PHILADELPHIA, July 24, 2015 — Polarized light can bring out an unexpected electrical property in silicon, potentially unlocking new possibilities for computing.
University of Pennsylvania researchers examining a silicon device caused current to flow in one direction by shining clockwise-polarized light on it; counterclockwise-polarized light caused current to flow in the opposite direction.
A silicon-based photonic device exhibits unexpected topological effects when illuminated with clockwise- and counterclockwise-polarized light. Courtesy of the University of Pennsylvania.
The phenomenon is a function of the geometric relationship between the pattern of atoms on the surface of silicon nanowires and how electrodes placed on those wires intersect them. The interaction between the semiconducting silicon and the metallic electrodes produces an electric field at an angle that breaks the mirror symmetry that silicon typically exhibits, lending it chiral properties.
"Whenever you change a symmetry, you can do new things," said professor Ritesh Agarwal. "In this case, we have demonstrated how to make a photodetector sensitive to a photon's spin. All photonic computers need photodetectors but they currently only use the quantity of photons to encode information. This sensitivity to photon spin would be an extra degree of freedom, meaning you could encode additional information on each photon."
The discovery grew out of work on topological insulators — materials whose surfaces conduct electricity but whose interiors do not. As a light, highly symmetric material, silicon was not thought to be able to exhibit this property.
"We expected the control experiment to give a null result; instead we discovered something new about nanomaterials," said professor Eugene Mele.
Silicon is at the heart of the computer industry, so finding ways to produce these effects in that material is preferable to learning to work with the heavier, rarer elements that naturally exhibit them, the researchers said.
Funding came from the U.S. Army Research Office, Department of Energy and National Science Foundation.
The research was published in Science (doi: 10.1126/science.aac6275).
For more information, visit www.upenn.edu.