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UV light makes microscopic cracks glow blue.

With lasers, materials scientists become surgeons, performing delicate operations with micrometer precision. They sculpt minute features into thin films (SN: 8/22/92, p.119) and zap materials to knock off atoms and deposit them elsewhere. This laser ablation yields ceramic coatings and high-temperature, superconducting thin films.

Now physicists have discovered an easy way to monitor a laser's interaction with inorganic crystals. In ultraviolet (UV) light, atomic-size nicks or defects glow bright blue, says Tom Dickinson of Washington State University in Pullman.

At the Materials Research Society meeting in Boston last week, he described how this simple visualization technique helps him understand laser ablation and possibly create tougher ceramic materials. "It's a new way to look at the [development of] fracture in brittle materials," Dickinson told Science News.

These results represent "a real change in the way that people look at how light is absorbed on surfaces," adds Richard F Haglund Jr., a physicist at Vanderbilt University in Nashville, Tenn.

Dickinson's group first noticed this luminescence when they aimed an intense laser through a material that was supposedly transparent to the laser. Over time, however, the material lost its transparency because the laser caused tiny fractures. Under UV light, those fractures showed up as many minute cleavages, all ringed with blue light.

"It's fairly spectacular to the eye," Dickinson says.

By bathing a sample in a low-intensity UV laser while processing and then photographing the luminescence through a microscope, he expects to be able to study the evolution of these defects. "We now have a way of finding spatially where the crystal has deformed," he adds.

These deformations mark vacancies in a crystal's orderly array of atoms. Such gaps occur when external energy causes a cluster of atoms in this array to shift, while one atom in the cluster lags behind. This atom falls into a space between two other atoms, leaving its spot in the array empty he says. Electrons flow into these spots; when light hits them, they get excited and give off the blue light.

A very intense laser can cause these dislocations to develop, but so can mechanical forces, such as scratching with a diamond tip. "And on a polished surface of these crystals, the luminescence is everywhere," says Dickinson. These microscopic cracks can make materials such as ceramics more likely to break.

"[Dickinson] has worked very patiently to demonstrate the link between microcracking and these defects," says Haglund. "It's a way of identifying what defects are there and away of cleaning them up." Scientists can then use a laser to smooth out the damage so it would be a more perfect surface," Dickinson says.

This nondestructive technique could also enable researchers to fine-tune laser ablation. While studying the effects of laser ablation on a magnesium oxide crystal, Dickinson and his colleagues observed that the laser chips away atoms along the blue-light-emitting cleavages in the crystal. "If we can control where the defects are, then we may be able to control where ablation occurs," says Dickinson. "I think we can get down to 1 micron resolution."
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Title Annotation:ultraviolet light
Author:Pennisi, Elizabeth
Publication:Science News
Date:Dec 12, 1992
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