Photon-assisted transport through ultrasmall quantum dots: Influence of intradot transitions

We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The ac field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. [Phys. Rev. Lett. 78, 1536 (1997)]. In addition, we propose an electron pump consisting of two serially coupled single-level quantum dots with a time-dependent interdot barrier.