  The effect of an ac electric field on quantum transport properties in a
system of three quantum dots, two of which are connected in parallel while the
third is coupled to one of the other two, is investigated theoretically. Based
on the Keldysh nonequilibrium Green's function method, the spin-dependent
current, occupation number and spin accumulation can be obtained in our model.
An external magnetic flux, Rashba spin orbit coupling (SOC) and intradot
Coulomb interactions are considered. The magnitude of the spin-dependent
average current and the positions of the photon assisted tunneling (PAT) peaks
can be accurately controlled and manipulated by simply varying the strength of
the coupling and the frequency of the ac field. A particularly interesting
result is the observation of a new kind of PAT peak and a multiple
electron-photon pump effect that can generated and controlled by the coupling
between the quantum dots. In addition, the spin occupation number and spin
accumulation can be well controlled by the Rashba SOC and the magnetic flux.
