Bright-exciton fine structure and anisotropic exchange in CdSe nanocrystal quantum dots

We report on polarization-resolved resonant photoluminescence (PL) spectroscopy of bright (spin +/- 1) and dark (spin +/- 2) excitons in colloidal CdSe nanocrystal quantum dots. Using high magnetic fields to 33 T, we resonantly excite (and selectively analyze PL from) spin-up or spin-down excitons. At low temperatures (< 4 K) and above similar to 10 T, the spectra develop a narrow, circularly polarized peak due to spin-flipped bright excitons. Its evolution with magnetic field directly reveals a large (1-2 meV), intrinsic fine-structure splitting of bright excitons, due to anisotropic exchange. These findings are supported by time-resolved PL studies and polarization-resolved PL from single nanocrystals.