Nonequilibrium theory of photon-assisted tunneling in nanostructures

The theory of the equation of motion for the nonequilibrium Green function, developed by the present authors using the Schwinger-Keldysh formalism, is adopted to treat the problem of photon-assisted tunneling through nanostructures. A quantum wire modeled as a two-level system and quantum dots with strong electron-electron interactions are considered. The density of states, electron occupation probability, tunneling current and conductivity are calculated for different cases with both diagonal and off-diagonal matrix elements of the interaction included. The electron population inversion is found due to the off-diagonal matrix elements for a wide range of the incident light frequency, suggesting a new mechanism for optical pumping. Negative resistance and other novel features of the tunneling current are also discussed.