SAN DIEGO University of California, San Diego (UCSD) researchers have announced the physical realization of negative-index-of-refraction materials that have been predicted by theory. Such "metamaterials" have optical properties that would be impossible with conventional materials and could lead to novel antennas or perfect optical lenses.
"We have demonstrated for the first time that metamaterials can be engineered to have specific electromagnetic behaviors that are physically impossible for natural materials," said UCSD physicist David Smith. His work, conducted with UCSD physicist Richard Shelby, was done under the guidance of Sheldon Schultz, a professor of physics at UCSD in a study supported by the Defense Advanced Research Projects Agency. The San Diego researchers have filed a patent application covering the construction of their metamaterial.
"We believe that engineers will find all types of wonderful applications for metamaterials," said Smith. "For instance, the cellular communications industry will be able to create novel new filters and solid-state antennas that fit right on a circuit board. And when we refine it to operate at visible frequencies, we will be able to create a perfect lens."
The researchers found a way to physically implement a longstanding theoretical analysis of how materials with a negative index of refraction might behave. All known materials have been found to have a positive index of refraction. But in 1968, Russian theorist V.G. Veselago predicted that metamaterials could someday be engineered that would interact with their environment in a manner precisely the opposite of the way natural materials react.
Metamaterials use repeated composite structures with properties specifically engineered to "break" inconvenient laws of nature. In particular, all naturally occurring materials exhibit two parameters: permeability and permitivity. Those two properties determine how a material will interact with electromagnetic (EM) radiation, including light, microwaves, radio waves, X-rays and all other EM wavelengths. The "right-hand rule" which all natural materials follow because their permeability and permitivity both have positive signs shows the direction in which the wave velocity propagates relative to the orthogonal electric and magnetic fields affecting it.
But the same wave equations can be solved when negative values are entered, a fact that allowed Veselago to predict the theoretical behavior of materials that would have a negative index of refraction. Those hypothetical materials would follow a "left-hand rule," thereby reversing their effect on EM radiation.
In the past 10 years, electromagnetic research has evolved a new class of "photonic bandgap" materials, which are essentially artificially constructed assemblages of materials with differing dielectric constants. For example, a piece of silicon is drilled with a regular pattern of holes, which have a different dielectric constant from the surrounding silicon. It was found that a periodic pattern would act on light waves just as the periodic atomic structure of silicon creates bandgaps for electrons.
The research actually began in the microwave region of the spectrum, because the long-wavelength radiation can be manipulated with macroscopic structures. Once in the visible range, the structures need to be organized on the micron scale, which is much more difficult to achieve, although it is now within grasp.
The UCSD experiment likewise uses microwave radiation, allowing the scientists to use circuit boards with copper patterns as the basic "optical" medium. In the UCSD metamaterial, a specific conductive pattern is lithographically imprinted on a vertically oriented substrate in a repeated planar pattern. The configuration represents a metamaterial from a repeating planar pattern of copper split-ring resonators (SRRs) and wires on a thin fiberglass circuit board. The SRRs and wires are arranged into a 2-D structure with a repeated 5-mm lattice, each composed of six SRRs and two wires. The SRRs are arranged in three groups of two resonators each, one slightly smaller and located inside the boundaries of the larger one. The wires are placed on the opposite side of the circuit board from the SRRs (see www-physics.ucsd.edu/lhmedia/how.html for a mathematical analysis of the structure).
To prove the concept, the UCSD researchers built an interlocking 3-D "lattice" of the circuit boards and then cut the assembly on a diagonal to create a prism. That allowed them to perform a microwave version of the classic prism, which deflects light at a positive angle to the surface normal. In this case, the emerging radiation was bent away from the normal exactly opposite the manner of all natural materials.
The team got a clue to the required geometry from work announced last year by John Pendry, a physicist at Imperial College in London, who showed that certain configurations of metal would respond to magnetic fields in a left-handed manner.
Further, Pendry's engineered magnetic metamaterial had a negative permeability, something not observed in natural materials and previously only hypothesized. The clear demonstration of negative permeability set the UCSD researchers looking for a metamaterial that had both negative permeability and negative permitivity the "unicorn" of the EM universe, a material that breaks the laws of nature by following a previously hypothetical left-handed rule.
Researchers at Imperial College and Marconi Caswell (London) this year announced a magnetic resonance imaging system using a magnetic metamaterial based on Pendry's design. The metamaterial appears to violate Snell's Law, which uses the right-hand rule to deduce that all materials bend light toward the normal by a factor called the index of refraction. Many of the other properties of the metamaterial are also expected to be proven to follow a new left-hand rule.
"We don't say that our metamaterial breaks Snell's Law; rather, we say that we have learned how to build a metamaterial with a negative index of refraction," said UCSD's Smith.
Metamaterials with a negative index of refraction would enable all sorts of new devices and should be especially useful, said Smith, to designers building smart antennas. By combining traditional materials with a positive index of refraction and metamaterials with a negative one, it should be possible to build highly sensitive antennas shaped to fit inside any enclosure.
Likewise, with a negative index of refraction, it should be possible to build a completely planar (flat) lens that nevertheless focuses light to a perfect geometric point. Instead of grinding the lens to specific convex or concave angles, metamaterials combined with traditional materials should be able to serve all lens needs with easy-to-make planar surfaces. "We think our principles can be extended to all wavelengths of electromagnetic radiation, eventually including visible light," said Smith.
The next step is to verify other predicted properties of the metamaterial, such as an ability to reverse the Doppler effect, which makes a train-whistle sound higher in pitch as it approaches and lower in pitch as it recedes. It is predicted that a reverse Doppler effect will result from a moving metamaterial source that is, shifting to lower frequencies as a source approaches and to higher ones as it recedes.