From Grimm's fairy tales to Harry Potter to Star Trek's Romulans, the cloak of invisibility has played a major role in fiction.

Now scientists have taken a small but important new step toward making it reality.

This computer model shows the structure of the cloaking device, with the bump in the layer of gold and invisibility cloaking structure underneath. (Science/AAAS)

Researchers at Germany's Karlsruhe Institute of Technology report they were able to cloak a tiny bump in a layer of gold, preventing its detection at nearly visible infrared frequencies.

Their cloaking device also worked in three dimensions, while previously developed cloaks worked in two dimensions, lead researcher Tolga Ergin said.

The cloak is a structure of crystals with air spaces in between, sort of like a woodpile, that bends light, hiding the bump in the gold layer beneath, the researchers reported in Thursday's online edition of the journal Science.

In this case, the bump was tiny, a mere 100 micrometres high and 30 micrometres across, so that a magnifying lens was needed to see it. A micrometre is a millionth of a metre.

"In principle, the cloak design is completely scalable; there is no limit to it," Ergin said. But, he added, developing a cloak to hide something takes a long time, "so cloaking larger items with that technology is not really feasible."

"Other fabrication techniques, though, might lead to larger cloaks," he added in an interview via email.

The value of the finding, Ergin said, "is that we learn more about the concepts of transformation optics, and that we have made a first step in producing 3-D structures in that field.

"Invisibility cloaks are a beautiful and fascinating benchmark for the field of transformation optics, and it is very seldom that one can foretell what practical applications might arise out of a field of fundamental research," he added.

Bounce off objects

In earlier research, a team led by David Schurig at Duke University developed a way to cloak objects in two dimensions from microwaves. Like light and radar waves, microwaves usually bounce off objects, making them visible to instruments and creating a shadow that can be detected.

The new research led by Ergin used infrared waves, which are close to the spectrum of visible light.

In cloaking, special materials deflect radar, light or other waves around an object, like water flowing around a smooth rock in a stream. It differs from stealth technology, which does not make an aircraft invisible but reduces the cross-section available to radar, making it hard to track.

Ergin's research was supported by the German Research Council, the state of Baden-Wuerttemberg, the European Commission and the German Ministry for Education and Research.