  We propose a lateral spin-blockade device that uses an InGaAs/InAlAs double
quantum well (DQW), where the values of the Rashba spin-orbit parameter
$\alpha_{\rm R}$ are opposite in sign but equal in magnitude between the
constituent quantum wells (QW). By tuning the channel length of DQW and the
magnitude of the externally applied in-plane magnetic field, one can block the
transmission of one spin (e.g., spin-down) component, leading to a
spin-polarized current. Such a spin-blocking effect, brought about by wave
vector matching of the spin-split Fermi surfaces between the two QWs, paves the
way for a new scheme of spin-polarized electric current generation for future
spintronics applications based on semiconductor band engineering.
