Michigan Physicists Built an 'Electron Lighthouse' That Aims a Current With Light Alone — No Electric Field

Electric current normally requires a push. Apply a voltage across a material, and electrons drift through it, ricocheting off atoms as they go. Physicists at the University of Michigan have now produced a directed current in a semiconductor without applying any electric field at all — using nothing but two colors of laser light — and they can steer the resulting electron beam by rotating the light.

"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," said U-M physicist Steven Cundiff, senior author of the report in Physical Review Letters. "Here, using light, you can actually sort of squirt the electrons in a specific direction without applying an electric field."

The effect runs on quantum interference. Light delivers energy in discrete packets called photons, which mobilize a semiconductor's charge carriers. In this setup, two phase-coherent laser fields of different colors drive the material through two different absorption pathways that arrive at the same final quantum state — and, as with any two routes to the same destination, the possibilities interfere. Cundiff compares it to overlapping ripples on water. For electrons heading in one direction the ripples reinforce each other; for electrons heading elsewhere they cancel. What survives is not a diffuse flow but a narrow stream pointed one way.

Earlier work had already shown that light by itself could set electrons in motion. The new experiment goes a step further by producing a tight beam and controlling where it points. Rotating the polarization of the two optical fields — the direction in which the light waves oscillate — swings the electron beam through different directions, exactly the way a lighthouse lamp sweeps its beam across the horizon. "The light no longer merely switches the current on; it also aims it," Cundiff said.

The idea had been sitting in the literature for years. J.E. Sipe of the University of Toronto, a previous collaborator of the group, had predicted that an "electron lighthouse" of this kind should be possible. Turning the prediction into hardware fell to Yiming Gong, who built the device as a doctoral student at U-M's Lurie Nanofabrication Facility with support from the U.S. National Science Foundation. The hardest part was not the optics but the fabrication: the team had to bond the device's materials together without introducing any stray electric fields of their own, since any residual field would undermine the entire claim that the light was doing all the work. "That was the biggest puzzle to solve for me, because there isn't a standard way to do that," Gong said.

The device was built to answer a question in fundamental physics rather than to fill a slot in a product, but the researchers see a path forward. "This electrical device that we manufactured at the Lurie Nanofabrication Facility has the potential to turn into something that measures different aspects of light," Gong said. "But this originates from a very fundamental level of physics, which is the interference between different optical absorption processes." A detector whose output current direction encodes properties of incoming light could be useful in optical sensing, imaging and telecommunications — and could offer a way to pack more information into a signal passing between chips.