First Demonstration Of Laser Action In A Powder

The Northwestern team of materials scientists, physicists and electrical engineers showed that powdery layers of zinc oxide and gallium nitride can produce blue laser light when it is "pumped" with light from another laser. The observation, reported in the Dec. 21 issue of Applied Physics Letters, changes scientists' understanding of solid-state lasers and suggests that these devices can be made much more simply than they have been.

A laser typically amplifies light inside a "cavity" formed by mirrors. Semiconductor lasers, in which the light is reflected back and forth between the end facets of a crystal, have been made only by complicated techniques to grow near-perfect crystal films on underlying template surfaces that precisely align the atoms of the crystal.

The expense of fabrication and the narrow choice of suitable substrate materials has limited the range of applications for these tiny lasers, says Robert P.H. Chang, director of the Materials Research Center at Northwestern, who led the study.

"We wanted to know if lasing could occur without mirrors and whether it is possible to make solid-state lasers with powder films," Chang said.

Chang, a professor of materials science and engineering in Northwestern's Robert R. McCormick School of Engineering and Applied Science, with graduate student Hock Ong and visiting scholar Ji Dai, grew thin films of zinc oxide on ordinary glass wafers. Zinc oxide is a semiconductor with a blue luminescence and a high optical gain to amplify light. It is a material that has been used extensively in electrical applications, such as resistors and diodes.

The powdered zinc oxide layer has a highly disordered structure. The researchers were able to show that this disorder paradoxically enhances the lasing effect. Traditional semiconductor lasers, with well-defined cavities, have perfectly ordered structures to minimize light scattering. In those devices, normal light scattering reduces laser output.

"We've shown something that is fundamentally new -- a laser without mirrors, in a totally disordered medium," said Hui Cao, an assistant professor of physics and astronomy. She and visiting scholar Yi-Guang Zhao evaluated the laser properties of the zinc oxide films.

"We're exploiting what had been seen as a drawback," Cao said. "People have been trying to eliminate scattering, which you always think of as bad for a laser. But we went to the other extreme. In a totally disordered medium, scattering is very strong, and it actually helps lasing because it forms closed-loop paths for the light and creates feedback. It makes its own laser cavities."

When they pumped the zinc oxide layer with a conventional laser at low power, Cao and Zhao found that the material gave off light with a broad band of wavelengths. But as the pump power was increased, the bands sharpened, and above a certain threshold, very sharp frequency bands appeared. The narrow frequency range of those bands and their strong polarization confirmed that the light was in fact a laser emission.

The researchers were actually able to observe the cavities forming and lasing in the zinc oxide powder film using a special camera at high magnification.

Cao said that the random orientation of the self-formed laser cavities in the film is also an advantage because it makes the laser output omnidirectional.

"Traditional lasers are highly directional, while LEDs shine in all directions," she said. "But if a zinc oxide laser device ultimately works, it could replace LEDs in luminescent display devices, because the light would go in all directions but with much greater efficiency.

"A laser at the same power consumption could be 1,000 times brighter than an LED," she said.

Another author on the Applied Physics Letters paper is Seng-Tiong Ho, an associate professor in electrical and computer engineering, who is modeling optical processes and developing new devices.

The research was funded by the National Science Foundation through the Northwestern University NSF Materials Research Science and Engineering Center.