We report on polarization-resolved resonant photoluminescence (PL) spectroscopy of bright (spin $\pm$1) and dark (spin $\pm$2) excitons in colloidal $\mathrm{CdSe}$ nanocrystal quantum dots. Using high magnetic fields to $33\mathrm{T}$, we resonantly excite (and selectively analyze PL from) spin-up or spin-down excitons. At low temperatures $(<4\mathrm{K})$ and above $\sim 10\mathrm{T}$, the spectra develop a narrow, circularly polarized peak due to spin-flipped bright excitons. Its evolution with magnetic field directly reveals a large $(1–2\mathrm{meV})$, 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.

• Received 13 March 2006

DOI:https://doi.org/10.1103/PhysRevB.73.241313