Research Sheds Exciting New Light on Semiconductor Potential

Lasers, nanostructures help move excitors — breakthrough in computer technology on the horizon?

A University of Pittsburgh physicist and his research team have found a way to create and move small bits of optical energy called excitons over relatively long distances, a development that could be an important step in creating semiconductors in which excitons are shuttled and controlled to form “excitonic circuits.”

David Snoke, associate professor of physics and astronomy and codirector of the Institute of NanoScience and Engineering, used lasers and a nanoscale structure called a quantum well to direct excitons to travel several millimeters. The researchers reported their work in a paper, “Long-Range Transport in Excitonic Dark States in Coupled Quantum Wells,” published in the Aug. 15 edition of Nature.

Learn more about excitons by clicking on the graphic (opens in a new window)

In conventional semiconductors, electrons or their absence (so-called holes) move in circuits to perform functions such as computation and storage of information. In his research, Snoke used laser light to separate an electron from an atom. The “excited” electron plus the hole remaining on the atom compose an exciton, which moves like an energy particle and could potentially carry information.

In most cases, excitons exist for only a few nanoseconds (billionths of a second) and travel only a few microns (millionths of a meter) before the electron and hole reunite and reemit the light.

By using the quantum wells—fabricated from gallium arsenide and indium gallium arsenide by his colleagues Loren Pfeiffer and K. W. West at Lucent Technologies’ Bell Laboratories — Snoke and his team were able to extend the amount of time the electron and hole were apart and, consequently, the distance the exciton traveled.

“A millimeter may not seem like a long distance, but with circuits now being designed on micron scales — that is, thousandths of a millimeter — a distance of a millimeter is tremendously long compared to typical circuit dimensions,” said Snoke. “Therefore, the exciton particles can easily travel over the distances needed for computer circuits.

“These results open up the possibility of using excitonic signals to carry information just as an electronic charge is currently used. Today, computer technology is based on sending electronic charges from place to place to carry information. Our experiments may open up the possibility of ‘excitonics’ as a new way of sending information. Instead of electrons, only energy excitations of the electrons, the excitons, would be sent from place to place.”

According to Snoke, previous experiments at the University of Pittsburgh already have shown that it is possible to apply a force on excitons to move them from one place to another using either pressure or an electric field. Given the recent results of Snoke’s research on long-distance exciton motion, the possibility now exists for excitonic circuits in which excitons are routed from one place to another.

The Pitt research project is supported by the National Science Foundation (NSF) and the United States Department of Energy.

“This research is an example of developments occurring at the interface of condensed matter physics, which produces semiconductor structures—starting with the transistor—and optical science,” said Hollis Wickman, a program manager in NSF’s materials research division. “These developments represent an emerging field called condensed matter optics.” • JF

Snoke’s papers are accessible at www.phyast.pitt.edu/~snoke; look for publications on excitons in quantum wells.