Glass semiconductor softens with low-power laser, then re-hardens

Columbus, Ohio, September 17, 2004--Scientists at Ohio State University have found that a special type of glass that is finding use in the electronics industry softens when exposed to very low-level laser light, and hardens back into its original condition when the light is switched off.

The discovery -- made by accident as physicists were trying to study properties of the material -- may one day enable new uses for the glass. Ratnasingham Sooryakumar said that he and former doctoral student Jared Gump thought they were working with a bad batch of germanium-selenium glass when Gump was testing the material's hardness in the laboratory and couldn't reproduce his results. They finally traced their strange results to the very low-power laser light that they were shining on the glass. Whether the laser was set to exactly the same power every time shouldn't have affected the experiment, but it did. The higher the laser power, the softer the glass. In fact, with the laser set to a mere 6 mW, the material became 50 percent softer than usual. In the journal Physics Review Letters, the physicists reported that the glass always hardened back into its original condition. Even the latticework of atoms that made up its structure appeared unchanged afterward. The glass is part of a family of glass semiconductors that are often used in electronics for DVDs and information storage technologies.

A combination of 80 percent selenium and 20 percent germanium is the "magic formula" where the material is neither too hard nor too soft, and well suited for forming a glass. Scientists call this point the rigidity transition. One way to determine the hardness of a material is to measure the speed of sound waves traveling through it; sound waves travel faster through harder materials. The physicists bounced a low-powered red laser beam off the sound waves to measure the speed. Here's why the physicists think the glass softened: When particles of light, called photons, hit the glass, they knocked some of the electrons that connect molecules in the latticework out of place. Such a change in bonding occurs most easily under conditions of minimum stress. With fewer supports holding up the structure, the glass became less stiff. Then, when the light was switched off, the electrons swung back into position, and the glass became stiff again.

Sooryakumar speculated that these types of glasses could have potential applications in re-writable computer memory. But right now, he and his colleagues are probing further to understand the rigidity transition and the remarkable response to light at this composition. They want to study what happens if the material is exposed to laser light of different color and higher power, and test different glasses besides selenium-germanium.