OPTICAL-PUMPING IN CDS1-XSEX NANOCRYSTALS

We report on the magneto-optical properties of large CdS0.4Se0.6 nanocrystals in a glass matrix. Optical pumping in the two high-energy bands of the photoluminescence spectrum is studied with resonant excitation and in the presence of a magnetic field. We also investigate magnetic field-induced polarization of luminescence for non-polarized excitation. We show that the higher energy band is most likely to be due to excitonic recombination. The Lande g-factor is found to be anisotropic Its maximum value is estimated from the study of magnetic field-induced circular polarization. The second band is attributed to donor-hole recombination. We show that electrons on the donor level are partly optically oriented by circularly polarized light and that the transition may be excited either from the A or from the B valence band. The maximum degree of circular polarization is calculated in the case of optical pumping with resonant excitation of the A exciton for an assembly of wurtzite nanocrystals with randomly oriented c axes. It is found to be equal to 5/7 instead of 1 in the bulk material. In the case of magnetic field-induced polarization, the effective Lande factors for the different luminescence bands are calculated and compared with the experimental results.