Light-controlled semiconductor device directs electrons

Researchers at the University of Michigan have created a device that enables them to control the flow of electrons through a semiconductor using only laser light, no electrical power source required.

This device was built to explore fundamental physics and realise a previously unobserved behaviour, but it could also open doors for new applications in areas that bridge optics and electronics, including sensing, imaging and telecommunications. The phenomenon could help improve how signals are sent through and between devices, as well as create new opportunities to store more information in those signals.

“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.”

The researchers showed they could induce the orderly flow of electrons through a semiconductor using two different colours of light. By rotating the polarisation of the two optical fields — the direction that the light waves wiggle — the researchers could also control the direction of the electronic current.

“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 Steven Cundiff, senior author of the team’s new report in the journal Physical Review Letters.

Although previous work had shown the ability of light to get electrons flowing with light alone, this work goes a step further and shows that light gets electrons flowing in a narrow beam in a specific direction.

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

Cundiff likens this to a lighthouse, which sweeps a beam of light by rotating its lamp. Here, the beam is made of electrons and that beam can be rotated by rotating the polarisation of the two phase-coherent optical fields.

The phenomenon relies on quantum interference, which arises when two colours of light drive different absorption pathways to the same final state. The light transfers energy to the semiconductor material in discrete, or quantised, packets called photons, which mobilise the material’s electric charge carriers.

In the U-M set-up, the incident light is absorbed by two routes at once, and the routes can be thought of as overlapping ripples, Cundiff said. Those ripples line up and enhance each other for electrons travelling a certain direction, but cancel each other out for electrons moving in other directions.

A collaborator on an earlier project, J.E. Sipe of the University of Toronto, had predicted it would be possible to create such an ‘electron lighthouse’. Now, working with the Lurie Nanofabrication Facility, Gong was able to realise such a device.

“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.

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

Image caption: When two pulses of different coloured lasers (the two waves at the top of the image) meet in a new device created at the University of Michigan, researchers create a beam of electrons (small golden particles) that flows in a controllable direction. By changing the laser colours, the electron beam can sweep through different directions like the beam of a lighthouse. Image credit: Yiming Gong