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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. Credit: University of Minnesota.

A new device is tapping into spinning light to control the flow of electrical current.

Researchers from the University of Minnesota have developed a new device that can control the direction of generated electrical current called photocurrent without deploying an electric voltage.

The researchers found that the control of current is effected by the direction in which the photons are spinning—either clockwise or counterclockwise.

Photocurrent is generated by the spinning light, which is also spin-polarized—where more electrons spin in one direction than the other.

The new device could possibly be used in the next generation of microelectronics using electron spin as the fundament unit of information, as well as being used for energy efficient optical communication in data centers.

“The observed effect is very strong and robust in our devices, even at room temperature and in open air,” Mo Li, Ph.D., a University of Minnesota electrical and computer engineering associate professor and a lead author of the study, said in a statement. “Therefore, the device we demonstrate has great potential for being implemented in next-generation computation and communication systems.”

In circularly polarized light, the electric field can spin in either the clockwise or the counterclockwise direction where the photon has either a positive or a negative optical spin angular momentum. The optical spin is analogous to the spin of electrons and endows magnetic properties to materials.

Scientists recently discovered topological insulators, which have electrons that spin pointed one way while always flowing in one direction. This effect—called spin-momentum locking—shows that the spin of the electrons is locked in the direction they travel.

By shining a circularly polarized light on a topological insulator (TI), the researchers were able to free electrons from its inside to flow on its surface in a selective way.

The researchers fabricated the device that can change the direction of a photocurrent without using an electric voltage with a thin film of a TI material—bismuth selenide—on an optical waveguide made of silicon.

The light is tightly squeezed in the waveguide, meaning that it tends to be circularly polarized along a direction normal to the direction in which it flows.

The scientists believed that integrating a TI material with the optical waveguide will induce strong coupling between the light in the waveguide and the electrons in the TI material, both having the same spin-momentum locking effect. This results in light flowing along one direction in the waveguide to generate an electrical current flowing in the same direction with electron spin polarized.

Reversing the light direction reverses both the direction of the current and its spin polarization.

"Our devices generate a spin-polarized current flowing on the surface of a topological insulator,” Li He, a University of Minnesota physics graduate student and an author of the paper, said in a statement. “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.

“Our research bridges two important fields of nanotechnology: spintronics and nanophotonics,” he added. “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.”

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