Explosive stripping of a material's surface.

An intense, ultrashort pulse of laser light can blast away a solid's surface. Known as laser ablation, this process is used industrially and surgically for removing thin layers of materials

Now, researchers have caught a glimpse of the radical restructuring that takes place on a surface in the brief interval between a laser pulse's arrival and the subsequent explosive boiling away of the material's atoms.

The solid's top layer initially turns into a dense, high-temperature, high-pressure liquid, says Klaus Sokolowski-Tinten of the University of Essen in Germany. The hot, pressurized liquid then expands very rapidly, forming bubbles and breaking up into a transparent, liquid-gas mixture.

Sokolowski-Tinten and his collaborators report their findings in the July 6 Physical Review Letters.

The researchers studied the effect of 120-femtosecond pulses of laser light at a wavelength of 620 nanometers on a variety of metals and semiconductors, including silicon, gallium arsenide, gold, and aluminum.

Using time-resolved microscopy, they observed an optical pattern known as Newton rings, which coincided precisely with the surface area heated by the laser pulse. Such a pattern is produced when light rays reflecting from the top of the ablating material interfere with light rays reflecting from the solid material that remains intact beneath the cloud, which is rapidly expanding upward.

The observations suggest that the ablating material is in a transparent state with a distinct front that typically moves upward at a few hundred meters per second.

Moreover, because different metals and semiconductors exhibit similar interference patterns when blasted by a laser pulse, the same physical mechanism appears to be at work in all these cases. "It is just boiling, but under very exotic conditions," Sokolowski-Tinten remarks.

"It's very accurate work using a rather clever technique," says Peter Pronko of the University of Michigan in Ann Arbor. However, the conclusion that a liquid--rather than a dense plasma of electrically charged particles--forms during the transition requires independent confirmation, he adds.

In one such effort, Ralph Jimenez and his coworkers at the University of California, San Diego are using X-ray diffraction techniques to measure changes in the spacing of atoms in laser-blasted gallium arsenide. However, Jimenez comments, it's too early in the data analysis to tell whether the lattice goes through a liquid state.