  We demonstrate a spin pump to generate pure spin current of tunable intensity
and polarization in the absence of charge current. The pumping functionality is
achieved by means of an ac gate voltage that modulates the Rashba constant
dynamically in a local region of a quantum channel with both static Rashba and
Dresselhaus spin-orbit interactions. Spin-resolved Floquet scattering matrix is
calculated to analyze the whole scattering process. Pumped spin current can be
divided into spin-preserved transmission and spin-flip reflection parts. These
two terms have opposite polarization of spin current and are competing with
each other. Our proposed spin-based device can be utilized for non-magnetic
control of spin flow by tuning the ac gate voltage and the driving frequency.
