Creation of dark exciton states in a semiconductor quantum dot by a light field with a strong longitudinal component

Depending on their spin configuration, excitons in quantum dots (QDs) are efficiently or poorly coupled to light, and so are called bright and dark, respectively. Ground-state dark excitons are long-lived states useful in quantum information technology. Here we predict the formation of ground-state dark excitons in QDs excited by light having a strong longitudinal component, e. g. a radially polarized beam. Our theoretical QD model includes asymmetry, light-matter interaction, Coulomb interaction between carriers, and valence-band mixing. Exciton states are calculated using Configuration Interaction and phonon relaxation processes are studied within a rate equation model. We find that the longitudinal field creates high-energy bright excitons with particular spin configurations that may relax with significant probability into ground-state dark excitons. For example, a possible optically excited bright-exciton is a mixture of non-interacting electron–hole (eh) pair states with one of its components being a dark eh pair state. Phonon-induced transitions do not alter the spin configuration, but cause this high-energy exciton to relax to a ground state in which the dominant non-interacting component is that dark eh pair, thus creating the desired dark exciton.