Miniature İnvisibility 'Carpet Cloak' Hides More Than İts Small Size İmplies

Invisibility cloaks are seemingly futuristic devices capable of concealing very small objects by bending and channeling light around them. Until now, however, cloaking techniques have come with a significant limitation—they need to be orders of magnitude larger than the object being cloaked. This places serious constraints on practical applications, particularly for the optoelectronics industry, where size is a premium and any cloaking device would need to be both tiny and delicate.

An international team of physicists from the Technical University of Denmark (DTU), the University of Birmingham, UK, and Imperial College London, however, may have overcome this size limitation by using a technology known as a "carpet cloaks," which can conceal a much larger area than other cloaking techniques of comparable size. The researchers achieved their result by using metamaterials, artificial materials engineered to have optical properties not found in nature. They describe their approach in the Optical Society's (OSA) open-access journal Optics Express.

Jingjing Zhang, a postdoctoral researcher at DTU's Fotonik Department of Photonics Engineering and Structured Electromagnetic Materials, and an author of the Optics Express paper, explains that the team's new carpet cloak, which is based on an alternating-layer structure on a silicon-on-insulator (SOI) platform, introduces a flexible way to address the size problem.

"This new cloak, consisting of metamaterials, was designed with a grating structure that is simpler than previous metamaterial structures for cloaks," she says.

Grating structures channel light of a particular wavelength around an object. A grating structure is simply a series of slits or openings that redirect a beam of light.

"The highly anisotropic material comprising the cloak is obtained by adopting semiconductor manufacturing techniques that involve patterning the top silicon layer of an SOI wafer with nanogratings of appropriate filling factor. This leads to a cloak only a few times larger than the cloaked object," says Zhang. In this case, filling factor simply refers to the size of the grating structure and determines the wavelengths of light that are affected by the cloak.

By precisely restoring the path of the reflecting wave from the surface, the cloak creates an illusion of a flat plane for a triangular bump on the surface—hiding its presence over wavelengths ranging from 1480nm to 1580nm (see figure).