Forbidden transitions in the emission spectrum of charged excitons in a single semiconductor quantum dot

New optical transitions in charged self-assembled InAs quantum dots have been investigated. The main optical emission is due to fast recombination of holes and electrons from their respective s levels. But photoluminescence (PL) experiments on quantum dots embedded in a vertical tunneling structure reveal a new group of weak lines blueshifted from the main s-s PL by 15 meV. These lines arise as the electronic p shell is first populated for the doubly negatively charged exciton, X^2-, induced by the vertical electric field. We therefore attribute these lines to s-p transitions, which are only possible to observe due to the breaking of the rotational symmetry in a real dot and provide a measure of its extent. These transitions show two remarkable effects. First, the population of the wetting layer splits the PL and accordingly the p sub-shells into two branches analogous to a magnetic field. Secondly the s-p PL retains in contrast to the s-s emission a small linewidth up to X^6- after the wetting layer is populated. This can be explained by an intra-dot interaction triggered by the s shell vacancy present in the s-s final state but not in the s-p final state. © 2005 American Institute of Physics.