Novel cloaking device makes 'larger' objects invisible

In the past year, there has been considerable progress in the development of cloaking technology, with several groups demonstrating workable devices that can conceal very small objects from optical light.

However, these devices were around 10–16 times larger than the objects that they were actually able to conceal, which places serious constraints on future practical applications when scaling up.

Now, a group of researchers from Denmark and the UK has tested a novel metamaterial cloaking device that is only around four times the size of the object it was able to conceal.

‘Instead of transforming the cloaked area to a point invisible to our eyes, a carpet cloak disguises the obstacle from light by making it appear like a flat ground plane,’ said Shuang Zhang of Birmingham University, who worked on the project alongside colleagues from Imperial College London and the Technical University of Denmark.

Like previous groups, the researchers used metamaterials, which are engineered to have optical properties not found in nature, but used a novel grating structure comprising a series of slits or openings to redirect a beam of light.

They achieved this by adapting semiconductor manufacturing techniques that involved patterning the top layer of a silicon-on-insulator (SOI) wafer with nanogratings of appropriate filling factor, which determines the wavelengths of light that are affected by the cloak.

By restoring the path of the reflecting wave from the surface, the cloak created an illusion of a flat plane for what was a triangular ramp object — hiding its presence over wavelengths ranging from 1,480nm to 1,580nm.

‘The majority of the invisibility cloaks reported so far have a spatially varying refractive index, which requires complicated design processes — our strategy eliminates this and, in terms of size, it allows for a very large obstacle-to-cloak ratio,’ Zhang said.