Polarization-selective magneto-photoluminescence study on CdSe self-assembled quantum dots by resonant excitation

We have performed polarization-selective magneto-photoluminescence (PL) experiments on a self-assembled CdSe quantum dot (QD) system to investigate the spin relaxation time of the exciton. The intensities of the PL peak exhibited significant differences for the two circular polarizations when a magnetic field was applied. The degree of polarization at each energy within the broad PL line strongly depends on the separation of that energy position from the LO-phonon resonance condition. We have used a simple two-level rate equation model, including spin-flip time τs and exciton recombination time τr, to analyze the observed polarization of the PL. In this model, we are able to fit the magnetic field dependence of the spin polarization within the QD (which produces the PL polarization) by using τs/τr as a fitting parameter. The fitting reveals that the parameter τs/τr ranges from 2 to 8, and that it strongly depends on the energy separation between the PL emitted by specific dots of the QD ensemble and the LO-phonon resonance line. Thus, the spin relaxation time is much longer than the exciton recombination time, but the value of the ratio of the two times is a sensitive function of how close the PL emission is to the resonant excitation.