QD 'Doughnuts' Control Light

Doughnut-shaped byproducts of quantum dots (QDs) have been used to slow and even freeze light. The discovery opens a range of possibilities, from reliable and effective light-based computing to "slow glass," a concept first suggested in science fiction.

The key to this new research, led by the University of Warwick, is the exciton, a particle essential to modern electronics. An exciton is a bound state of an electron and an imaginary particle called an electron hole. After both orbiting around the nucleus of the atom, the electron’s high-energy state decays, it is drawn back to the hole, and light is emitted.

That cycle usually happens very quickly, but researchers thought that if one could find a way to freeze or hold an exciton in place for any length of time, one could delay the reemitting of a photon and effectively slow or even freeze light.

The researchers, led by PhD researcher Andrea Fischer and Dr. Rudolf A. Roemer from the University of Warwick’s department of physics, looked at the possibilities presented by some tiny rings of matter accidentally made during the manufacture quantum dots. When creating these very small QDs (10-100 nm in size), physicists some times cause the material to splash when depositing it onto a surface, leaving not a useful dot, but a doughnut-shaped ring of material.

Though originally created by accident these “Aharonov-Bohm nanorings” are now a source of study in their own right and in this case seemed just the right size for enclosing an exciton. However simply being this useful size does not, in itself, allow them to contain or hold an exciton for any length of time.

Remarkably, the team have found that if a combination of magnetic and electric fields is applied to these nanorings, they can actually then simply tune the electric field to freeze an exciton in place or let it collapse and re-emit a photon.

While other researchers have used varying exotic states of matter to dramatically slow the progress of light, this is the first time a technique has been devised to completely freeze and release individual photons at will.

“This has significant implications for the development of light-based computing which would require an effective and reliable mechanism such as this to manipulate light," said Roemer.

The technique could also be used to develop a “buffer” of incoming photons which could re-release them in sequence at a later date thus creating an effect not unlike the concept of “slow glass” first suggested by science fiction author Bob Shaw several decades ago.

The research paper, “Exciton storage in a nanoscale Aharonov-Bohm ring with electric field tuning" by Fischer, Roemer, Vivaldo L. Campo Jr. (Universidade Federal de Sao Carlos-UFSCar, Brazil), and Mikhail E. Portnoi (University of Exeter), was recently published in Physical Review Letters.

For more information, visit: www2.warwick.ac.uk