Portland, Ore. -- As recently as this summer, invisibility cloaks were only a theoretical possibility. Now the world's first cloaking demonstration has bent microwaves around a 7.2-cubic-inch enclosure, effectively hiding it from detection. The proof of concept suggests how engineers might pattern split-ring resonators to create "designer" metamaterials.
"There is going to be a need for EEs to figure out what patterns to use" for such materials, said David Schurig, a postdoctoral fellow at Duke University. "I advise all EEs to learn how to implement a material design specification by constructing these SRR [split-ring resonator] patterns." Schurig did the proof-of-concept work with professors David Smith at Duke and John Pendry at London's Imperial College.
Split-ring resonators--free-space rings of metal with a gap that prevents them from being a complete ring--are usually patterned on a fiberglass circuit board. When microwave radiation passes through them, they act as a dielectric whose magnetic permeability and electrical permittivity can be custom-tailored by adjusting the size and shape of the resonator.
"The trick is to design a repeating pattern [of split-ring resonators] in such a way that these little resonant objects respond to give us the [magnetic] permeability and the [electrical] permittivity that we are after [for bending waves around objects]," said Schurig. "We tune the magnetic resonance by changing the width of the split in the ring, and we tune the electric resonance by rounding off the corners of our [otherwise] square-ring [shape]."
The cloak demonstrated earlier this month was designed to bend microwaves. But the researchers claim that with more engineering effort, EEs could create invisibility cloaks for any type of electromagnetic radiation, even visible light.
Snell's Law, or the "right-hand rule," relates the magnetic element of propagating waves to their electrical element, stating that magnetism curls around a wire in the direction of your fingers when you point your thumb in the direction of current flow. The right-hand rule predicts that light will bend toward the normal when passing through a material, because both magnetic permittivity and electrical permeability are positive values in all natural materials.
Metamaterials constructed of free-space split-ring resonators, however, can exhibit negative permeability and negative permittivity and thereby bend waves away from the normal. That allows even a flat lens to focus light to a point.
The demonstration performed just such a manipulation, by bending microwaves around a 110-cm2 cloaked enclosure and then bending them back into their original direction on the other side. Three resonators at the front lip, middle section and back lip of the cylinder-shaped enclosure enabled the microwaves to bend up, over and then down, respectively, "similar to water flowing around a smooth rock," said Schurig.
The demo used 10 concentric cylinders. The innermost measured 2.4 inches in diameter and 0.4 inch deep; its 7.2-cubic-inch interior became invisible to micro-waves. The outermost cylinder measured 4.8 inches in diameter--small enough to fit inside of a microwave guide for testing.
The cylinders' magic was wrought from the patterns inscribed on their surfaces: traces manifesting arrays of split-ring resonators. Each resonator was custom-tailored to deflect the microwaves around the central region, with resonators in the central rings having a stronger effect than those on the outer rings.
"The patterns must change a little bit [from cylinder to cylinder] to get the material properties that we want at that radius," said Schurig. By adjusting the index of refraction of the material for the wavelength to be cloaked, electromagnetic energy "flows" around the hidden object and continues, undistorted, on the other side. The cloak neither reflects nor casts a shadow.
Assisting in the work were research associate Jack Mock and student Bryan Justice, both of Duke, and Anthony Starr, president of SensorMetrix (San Diego).