Thin-film mirror changes into a window.

Mirror, mirror, on the wall, who're the cleverest of them all?

This week, the answer appears to be Ronald P. Griessen, a physicist at Free University in Amsterdam, and his colleagues. The group has devised a metallic thin film that can be switched from a reflective mirror to a transparent window.

Describing their switchable mirror in the March 21 Nature, the physicists explain that thin films of yttrium or lanthanum hydrides, when exposed to hydrogen, abruptly become insulators rather than metals. At the same moment, such a film loses its reflective metallic sheen and changes into a see-through sheet.

The process is fully reversible, they add.

The researchers made their discovery while working on an entirely different project, a study of superconductivity in hydrogen under extreme pressure and low temperature. "One day, while we were exposing yttrium hydride to hydrogen in the laboratory, the sample just disappeared," says Griessen. "It became almost invisible while the film was absorbing hydrogen."

After months of investigation, the team discovered they could reproduce the strange effect at room temperature and lower pressure. Their switchable mirror consists of a film of yttrium dihydride only 500 nanometers thick, coated with a 20 nm protective layer of palladium.

To see whether the thin film's change in appearance is linked to its electrical transition, the physicists measured various characteristics, including resistance, photoconductivity, and optical transmission. They found that the switch from reflectivity to transparency indeed correlates with the change in its electrical properties.

"We're not absolutely certain why this optical effect occurs," Griessen says. "There are several possible explanations, making this phenomenon interesting from a fundamental point of view."

Louis Schlapbach, a physicist at the University of Fribourg in Switzerland, calls Griessen's original experiment "a clever idea, which proves that the [current] theory of metal-to-nonmetal transition in rare earth hydrides is correct."

According to J.J. Huub Eggen, a scientist at the Dutch national science foundation in Utrecht, the electronics manufacturing company Philips is seeking to patent the technique. Though applications remain years away, the material may ultimately prove useful for solar cells, electronic switches, optical sensors, and even windows in houses.

"This switchable mirror is a good example of an inadvertent spin-off from basic research," says Griessen. "We set out to do fundamental science and stumbled into this discovery, which may have great technical relevance. "Sometimes, to discover something really new," says Griessen, "you just have to play around."