Photon assisted tunnelling in single and coupled quantum dot systems

We describe photon assisted tunneling (PAT) in single and coupled quantum dot systems which have a relatively large zero-dimensional (OD) level separation. A microwave electric field applied across the tunneling barrier leads to the formation of a sideband structure. For a single quantum dot system, a PAT current peak is clearly observed in addition to the Coulomb oscillation peak. For a weakly-coupled double-dot system, we observe resonant OD-OD PAT. The resonant PAT current appears only when two discrete quantum levels in the neighboring dots are exactly separated by the microwave photon energy. The PAT current shows a narrower peak than the main resonant tunneling current in accordance with time-dependent tunneling theory. The photon stimulated emission associated with PAT from a higher-lying occupied level to a lower-lying empty level is obtained at a drain voltage sufficient that population inversion takes place. (C) 1997 Academic Press Limited