Researchers Generate a Directional, Spin-Polarized Photocurrent Using Topological Insulator

Researchers have developed a device for controlling the direction of photocurrent without using an electric voltage. This device, according to researchers, could provide a way of implementing coupled spin-orbit interaction between electrons and photons and could lead to applications in opto-spintronics and quantum information processing.

The optoelectronic device integrates a topological insulator (TI) with a photonic waveguide. Interaction between photons in the waveguide and surface electrons in a Bi₂Se₃ (TI) layer generates a directional, spin-polarized photocurrent. Because of spin-momentum locking, changing the light propagation direction reverses the photon spin, and thus the direction of the photocurrent.

This image shows a false-colored electron microscope image of the University of Minnesota device. The blue area marks the topological insulator on top of the optical waveguide in red. Courtesy of University of Minnesota.

In the research that led to fabrication of the device, the University of Minnesota team observed that control of the current was affected by the direction in which the photons were spinning, and that the photocurrent generated by the spinning light was spin-polarized.

The team surmised that integrating a TI material with the optical waveguide would induce strong coupling between the light in the waveguide and the electrons in the TI material, due to a spin-momentum locking effect. The team further theorized that this coupling would result in an unusual optoelectronic effect, i.e., light flowing along one direction in the waveguide would generate an electrical current flowing in the same direction with electron spin polarized. Reversing the light direction reversed both the direction of the current and its spin polarization. Other possible causes for this effect, such as heat generated by the light, were experimentally ruled out.

“The observed effect is very strong and robust in our devices, even at room temperature and in open air,” said professor Mo Li.

Li said that the research holds potential for use in next-generation computation and communication systems.

“Our devices generate a spin-polarized current flowing on the surface of a topological insulator. They can be used as a current source for spintronic devices, which use electron spin to transmit and process information with very low energy cost,” said researcher Li He.

“Our research bridges two important fields of nanotechnology: spintronics and nanophotonics. It is fully integrated with a silicon photonic circuit that can be manufactured on a large scale and has already been widely used in optical communication in data centers,” said He.

The research was published in Nature Communications (doi:10.1038/s41467-017-02264-y).