Exciton dynamics probe the energy structure of a quantum dot-in-a-well system: The role of Coulomb attraction and dimensionality

We investigate the energy structure of a quantum dot-in-a-well system by tracing the population of optically created excitons in pump-probe experiments and numerical simulations. The combination of a zero-dimensional and a two-dimensional substructure in this system gives rise to crossed excitons, with the electron confined in the quantum dot and the hole in the quantum well. An analysis of a large set of experiments at low carrier density and variable extraction potential shows that excitons created with no excess energy remain Coulomb-coupled and equilibrate and diffuse as correlated pairs. Individual quantum dots reach their quasi thermal equilibrium after few picoseconds, spatial equilibration of the whole system is reached after 300 ps.