Theory of exciton doublet structures and polarization relaxation in single quantum dots

The mechanism of exciton doublet structures in quantum dots is identified as the long-range part of the electron-hole exchange interaction which is emphasized by the anisotropic shape of quantum dots. The physical origin of the energetic order of the orthogonally polarized exciton states of each doublet is clarified by inspecting the spatial distribution of the exciton polarization. The key concepts to understand the energetic order are the node configuration of the distribution function of exciton polarization and the dipole-dipole interaction energy originating from the long-range electron-hole exchange interaction. The population relaxation and the polarization relaxation of excitons are studied and the extremely slow polarization relaxation within exciton doublet states is predicted. It is also found that the inter-doublet cross-relaxation between orthogonally polarized exciton states occurs as efficiently as the population relaxation.