We consider theoretically a conical Ag tip coupled to surface states of a three-dimensional topological insulator (3D TI) such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Generation of propagating surface plasmon polaritons (SPPs) in a metal cone by a laser field produces an intense optical field near the tip of the cone and high concentration of hot carriers in the metal near the surface of the topological insulator. Such hot carriers are transferred through a Schottky contact between tip and TI to the chiral spin surface states of TI, resulting in finite spin polarization and spin current at the surface of TI. SPPs in the metal cone are excited with a THz laser with energy in the optical band gap of 3D TI to avoid photoexcitations in the bulk of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. We also propose a detail scheme for experimental measurements of such currents. Our results can find applications in spintronics.

• Received 5 September 2017
• Revised 12 April 2018

DOI:https://doi.org/10.1103/PhysRevB.98.075428