PHOTON-ASSISTED TUNNELING IN SINGLE AND COUPLED QUANTUM-DOT SYSTEMS

We describe photon assisted tunneling (PAT) in single and coupled quan tum dot systems which have a relatively large zero-dimensional (OD) le vel separation. A microwave electric field applied across the tunnelin g barrier leads to the formation of a sideband structure. For a single quantum dot system, a PAT current peak is clearly observed in additio n to the Coulomb oscillation peak. For a weakly-coupled double-dot sys tem, we observe resonant OD-OD PAT. The resonant PAT current appears o nly when two discrete quantum levels in the neighboring dots are exact ly separated by the microwave photon energy. The PAT current shows a n arrower peak than the main resonant tunneling current in accordance wi th time-dependent tunneling theory. The photon stimulated emission ass ociated with PAT from a higher-lying occupied level to a lower-lying e mpty level is obtained at a drain voltage sufficient that population i nversion takes place. (C) 1997 Academic Press Limited