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 nanostru ctures. 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 c alculated for different cases with both diagonal and off-diagonal matrix el ements of the interaction included. The electron population inversion is fo und due to the off-diagonal matrix elements for a wide range of the inciden t light frequency, suggesting a new mechanism for optical pumping. Negative resistance and other novel features of the tunneling current are also disc ussed.