Scientists have devised an invisibility cloak that works in the microwave region of the spectrum. A cloak that works with all wavelengths of light has yet to be developed.

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SYDNEY: An invisibility cloak that works in the microwave region of the electromagnetic spectrum has been developed by researchers in the USA.

The cloak works by deflecting microwave radiation around an object, making it appear to an receiving instruments as though there were nothing obstructing it at all, rendering it, effectively invisible. The technology is described in a paper published today in the U.S. journal Science.

Co-author David Smith of Duke University in North Carolina explains: “Imagine a boulder that protrudes at some location in a stream. The water flows around the boulder and then regroups on the other side. It’s not a perfect analogy, but it does help you to picture what the [electromagnetic] fields are doing.”

The cloak was constructed using ‘metamaterials’ to confer the specific electromagnetic properties required. Materials with such exotic optical properties do not occur naturally, so have to be engineered from scratch.

While the properties of natural materials are determined by their chemistry, the properties of metamaterials depend on their physical structure. In the invisibility cloak, that structure consists of copper rings and wires patterned onto sheets of fibreglass composite that are traditionally used in computer circuit boards.

The device is about the size of a frisbee and – to simplify fabrication – has been designed to work only in two dimensions rather than three. According to the researchers, however, a cloak that works in three dimensions should be possible in the future.

To assess the cloak's performance, the researchers directed a microwave beam at the cloak, which was situated between two metal plates inside a test chamber. They then measured the electromagnetic fields that developed inside and outside the cloak.

An animated representation of the data revealed that the wave fronts of the beam separated and swept around the centre of the cloak.

According to Smith, “Any object could be hidden so long as it fits within the available volume.” This could include humans, if a large enough cloak were built, he said.

But the next invisible man could be a long time coming. Even though all light, radio waves and microwaves are forms of electromagnetic radiation, they operate at different frequencies and wavelengths. To make an object literally vanish before a person's eyes, a cloak would have to simultaneously interact with all the wavelengths that make up visble light, said Smith. That means the cloak would have to be constructed to operate on all the colours of the rainbow at the same time.

That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise.

According to Smith, "It's not yet clear that you're going to get the invisibility that everyone thinks about with Harry Potter's cloak or the Star Trek cloaking device."