Metamaterial lens focuses radio waves with extreme precision
15 November 2012
Metamaterials, with their intricately designed structures, are capable of bending electromagnetic waves in ways that are impossible for materials found in nature.
The orientation of 4,000 S-shaped units forms a metamaterial lens that focuses radio waves with extreme precision, and very little energy lost (photo: Dylan Erb)
Scientists are investigating metamaterials for their potential to engineer invisibility cloaks — materials that refract light to hide an object in plain sight — and 'super lenses' which focus light beyond the range of optical microscopes to image objects at nanoscale detail.
Researchers at MIT have now fabricated a three-dimensional, lightweight metamaterial lens that focuses radio waves with extreme precision. The concave lens exhibits a property called negative refraction, bending electromagnetic waves — in this case, radio waves — in exactly the opposite sense from which a normal concave lens would work.
Concave lenses typically radiate radio waves like spokes from a wheel. In this new metamaterial lens, however, radio waves converge, focusing on a single, precise point — a property impossible to replicate in natural materials.
A metamaterial’s extraordinary properties are determined largely by its structure, similar to how a diamond’s crystals impart strength. A material can refract light differently depending on the shape of individual units within a material, and the arrangement of those units as a whole.
The MIT team has come up with a blocky, S-shaped 'unit cell' whose shape refracts radio waves in particular directions. This unit shape is used as the basis for the concave lens, the rough shape being created from more than 4,000 unit cells, each only a few millimeters wide.
3-D printing was used to build a lens layer by layer from a polymer solution, each layer being subsequently coated with a fine mist of copper to give the lens a conductive surface.
To test the lens, the researchers placed the device between two radio antennas and measured the energy transmitted through it. Most of the energy was able to travel through the lens, with very little lost within the metamaterial — a significant improvement in energy efficiency when compared with past negative-refraction designs. The team also found that radio waves converged in front of the lens at a very specific point, creating a tight, focused beam.
The device, which weighs less than a pound, may be used to focus radio waves precisely on molecules to create high-resolution images — images that are currently produced using bulky, heavy and expensive lenses. Such a lightweight device could also be mounted on satellites to image stars and other celestial bodies in space.
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