A team of researchers in China has manufactured an artificial surface which is capable of bending electromagnetic waves, an advancement in metamaterials that will go far beyond the creation of new antennas that will conform to any shape. The breakthrough that is being called “broadband transformation optics” was reported in the American Institute of Physics journal, called Applied Physics Letters, as being built with small-scale metallic U-shaped structures on a dielectric material that mimic the Luneburg Lens.
Dielectric materials are insulators which can be polarized with an electrical field, and because they do not conduct electricity they can be used to shift the equilibrium in either direction. When a positive charge moves towards an applied electrical field, the negative charge moves into the opposite position. This creates another field within the material that redistributes weakly bonded molecules into symmetrical alignment with the original charge.
The artificial surface was created by Tie Jun Cui and a team at the Southeast University of Nanking, China and it marks a stepping stone in metamaterial advancement, which contain properties not found in nature. Most lenses are made of glass or plastic that bend light in a uniform way, called the “index of refraction.” In 1944, Rudolf Luneburg invented a lens with a gradient index that was parallel on either side, creating a mutual focal point of refraction from opposite directions. This technology has been used to adjust microwave frequencies for antenna and radar calibration, and cylindrical versions have been used to narrow the rays of laser diodes.
The Luneburg lens contains a variable refractive index, making it capable of focusing electromagnetic radiation such as radio waves and microwaves, and will eventually be used with infrared light and visible light. A normal lens transfers waves through the surface, but the Luneburg lens can bend this electromagnetism to an off-axis point at the edge, uniformly channeling it from one side to another in a single path. The shape of the lens was the last obstruction because it had to be curved, but for applications such as radar reflectors this was an inconvenience, so the Chinese developed a metamaterial that could perform the same tasks but with any shape.
With the manipulation of surface wavelengths through geometric optics, holographic optics, and now transformation optics, this metamaterial in other applications in reference to their capacity to guide light around an object will be the next stage of development for invisibility. If light waves bend around an object and proceed in a straight line, the object itself would not cast a shadow. Using this with microwaves from radar, if an airplane can be coated in a layer of the metamaterial, it would be rendered undetectable. Cloaking devices for the human eye will be more difficult to achieve because the electromagnetic radiation that makes up light waves are measured in nanometers, or billionths of a meter, and therefore the material would have to be manufactured on a nano-scale and because different colors are different wavelengths it would have to account for every variation. The United States Department of Defense agency called DARPA, or Defense Advanced Research Projects Agency, will be looking into this artificial surface in a way that will likely create competition equivalent to a technological arms race.
