Efforts to create a working "invisibility cloak" have generally involved the use of artificial materials with a negative refractive index known as metamaterials. Another promising technique involves the use of a natural crystal called calcite that boasts an optical property known as birefringence, or double-refraction. While both methods have proven successful in rendering very small objects invisible in specific wavelengths of light by bending and channeling light around them, both techniques require the "cloak" to be orders of magnitude larger than the object being concealed. Researchers are now reporting progress in overcoming this size limitation using a technology known as a "carpet cloak."
The carpet cloak technology developed by an international team of physicists from the Technical University of Denmark (DTU), the University of Birmingham, UK, and Imperial College London (which is the same team responsible for the calcite-based invisibility cloak) doesn't refer to a cloak that can conceal fabric floor covering, but technology that can conceal a much larger area than other cloaking techniques of comparable size. Metamaterials are once again central to the technology, but the researchers have employed a new grating structure that consists of a series of slits or openings that redirect a beam of light.
The cloaking material, which is highly anisotropic – meaning it has different physical properties along different axes – was created using semiconductor manufacturing techniques that involve patterning the top silicon layer of a silicon on insulator (SOI) wafer with nanogratings of an appropriate size and structure, referred to as the filling factor, which determines the wavelengths of light that are affected.
"This leads to a cloak only a few times larger than the cloaked object," says Jingjing Zhang, a postdoctoral researcher at DTU's Fotonik Department of Photonics Engineering and Structured Electromagnetic Materials. "The cloak parameters can be tweaked by tuning the filling factor and the orientation of the layers. Therefore, layered materials bypass the limitation of natural materials at hand and give us extra freedom to design the devices as desired."
By precisely restoring the path of the reflecting wave from the surface, the researchers have created the illusion of a flat plane that conceals the presence of a triangular bump on the surface over wavelengths ranging from 1,480 nanometers to 1,580 nanometers. Additionally, the researchers say the cloaking carpet is also easier to design and fabricate than other nanostructure-based cloaking techniques.
Being made exclusively of dielectric materials that are highly transparent to infrared light, the cloak is also very efficient and absorbs a negligible fraction of energy. However, although the cloak ensures that the beam shape is unaffected by the presence of the object, the beam intensity is slightly reduced. The researchers attribute this to reflection at the cloak's surface as well as imperfections of the fabrication, but they say that adding an additional layer of material around the cloak and improving the uniformity of the grating would help eliminate reflection and scattering issues.
"Although our experiment was carried out at near-infrared frequencies, this design strategy is applicable in other frequency ranges," notes Zhang. "We anticipate that with more precise fabrication, our technique should also yield a true invisibility carpet that works in the microwave and visible parts of the spectrum and at a larger size – showing promise for many futuristic defense and other applications."
The researchers describe their carpet cloak technology in a paper titled "Homogeneous optical cloak constructed with uniform layered structures" published in the journal Optics Express.