In Spring 2008, Jason Valentine, a graduate student in the nano-engineering lab at the University of California at Berkeley, US, claimed to have made 3D metamaterial with a negative refractive index. Previous matamaterials have only worked in 2D.
Valentine's "prism" is made from 21 alternating layers of silver and magnesium fluoride, arranged in a "fishnet" structure. He claims that the refractive index is negative in a small region of the near-infrared spectrum. A paper on his negative prism was presented on 8 May at the Quantum Electronics and Laser Science Conference in San Jose, California, US.
However, Gunnar Dolling of the University of Karlsruhe, Germany, who has made a flat negative refractive-index material for light on the boundary between red and infrared light is unconvinced by Valentine's claims.
Valentine based his figures on measuring that light was bent backwards by the prism. But Dolling says that he showed last year that, for metamaterials similar to Valentine's, that method does not clearly indicate the material's true properties.
Dolling and colleagues observed that the complex interactions between light waves and such metal-based metamaterials can deflect a light beam the "wrong way" without a negative refractive index. He told New Scientist, "You can only measure a negative refractive index by measuring the phase velocity", meaning the actual speed of light in the medium."
In 2003, scientists at the University of Tokyo in Japan first demonstrated a different approach - called "optical camouflage". Rather than bending waves around an object, this approach uses virtual reality.
The world's first cloaking demonstration bent microwaves around a 110 cubic centimeter enclosure, effectively hiding it from detection.
For further information also read Invisible electronics, Metamaterials: Printing the cloak of invisibility, Inorganic and Composite Printed Electronics 2008-2018 and attend Printed Electronics Asia 2008 or Printed Electronics USA 2008.