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Unleashing next-generation technologies with electron-detection

Visible red light can be produced when applying a voltage across gallium arsenide crystals whilst simultaneously illuminating the material with an invisible infrared laser pulse


Physicists at the University of Kansas have discovered a new method of detecting electric currents.

The process is based on “second-harmonic generation," and is similar to the process taking place in a radar gun for electrons that can remotely detect their speed.

The researchers say this new idea could improve many present-day renewable-energy technologies, like solar cells and batteries, which all rely on detection of electric currents. In the future, sensors that better read the motion of electrons could underpin next-generation cell phones and computers.

“So far, most techniques to detect electric currents are very much like measuring the speed of a car by tracking it with a camera, and later analysing how the position changes with time,” said Hui Zhao, assistant professor of physics at the University of Kansas (KU). “But for moving cars, a radar gun is a much better tool, since radar allows us to instantaneously measure the speed. Yet, for electrons, there has been no tool available that allows us to directly ‘see’ the motion like this.”

Zhao collaborated on the research at KU’s Ultrafast Laser Lab with Judy Wu, University Distinguished Professor of Physics, and graduate students Brian Ruzicka, Lalani Werake, Guowei Xu.

The researchers discovered that by shining light from a high-power laser onto a material that contains moving electrons, light of a different colour is generated.

By applying a voltage across thin GaAs crystals, they set electrons to move through it with a specified speed. When illuminating the crystal with an infrared laser pulse, invisible to the human eye, they found that visible red light was produced — a signature of the second-harmonic generation process.



What's more, they observed that the brightness of the red-light increases with the speed of the electrons. In other words, when the electrons have no directional motion, no red light comes out.

“By detecting the red light, one can accurately determine the speed of electrons without making any contact with the sample and without disturbing the electrons,” Zhao said. “Before this study, it was generally known that an electric current has three effects: It can charge the system, change its temperature and produce a magnetic field. As a result, all experimental techniques of current detection were based on these effects. This newly discovered optical effect of currents opens up a new way of using lasers to study currents.”

The KU researchers’ experimental results are consistent with theoretical studies performed by professors Jacob Khurgin of John Hopkins University and Eugene Sherman from Spain.

This research was jointly funded by a five-year CAREER award from NSF and the NSF EPSCoR Kansas Centre for Solar Energy Research. The experimental equipment was developed under support of the KU College of Liberal Arts and Sciences new faculty start-up funds.

Further details of this research are detailed in a recent paper published in Physical Review Letters.
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