  Recently it has been shown that ac-driven double quantum dots can act as spin
pumps and spin filters. By calculating the current through the system for each
spin polarization, by means of the time evolution of the reduced density matrix
in the sequential tunneling regime (Born-Markov approximation), we demonstrate
that the spin polarization of the current can be controlled by tuning the
parameters (amplitude and frequency) of the ac field. Importantly, the pumped
current as a function of the applied frequency presents a series of peaks which
are uniquely associated with a definite spin polarization. We discuss how
excited states participating in the current allow the system to behave as a
bipolar spin filter by tuning the ac frequency and intensity. We also discuss
spin relaxation and decoherence effects in the pumped current and show that
measuring the width of the current vs frequency peaks allows to determine the
spin decoherence time $T_{2}$.
