Nanocrystals Show Potential for Cheap Lasers

For more than a decade, researchers have been frustrated in their attempts to create continuously emitting light sources from individual molecules because of an optical quirk called "blinking", but now scientists at the University of Rochester have uncovered the basic physics behind the phenomenon.

Many molecules, as well as crystals just a billionth of a meter in size, can absorb or radiate photons. However, they also experience random periods when they absorb a photon, but instead of the photon radiating away, its energy is transformed into heat. These "dark" periods alternate with periods when the molecule can radiate normally, leading to the appearance of them turning on and off, or blinking.

"A nanocrystal that has just absorbed the energy from a photon has two choices to rid itself of the excess energy - emission of light or of heat," says Todd Krauss, professor of chemistry at the University of Rochester. "If the nanocrystal emits that energy as heat, you've essentially lost that energy." Krauss and his colleagues succeeded in developing a new, non-blinking nanocrystal.

After a lengthy investigation, they concluded that the reason the blinking didn't occur was due to the unusual structure of the nanocrystal. Normally, nanocrystals have a core of one semiconductor material wrapped in a protective shell of another, with a sharp boundary dividing the two. The new nanocrystal, however, has a continuous gradient from a core of cadmium and selenium to a shell of zinc and selenium. That gradient squelches the processes that prevent photons from radiating, and the result is a stream of emitted photons as steady as the stream of absorbed photons.

With blink-free nanocrystals, Krauss believes lasers and lighting could be incredibly cheap and easy to fabricate. Currently, different color laser light is created using different materials and processes, but with the new nanocrystals a single fabrication process can create any color laser.; Source: University of Rochester