Disorderly crystals emit laser light.

In order to shine perfectly synchronized photons--the sine qua non of laser light--semiconductor crystals must have perfectly arranged atoms. Any defects in the crystal scatter emitted photons, degrading the performance of the laser.

Now, researchers at Northwestern University in Evanston, Ill., have demonstrated a lasing material that actually uses scattering to its advantage. They found that a thin film of tiny, light-scattering zinc oxide crystals can emit ultraviolet laser light. Their results open the door to fabricating lasers simply by spreading a powder on a surface, says R.P.H. Chang. He and his colleagues report their findings in the Dec. 21, 1998 APPLIED PHYSICS LETTERS.

Semiconductor chips that form the basis of traditional lasers contain reflective surfaces that bounce emitted photons back and forth through the chip. These photons stimulate the release of additional photons that move in lockstep with each other, eventually emerging as a coherent beam of light.

The disordered zinc oxide film, on the other hand, doesn't need mirrors, says Chang. The emitted photons scatter from one tiny particle to another in closed loops, "like a dog chasing its tail." The irregularly shaped zinc oxide grains range in size from 50 to 150 nanometers and have numerous internal defects, which promote light scattering. If a critical number of photon circuits arises in a patch of grainy film, it emits laser light. Theorists predicted this phenomenon 3 decades ago, but his team's study is the first experimental demonstration, Chang says.

Because the particles are oriented randomly, the light shines out in all directions. This characteristic could make such films useful in flat-panel displays, whose images often fade when viewed from an angle, Chang notes. A display made with a lasing film could be a thousand times brighter than one made with light-emitting diodes, he adds.