Now you see it, now you don’t – or do you??

David Smith (left) is philosophical about metamaterials

By James Dacey

In March, a group of researchers in the UK and Germany grabbed the science headlines when they unveiled the world’s first optical cloak that could hide an object in three dimensions. Okay, the thing they hid may only be a microscale bump but the researchers say their design could in principle be scaled up into a full Harry Potter-style invisibility cloak.

A new paper, however, on the arXiv preprint server has poured cold water on the breakthrough by pointing out a slight flaw in the cloak design – namely that it doesn’t really hide things.

Cloaking requires special materials known as metamaterials, which possess unique optical properties. The trick is to create a material whose optical properties are constantly changing so that it “steers” waves around an object as if it were not there.

This recent breakthrough involves a type of cloak known as a “carpet cloak” because it involves smoothing out a bump on a surface as if flattening out a ruck in a rug. The researchers stacked nanofabricated silicon wafers on top of one another to produce a distribution of refractive indices. As light reflected off the surface, it appeared as if the device (and the bump it was hiding) was not there.

To engineer the carpet cloak, the researchers had to modify a conducting surface to create a grid in which the mechanical and optical properties are uniform in all directions. And this is the point where the technology breaks down – claim a trio of researchers in this arXiv paper.

Bae-Ian Wu and his colleagues at Massachusetts Institute of Technology are concerned that a cloak that is isotropic could never truly hide an object. Instead, it shifts an object to the side by an amount related to the angle of incoming light: change your viewing angle, you still see the object.

To illustrate their point, they trace a ray of light as it approaches a bump of 0.2 units from an angle of 45 degrees. According to the calculations, the ray is shifted laterally by 0.15 units.

The solution, they say, is to create a cloak whose optical properties are anisotropic, i.e. they vary depending on orientation.

I got in touch with John Pendry of Imperial College in London who was involved in the recent cloak design to get his reaction. “We already knew that the isotropic cloak was not exact, the question is by how much,” he said. “The first step [in the research] was the exact specification of the cloak which made objects truly invisible when viewed against a mirror background. However this exact prescription requires anisotropic optical materials and would be very difficult to manufacture.”

I also got in touch David Smith, at Duke University in the US, who is credited with creating the first metamaterial cloak back in 2006. “The carpet cloak is not really a cloak, as we know, but a different example of a transformation optical structure,” he said.

Smith was philosophical, however, about the achievements in metamaterial research so far. “The optics world has lived for 100s of years with imperfect optics! If our goal is not to make something perfect, but make something that is less poor than what exists, our chances of success are much greater. Transformation optics and quasi-conformal techniques help us do just that.”