Optical Spectroscopy of Dark and Bright Excitons in CdSe Nanocrystals in High Magnetic Fields

Low-temperature polarized and time-resolved photoluminescence spectroscopy in high magnetic fields (up to 30 T) has been used to study the spin-polarization, spin-relaxation, and radiative lifetimes of excitons in wurtzite semiconductor (e.g., CdSe) colloidal nanocrystals. The applied magnetic field leads to a significant degree of circular polarization of the exciton photoluminescence, accompanied by a reduction in the photoluminescence decay time. The circular polarization arises from the Zeeman splitting of exciton levels, whereas lifetime reduction results from a polarization-preserving field-induced mixing of exciton levels. We analyze these experimental findings in terms of a simple model that combines both Zeeman effect and exciton-level mixing, as a function of the relative orientation of the nanocrystal c-axis and the magnetic field. This model is able to simultaneously describe the degree of circular polarization and lifetime reduction of the exciton photoluminescence, permitting us to quantify ...