Sign reversal and light controlled tuning of circular polarization in semimagnetic CdMnSe quantum dots

Circularly polarized luminescence of CdMnSe quantum dots in magnetic fields up to 5 T is studied for nominal Mn concentrations of 0%, 1%, and 2% by using a photoelastic modulator technique. The exciton g factors as well as spin relaxation times were determined from the polarized luminescence taking into account the exciton lifetimes, which were also extracted by means of time-resolved photoluminescence spectroscopy. For quantum dots without Mn and with 2% Mn exciton g factors of −1.62 and +1.32, respectively, were found. The quantum dots with 1% Mn show a vanishing small value of g for small excitation powers. For this structure the polarization properties are dominated by the optical orientation. Interestingly, for the 1% Mn quantum dots with increasing excitation power considerable changes of the polarization and the exciton g factor were observed which are interpreted in terms of heating effects. From the power dependence indirect heating via phonons and above a critical value direct heating due to photocarriers were identified to result in drastic changes of the circular polarized quantum dot emission.