Scientists create an “electron lighthouse” with laser light

The device was primarily designed to investigate fundamental physics and demonstrate a behavior that had never previously been observed. However, the discovery could eventually support technologies that combine optics and electronics, including advanced sensing, imaging, and telecommunications. It may also lead to better ways of transmitting signals between devices and encoding more information within them.

"This electrical device that we manufactured at the Lurie Nanofabrication Facility has the potential to turn into something that measures different aspects of light," said Yiming Gong, who helped lead the project as a doctoral student in the U-M Department of Physics. "But this originates from a very fundamental level of physics, which is the interference between different optical absorption processes."

Two Colors of Light Direct Electron Flow

With federal support from the U.S. National Science Foundation, the team demonstrated that two different colors of light could produce an organized flow of electrons through a semiconductor. The researchers could also change the direction of that current by rotating the polarization of the two optical fields, which describes the direction in which the light waves oscillate.

"This isn't the way things normally work. When you think about electrons moving through a material, they're moving because you've applied an electrical field and they actually bounce around and drift across the materials. Here, using light, you can actually sort of squirt the electrons in a specific direction without applying an electric field," said U-M physicist Steven Cundiff, senior author of the team's new report in the journal Physical Review Letters.

Earlier studies had already shown that light alone could cause electrons to move. The new experiment goes further by producing a narrow stream of electrons and controlling the precise direction in which it travels.

"The light no longer merely switches the current on; it also aims it," Cundiff said.

A Lighthouse Beam Made of Electrons

Cundiff compares the effect to a lighthouse that sweeps a beam across the horizon as its lamp rotates. In this experiment, however, the moving beam consists of electrons. Scientists can rotate that electron beam by adjusting the polarization of the two phase-coherent optical fields.

The effect depends on quantum interference. It occurs when two colors of light send the semiconductor through different absorption pathways that ultimately lead to the same final state. Light delivers energy to the material in discrete, or quantized, packets known as photons. Those photons then mobilize the semiconductor's electric charge carriers.

In the U-M setup, the material absorbs the incoming light through two pathways at the same time. Cundiff compared the process to overlapping ripples. For electrons moving in one direction, the ripples align and reinforce each other. For electrons traveling in other directions, they cancel one another out.

This selective reinforcement produces a current that is concentrated into a particular direction rather than spreading evenly through the material.

A Long-Predicted Quantum Device Becomes Reality

J.E. Sipe of the University of Toronto, who collaborated with the researchers on an earlier project, had predicted that such an "electron lighthouse" could be created. Gong ultimately turned that theoretical idea into a working device with help from the Lurie Nanofabrication Facility.

"The LNF is an amazing facility," Gong said, adding it was a painstaking process to meld the device's materials together in a way that didn't introduce any extraneous electric fields.

Preventing unwanted electric fields was especially important because the researchers needed to confirm that the electron flow came entirely from the light. Producing the device required Gong and the facility's staff to experiment with different fabrication methods.

"That was the biggest puzzle to solve for me, because there isn't a standard way to do that. So I worked with the LNF staff to play around with different recipes and temperatures to come up with a manufacturing process."

Kai Wang, a former postdoctoral scientist who worked with Cundiff and is now a professor of electronics and information at Sun Yat-Sen University in China, also advised Gong during the project.

Gong has since earned his doctorate from U-M and now works in Chicago as a machine learning scientist. In that role, he applies the quantitative and analytical abilities he developed during his physics research to problems in machine learning.