Turning a Powder Into a Laser

Physicists have for the first time created a laser in a powder that "traps" light, they report in a paper in this week's issue of Physical Review Letters.

Conventional lasers use a pair of mirrors to bounce a light wave back and forth through a cavity containing a material or gas whose atoms are "pumped" into a higher energy state by an external light source. Each time a photon hits an excited atom, the atom falls back to a lower energy state, while emitting a photon with the same wavelength and direction. When enough atoms in the laser cavity are excited, the process can amplify a light beam.

A team led by Hui Cao from Northwestern University in Evanston, Illinois, has produced a similar effect in a finely ground semiconductor powder. In 1997, a team of Dutch and Italian scientists demonstrated that such a powder can trap or "localize" light. Because of its high refractive index, the powder strongly scatters light waves, bouncing photons back and forth like a pin-ball machine. But if the grains are close enough--less than the wavelength of the scattered light--the paths of the photons form closed loops. They pass many times through the same grains, just as an ordinary laser's light passes many times through the cavity between mirrors. If the atoms in the grains have been pumped to a higher energy state, the process could amplify the light.

To test this idea, the team at Northwestern University prepared powder films of zinc oxide and gallium nitride, with particles of about 100 nanometers in diameter. They shined laser light onto the films to "pump" their atoms. Then they directed a probe beam at the sample and measured the total intensity of the scattered light. The team noticed that when the pump laser reached a certain power, the intensity of the light emitted by the sample increased sharply. They concluded that the light was amplified; the powder had become a laser. "What we think happens is that light travels in those loops, and each of these closed loops form a cavity," says team member Eric Seelig.

Physicist Roberto Righini of the European Laboratory for Non-Linear Spectroscopy in Florence, one of the scientists who demonstrated that powders can trap light, says it's the first time researchers have demonstrated that laser amplification can also take place in a powder. "The paper is rather convincing," he comments. "This research will trigger more experiments."