“Carpet cloak” hides more than its size implies

Compiled by Photonics Spectra staff

Optical cloaking is getting a boost from metamaterials in a new technology called “carpet cloaking,” which can conceal a much larger area than other cloaking techniques of comparable size.

The new carpet cloak, based on an alternating-layer structure on a silicon-on-insulator (SOI) platform, introduces a flexible way to address the size problem. It was developed by an international team of physicists from Technical University of Denmark (DTU), the University of Birmingham in the UK and Imperial College London. Their approach appeared in OSA’s open-access journal Optics Express (doi: 10.1364/OE.19.008625).

The cloak was designed with a grating structure that is simpler than previous metamaterial structures for cloaks, said Jingjing Zhang, a postdoctoral researcher at DTU. The grating structure – a series of slits or openings that redirect a beam of light – channels light of a particular wavelength around an object.

“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,” Zhang said.

The measured output image from a flat surface (left) and a cloaked protruded surface (right) at (a) 1480 nm, (b) 1550 nm and (c) 1580 nm. Courtesy of Technical University of Denmark and Optics Express.

“Filling factor” 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 1480 to 1580 nm. This means the carpet cloaks essentially distinguish an object from light, making it appear like a flat ground plane.

The parameters of the cloak could be tweaked by tuning the filling factor and the orientation of the layers, which would allow the layered materials to bypass the limitation of natural materials at hand and give the team extra freedom to design the devices as desired, Zhang said.

The cloak absorbs a negligible fraction of energy because it is made exclusively of dielectric materials that are highly transparent to infrared light.

The researchers still want to improve the technology: They reported in their paper that, although the cloaking ensures that the beam shape is unaffected by the presence of the object, the beam intensity is slightly reduced. Attributing this to reflection at the cloak’s surface and partly because of imperfections in the fabrication, they discovered that adding a layer of material around the cloak to improve the uniformity of the grating would help to eliminate the reflection and scattering issues.

With more precise fabrication, the team hopes, the cloak will work in the microwave and visible parts of the spectrum, opening doors for futuristic defense applications.