Toronto Composite metamaterials that exhibit a negative index of refraction are being harnessed to enable a variety of hitherto impossible applications, promising to reduce size and cost while simultaneously increasing accuracy and range.
"Electrical engineers will be interested to know that our metamaterial technology is now being adapted to make microwave devices and antennas with unprecedented levels of performance and functionality," said University of Toronto professor George Eleftheriades, himself an EE. "Our latest results are very promising for both basestation and handheld hardware. . . . Now is the time for electrical engineers to really start creating a whole new range of useful devices for the cell phone industry."
Metamaterials enable lenses without an optical axis, despite their planarity, to focus waves by means of refraction. Metamaterials substitute macroscopic objects for atoms in a giant, crystalline-like lattice. The Toronto team's lattice was constructed of perpendicular wires that defined a grid whose spacing was set to a subwavelength of the wavelength affected. (Another advance in metamaterials was recently reported at the University of Manitoba; see story, page 55.)
In 2002, Eleftheriades proved that a planar lens could be made from metamaterial at microwave wavelengths. To achieve subwavelength focusing of microwaves in the more recent work, Eleftheriades set the spacing to 1/20 of the wavelength he wanted to filter.
"We showed for the first time that you can make a flat, planar lens with a negative index of refraction that can focus microwaves," said Eleftheriades, who performed his pioneering proof with graduate students Anthony Grbic and Ashwin Iyer. "As far as whether focusing with negative-index-of-refraction metamaterials is possible, we have put that controversy to rest. We have clear experimental evidence that our lens is focusing.
"Now we are working on real applications by collaborating with Canadian telecommunications companies."
Eleftheriades foresees "enhancing the performance of devices that go into cell phones and basestations including the antenna, couplers, multiplexers and filters. There is a whole range of ways to harness a negative index of refraction to make smaller, multifunctional broadband devices."
Planar composite metamaterials with a negative index of refraction can enable the "perfect" lens one that achieves subwavelength, diffraction-free, near-field focusing by virtue of their ability to converge or focus waves passing through them, rather than diverge or diffract them, as natural materials do.
Materials that would exhibit a negative permittivity and permeability, and thus enable a negative index of refraction, were first hypothesized in 1968 by Russian theorist Victor Veselago. Such hypothetical materials, Veselago theorized, would interact with their environment in the opposite way from natural materials, which all have positive permeability and permittivity. Using mathematical models, Veselago predicted that such metamaterials would follow a "left-hand rule," which would reverse their effect on electromagnetic radiation, as opposed to the universal "right-hand rule" governing all normal materials.
One intriguing prediction was that composite metamaterials following the left-hand rule would allow a flat lens to focus electromagnetic waves.
Veselago's prediction that such lenses could be made lay dormant until 2000, when John Pendry, a physicist at the Imperial College in London, extended
the work and predicted that such lenses could allow perfect imaging. Pendry also showed that certain metals could be engineered to respond to electrical fields as though the field parameters were negative. Pendry demonstrated various metal configurations that exhibited a left-hand rule for magnetic fields.
In 2001, researchers at Imperial College and Marconi Caswell (London) announced a magnetic-resonance imaging system that used a magnetic metamaterial based on Pendry's design. The demonstration, however, did not prove the negative-refraction, perfect-lens hypothesis.
Then, in 2001, physicists at the University of California-San Diego demonstrated a left-handed composite metamaterial that exhibited a negative index of refraction for microwaves. The simple arrangement consisted of a planar pattern of copper split-ring resonators (SRRs) and wires on printed-circuit boards (www.eetimes.com/story/OEG20010430S0110). The SRRs and wires were arranged into a two-dimensional structure with a repeated 5-mm lattice, with the wires located on the opposite side of the board from the SRRs.
Using a prism made out of these metamaterials, the UCSD team produced the first experimental evidence of negative refraction of microwaves. The work was done by David Smith and Richard Shelby under the guidance of physics professor Sheldon Schultz in a study supported by the Defense Advanced Research Projects Agency.
Last year, Vladimir Shalaev, a professor at Purdue University, demonstrated a scaled-down version of Smith and Shelby's metamaterial that reduced the pitch of the SSR lattice from 5 mm to 15 nanometers so that light could take advantage of negative permeability and permittivity (www.eetimes.com/at/news/OEG20020826S0041).
The UCSD approach was pioneering, but it created controversy. Some EEs said the experimental data was weak, and they were skeptical about the physics of negative refraction and associated focusing.
To quell the debate, Eleftheriades took an electrical engineer's approach to making metamaterials. His work with Iyer on planar metamaterials demonstrated the focusing of microwaves over an octave bandwidth, and his work with Grbic demonstrated a focusing capability beyond the classical diffraction limit.
Eleftheriades also invented a manufacturing method for composite metamaterials that he said holds "the potential for commercial success."
The Eleftheriades material looks like a circuit board; but instead of the planar pattern of copper SRRs and wires used at UCSD, the Toronto approach lays down a mesh of metal lines whose pitch is smaller than the wavelength to be focused. The overlapping perpendicular lines in the mesh define square cells that are loaded with integrated inductors and capacitors. The periodic two-dimensional structure locates a dual-LC-loaded transmission-line network inside each unit cell.
The structure not only corrects the phase of propagating microwaves through negative refraction but also enhances the amplitudes of the "evanescent waves" that carry the subwavelength details of the object to be imaged.
The dual-loaded transmission line, according to Eleftheriades, provided undeniable experimental evidence: It not only demonstrated diffraction-free, near-field focusing by a flat lens but also affected an octave of wavelengths an order of magnitude more bandwidth than was possible with split-ring resonators.
"The key to our success is that we achieve a negative index of refraction over an octave for example, from 1 GHz to 2 GHz. Hence, our approach is much more tolerant to manufacturing variations, which we think is why we obtained focusing results that are reliable," Eleftheriades said. "More recently, we have double-checked that our original focusing experiments were valid by making an even larger prototype lens that more clearly focuses microwaves."
One unique feature of the dual-loaded transmission-line approach is that incident microwaves are focused within the plane of the single pc board holding the transmission line. "Everything happens on a plane. You have incident microwave power on the board, and it focuses on the board again," said Eleftheriades. "For microwave applications, this is an advantage, because electronic devices are not three-dimensional but planar. It is really very compatible with the way integrated circuits are fabricated."
Eleftheriades said his dual-loaded transmission-line approach is ready for commercialization now. But harnessing these techniques at optical frequencies, which have much shorter wavelengths than microwaves, will require the use of nanoscale fabrication techniques.
"We have found that our unit cells determine the resolution: If you want your focused resolution to be subwavelength, then you have to use subwavelength cells. If you relax the resolution requirement, then you can use larger cells," said Eleftheriades.
Still, the plan is to work with optical frequencies eventually. The recent experiments proved that the dual-loaded transmission-line technique works from 900 MHz up to 10 GHz, and Eleftheriades expressed confidence that it could be stretched to as high as 100 GHz, given the appropriate nanoscale techniques.
"What is still not clear is to how high a frequency one can go while still maintaining super-resolution," said Eleftheriades. "We have achieved a sixth-of-a-wavelength resolution with microwaves, but whether one can maintain this at optical frequencies needs to be proven."