Shedding Light on Metamaterials

“I remember it was a Sunday in August 2001, and I was outside in the backyard at the BBQ. In that moment, I just knew something was true. I saw the light,” says Dr. Eleftheriades, an associate professor in the Department of Electrical and Computer Engineering at the University of Toronto, and one of six recipients of a 2004 NSERC Steacie Fellowship.

But what a weird light it was. Dr. Eleftheriades envisioned a new way to create artificial materials, called metamaterials, that do what until now is the seemingly impossible: they bend electromagnetic waves – such as microwaves, radio waves and visible light – the “wrong” way.

Dr. Eleftheriades' metamaterials are artificial composites of tiny photolithographed wires forming the squares of a quilted pattern on what looks like a conventional circuit board, about 10 cm by 5 cm. But the collective properties of this metamaterial pattern defy conventional wisdom.

Shine a flashlight through a conventional window on any angle other than straight-on, and inside the glass, the beam of light (called the incident beam) refracts, or bends, away from the source. Known as Snell's Law, it's a standard of high-school physics textbooks, and a law of physics that's been considered inviolate from its articulation in 1621.

But shine a beam of light, or in this case a radio wave, through one of Dr. Eleftheriades' flat metamaterials and the beam bends in the opposite direction, towards the source. Welcome to the world of left-handed or negative-refractive-index metamaterials.

“You can't say it's the correct or the wrong way for light to bend,” says Dr. Eleftheriades. “It's the right way for left-handed materials. These phenomena were predicted by the Russian physicist Victor Veselago in the 1960s, based on Maxwell's equations created in the 1870s. But it's only recently that people managed to make materials that would behave like this.”

The practical importance of this Alice-in-Wonderland bending is enormous: combined with traditional dielectric materials, the metamaterials cause electromagnetic waves to be focused on a point instead of diverging outwards. While normally a lens has to be curved to focus light, these flat metamaterials focus electromagnetic waves with unparalleled precision.

In December 2002, Eleftheriades produced the first experimental demonstration of “left-handed” metamaterial lenses. (Negative refraction had previously been observed by a group at UC San-Diego, using a different kind of metamaterial.)

In June 2003, Dr. Eleftheriades took the work a remarkable step further and reported the first experimental evidence that these flat left-handed metamaterials can be used to form lenses that “see” previously invisible detail.

All present day lenses – whether optical ones for telescopes, or the antennas used to capture radio and radar waves – are limited by one key factor: they can't “see,” or resolve, details that are smaller than the wavelength of the electromagnetic wave. For example, atoms are smaller than the wavelengths of visible light and thus can't be seen using optical microscopes.

However, just as left-handed materials cause light to bend negatively, they also confound our understanding of lenses by providing super-resolution. These flat left-handed lenses are able to resolve sub-wavelength details by focusing the weak wavelets, or evanescent waves, that carry an object's sub-wavelength details.

“With left-handed lenses you actually take the evanescent waves and they grow – their amplitude increases inside the lenses, thereby enhancing the resolution,” says the Cyprus-born Dr. Eleftheriades, who was recruited to Canada and his University of Toronto post in 1997.

Left-handed materials open up a range of new practical possibilities. Their ability to focus waves on sub-wavelength details means that they can be used to miniaturize antennas, making for significantly smaller wireless devices with ultrawide bandwidth. He's currently working with a major multinational telecommunications company to do just this.

As part of his NSERC Steacie research, and in collaboration with the Department of National Defence, he's also working to make metamaterials to enhance radar resolution, as well as to avoid radar detection through stealth technology. If super-resolving metamaterials are extended to optical frequencies, they could enable new photolithography techniques, whose sharper beams could burn more information onto CDs or allow the fabrication of nanodevices.

For Dr. Eleftheriades one of the most inspiring applications is medical. He's determined to see his left-handed metamaterials applied to medical imaging devices, such as MRI machines. This would increase their resolution, providing the ability to see much smaller details, including, potentially, tiny cancerous tumours.

Contact:

Professor George V. Eleftheriades
Tel.: (416) 946-3564
E-mail: gelefth@waves.utoronto.ca

For more information on Dr. Eleftheriades' work see http://www.waves.utoronto.ca/prof/gelefth/main.html