Spin-momentum locking, a key property of the surface states of three-dimensional topological insulators (3DTIs), provides a new avenue for spintronics applications. One consequence of spin-momentum locking is the induction of surface spin accumulations due to applied electric fields. In this work, we investigate the extraction of such electrically-induced spins from their host TI material into adjoining conventional, hence topologically trivial, materials that are commonly used in electronics devices. We focus on effective Hamiltonians for bismuth-based 3DTI materials in the ${rm Bi}_2{rm Se}_3$ family, and numerically explore the geometries for extracting current-induced spins from a TI surface. In particular, we consider a device geometry in which a side pocket is attached to various faces of a 3DTI quantum wire and show that it is possible to create current-induced spin accumulations in these topologically trivial side pockets. We further study how such spin extraction depends on geometry and material parameters, and find that electron-hole degrees of freedom can be utilized to control the polarization of the extracted spins by an applied gate voltage.
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