PHOTON-ASSISTED TRANSPORT THROUGH ULTRASMALL QUANTUM DOTS - INFLUENCEOF INTRADOT TRANSITIONS

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