Intersubband transitions in photon-assisted tunneling through a quantum wire

The photon-assisted coherent tunneling is treated by the equation of motion for nonequilibrium Green functions. The effects of intersubband transitions on the tunneling through a two-level quantum wire are investigated under different conditions by calculating the off-diagonal matrix elements of the electron-photon interaction in addition to diagonal matrix elements. The differences in density of states and current for diagonal and off-diagonal interaction are examined. It is found that electronic transitions due to multiphoton absorption are more or less suppressed when the off-diagonal interaction dominates, while both multi- and single-photon transitions are present when diagonal matrix elements dominate the interaction. Depending on the equilibrium position of Fermi surface in electrode leads, the tunneling current is found to display either peak or plateau structures in the $I-V$ curve. The necessary condition for the appearance of absolute negative resistance in either diagonal or off-diagonal interaction is discussed.