Exciton fine structure in wurtzite nanorods: Is exciton ground state always dark?

Colloidal semiconductor nanorods have demonstrated potential as bright, stable, polarized light sources. Emission of light in these and other nanocrystals proceeds through recombination of confined electron-hole pairs, or excitons, with tunable size-dependent resonant frequencies. Usually, their brightness is reduced by the "dark exciton" -- a non-emissive state into which electron-hole pairs relax before recombining radiatively. Here we analyze the fine structure of exciton states in wurtzite CdSe nanorods and demonstrate that, for cylindrical nanorods of radii in a certain range and aspect ratios exceeding some critical value, the exciton ground state is the bright emissive state polarized parallel to the axis of the nanorod. To the best of our knowledge, this is the first known nanostructure where the ground exciton state is bright. Our results are in good agreement with the experiment.