in a glass matrix proved by four-wave mixing and pump-and-probe spectroscopy

The polarization properties of time-integrated four-wave mixing (FWM) and pump-and-probe transmission (PPT) signal spectroscopy were investigated for a system of excitons confined in CdSSe quantum dots (QD’s) embedded in a glass matrix. A strong FWM signal was observed by scattering ${\sigma }^{-}$ circularly polarized light on the population grating created by two ${\sigma }^{+}$ pulses (forbidden geometry according to the noninteracting oscillator’s model). A similar depolarization effect is observed in the PPT studies: the difference in the signals in the ${\sigma }^{+}{\sigma }^{+}$ and ${\sigma }^{+}{\sigma }^{-}$ geometries decays for delay times of about 5 ps. No marked scattering is observed if the population grating is created by two pulses of opposite $({\sigma }^{+}$ and ${\sigma }^{-})$ polarizations. We demonstrate that these facts are associated to a strong exciton-exciton coupling in the QD and to the inhomogeneous spin dephasing of the QD excitons due to a random exchange splitting of the $J=1\mathrm{}$ exciton state, originating from the lowered QD symmetry.