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We study single-electron tunneling in a two-junction device in the presence of microwave radiation. We introduce a model for numerical simulations that extends the Tien-Gordon theory for photon-assisted tunneling to encompass correlated single-electron tunneling. We predict sharp current jumps which reflect the discrete photon energy h f, and a zero-bias current whose sign changes when an electron is added to the central island of the device. Measurements on split-gate quantum dots show microwave-induced features that are in good agreement with the model. An oscillating potential with frequency f changes the energy F. of an electron state into a set of energies E+nhf with n=0,~1,~2, . . . . These so-called sideband energies can lead to electron tunneling that involves the emission (n&0) and absorption (n)0) of photons. The formation of sidebands has been important for many time-dependent transport studies addressing issues such as photon-assisted tunneling of quasiparticles in superconducting junctions, the tunneling time, and time-dependent resonant tunneling. Recent theoretical work has begun to focus on the effects of an oscillating potential on transport through small capacitance devices where the charging energy regulates the tunnel processes. For instance, Bruder and Schoeller have calculated the photoresponse of a two-level system. However, little theoretical work exists on realistic systems, e.g. , quantum dots with many quantum levels, and no experiments have been reported except at very low frequencies. "In this paper, we present numerical simulations and experiments on photon-assisted tunneling through a quantum dot. We assume a continuous single-particle density of states, i.e. , a metallic system with an equivalent single electron circuit shown in the inset of Fig. 1 . Our model extends theTien-Gordon theory' to include the correlated tunneling of single electrons through a two-junction device. This model predicts discrete photon features, and is in agreement with measurements on split-gate quantum dot devices irradiated by 19-GHz microwaves.We model the microwaves as an oscillating potential V cos(2mft) of the central island relative to the source and drain leads. To allow for an asymmetry in the ac coupling, we model the microwave amplitude by two parameters a; =eV;/h f, i =s,d where V, and Vd are the ac voltage drops across the source and drain junctions. Analogous to the Tien-Gordon description, we write the tunnel rate I; in the presence of microwaves in terms of the rate 1; without microwaves as

We present results on microwave-assisted transport through quantum dots. First, the important energy/frequency scales are discussed. Then, measurements of the current versus gate voltage characteristics in the presence of microwaves are presented. At finite source-drain bias, microwave-induced features are observed, and at zero source-drain bias, an oscillating photocurrent is observed. A model of photon-assisted transport is discussed that can account for the experimental observations.