Abstract
The propagation of electrons and photons can respectively have the spin-momentum locking effect which correlates spin with linear momentum. For surface electrons in three-dimensional topological insulators (TIs), their spin is locked to the transport direction. Analogously, photons in optical waveguides carry transverse spin angular momentum which is also locked to the propagation direction. A direct connection between electron and photon spins occurs in TIs due to spin-dependent selection rules of optical transitions. Here we demonstrate an optoelectronic device that integrates a TI with a photonic waveguide. Interaction between photons in the waveguide and surface electrons in a Bi2Se3 layer generates a directional, spin-polarized photocurrent. Because of spin-momentum locking, changing light propagation direction reverses photon spin and thus the direction of the photocurrent. Our device represents a way of implementing coupled spin–orbit interaction between electrons and photons and may lead to applications in opto-spintronics and quantum information processing.
Highlights
The propagation of electrons and photons can respectively have the spin-momentum locking effect which correlates spin with linear momentum
In the presence of a surface, Bi2Se3 thin film has a C3v crystalline structure which has a threefold rotation symmetry around the z-axis (Fig. 2b), and we investigate the anisotropy of its photogalvanic effects
To minimize the thermoelectric effect due to non-uniform heating of the sample, we focused the laser to a spot of 330 μm in diameter, much larger than the size of the Bi2Se3 sample which has a diameter of 10 μm
Summary
The propagation of electrons and photons can respectively have the spin-momentum locking effect which correlates spin with linear momentum. The handedness of the polarization and the transverse SAM are locked to the propagation direction, meaning that their signs reverse with the direction of propagation, as schematically illustrated in Fig. 1a for a waveguide mode[4,5,6,7] This effect is analogous to the quantum spin Hall effect and the spinmomentum locking effect occurring for the surface electrons in topological insulators[8,9,10,11,12], as depicted, albeit the optical systems (waveguides, fibers) are topologically trivial. Because the photons in the TM mode are elliptically polarized, through the CPGE effect they will induce a photocurrent flowing in the longitudinal direction (the z-axis) with spin polarization along the transverse direction (the x-axis) In this way, the CPGE interaction couples the spin-momentum locking of the TM mode photons with the surface electrons in the TI. Such a TI-waveguide integration makes an opto-spintronic device, the first of its kind, that converts the propagation path information (forward or backward) of photons ayxbzy TI c z d
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