We propose an alternative mode of operation of an electron pump consisting
of two weakly coupled quantum dots connected to reservoirs. An electron can
be transferred within the device at zero bias voltage when it is subjected
to electromagnetic radiation, thereby exciting the double dot. The excited
state can decay by transferring charge from one lead and to the other lead
in one direction. Depending on the energies of the intermediate states in
the pumping cycle, which are controlled by the gate voltages, this transpor
t is either incoherent via well-known sequential tunneling processes, or co
herent via an inelastic co-tunneling process. The latter mode of operation
is possible only when interdot Coulomb charging is important. The de transp
ort through the system can be controlled by the frequency of the applied ra
diation. We concentrate on the resonant case, when the frequency matches th
e energy difference for exciting an electron from one dot into the other. T
he resonant peaks in the pumping current should be experimentally observabl
e. We have developed a density matrix approach that describes the dynamics
of the system on time scales much larger than the period of the applied irr
adiation. In contrast to previous works we additionally consider the case o
f slow modulation of the irradiation amplitude. Harmonic modulation produce
s additional sidepeaks in the photoresponse, and pulsed modulation can be u
sed to resolve the Rabi frequency in the time-averaged current.