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Unnatural optics create precise photonic lens

Posted: 28 Aug 2002 ?? ?Print Version ?Bookmark and Share

Keywords:Optical experiments using arrays of nanowires are demonstrating that the concept of a negative refractive index could be realized in practical systems.?

Optical experiments using arrays of nanowires are demonstrating that the concept of a negative refractive index could be realized in practical systems. The work, done at Purdue University, attempts to reproduce results similar to those shown last year at the University of California using microwave radiation. A negative refractive index, which is not found in nature, would allow scientists to construct new types of microscopes with unprecedented resolution and could allow the creation of novel photonic devices.

Since the first demonstration of a negative refractive index material, research groups around the world have been pursuing photonic technologies that appear to break the laws of nature. "The race is on," said Vladimir Shalaev, a professor in Purdue's School of Electrical and Computer Engineering. "We think there are about 20 other labs around the world are rushing to create the first working prototypes at visible and communications wavelengths. We hope to have a prototype by early next year."

Shalaev is assisted by Viktor Podolskiy, a post-doctoral fellow at Princeton University, and Andrey Sarychev, a senior research scientist at Purdue.

A transmission medium with a negative index of refractions would enable a flat planar lens to focus light to precisions that are smaller than the wavelength of the light itself. With tunable versions of such photonic materials now being rushed into prototypes by labs worldwide, it is conceivable that not only could a "perfect" lens be created but that known electron effects could be translated into photonic operations to create sensors that could detect a single molecule.

"Conventional lenses cannot focus light in an area smaller than the wavelength of the light, but with our nanomaterials you can focus light down much smaller than its own wavelength," said Shalaev. "These metallic nanostructures might even be able to detect a single molecule of a substance, which will never be possible for conventional optics."

All materials have two fundamental electromagnetic parameters: permeability and permittivity, which respectively measure the capacities of a medium to form magnetic and electrical fields. The values of those parameters produce the characteristic bending of a light beam when it travels from one medium into another. In addition, since both parameters are always positive in nature, the electric and magnetic vector field components are directed according to the "right-hand rule," which can be represented by pointing the index finger of the right hand in the direction of propagation. The thumb and middle finger are then oriented at right angles to the index finger, showing the field vector directions.

With the photonically engineered materials, everything is reversed: The field parameters are negative, and the field vectors are described by a corresponding "left-hand rule." In addition, the electromagnetic direction bends away from the normal to the interface between two media, rather than toward the normal, as in Snell's Law.

Perfect-lens recipe

In 1968, Russian theorist Victor Veselago predicted that composite metamaterials might be engineered to have negative permeability and permittivity. Such materials, Veselago theorized, would interact with their environment in exactly the opposite way from natural materials. Using mathematical models, Veselago predicted that such metamaterials would follow a "left-hand rule," which would reverse their effect on electromagnetic radiation. One intriguing prediction was that the left-hand rule would nevertheless allow a flat lens to focus light to a point.

Veselago's prediction that such perfect lenses could be made from metamaterials lay dormant until 2000, when John Pendry, a physicist at Imperial College in London, showed that certain metals could be engineered to respond to electric fields as though the field parameters were negative. Pendry demonstrated different configurations of metal that created a left-hand rule for magnetic fields. In 2001, researchers at Imperial College and Marconi Caswell (London) announced a magnetic resonance imaging (MRI) system using a magnetic metamaterial based on Pendry's design.

Last year, physicist Richard Shelby's group at the University of California in San Diego demonstrated a left-handed composite metamaterial that exhibited a negative index of refraction for microwave EM. The simple arrangement consisted of a planar pattern of copper split-ring resonators (SRRs) and wires on a thin fiber glass circuit board. The SRRs and wires were arranged into a two-dimensional structure with a repeated 5mm lattice, with the wires located on the opposite side of the circuit board from the SRRs.

Now Salaev's group is working to scale down Shelby's 5mm pitch to 15nm so that instead of microwaves, light at visible and communications wavelengths can take advantage of negative permeability and permittivity.

To simulate their nanoscale metamaterial, Salaev's group had to model the behavior of left-handed metamaterials at the nanoscale. To do that, they had to turn to the study of nanoscale metallic structures that produce electron configurations called surface plasmon polaritons (SPPs).

SPPs are a "higher-order" object since they are part light (photons) and part plasmon. To complicate matters, plasmons are themselves higher-order objects, composed of free electrons behaving as a wave across the surface of a metal.

Optical amplification

Three years ago, Thomas Ebbesen at the NEC Research Institute in New Jersey reported that some wavelengths of light could be transmitted by a nanoscale metal grid with an efficiency of greater than one. That implied that the photons were being accelerated, rather than retarded, by the metal grid. Even stranger was the fact that the grid spacing was smaller than the wavelength of the photons, which normally would have blocked out most of the radiation. Theory stated that almost no light should go through a hole smaller than its own wavelength, but Ebbesen reasoned that resonant waveforms across the surface of the grid, called surface plasmons, were performing a type of optical amplification on the incident photons.

Surface plasmons are collective oscillations of electrons at the boundary between conductors and insulators. Plasmons, themselves a collection of electrons, then meld with photons to form a new order of object, called a surface plasmon polariton. SPPs produce a reverse effect to a photonic crystal: Whereas the crystals exclude light at special wavelengths (so-called optical "bandgap" materials), SPPs enhance transmission in certain bands, creating the negative refractive index effect.

Resonant SPPs on the metal surface accumulate electromagnetic energy, operating like an antenna when the grid pitch is close to the resonant wavelength of the light. Thus, by changing the pitch of the grid, the wavelength of enhanced transmission can be tuned to a desired wavelength of light. An optical "near field" is generated when localized SPPs are excited by light. The resonating SPPs enhance the transmission of specific light wavelengths by several orders of magnitude, according to Salaev's model.

Now all the researchers have to do is build it.

"In the simulations, we took metal wires and spheres about 10nm thick - about 100 atoms wide - and they functioned like nano-antennas for certain wavelengths," said Shalaev.

Clouds and waves

The tiny wires, 10nm thick and as long as the wavelength of light they are tuned to enhance, were arranged in pairs parallel to each other. When the resonant wavelength of light hit the wires, they resonated, transforming a cloud of electrons into a wave (plasmon), which enhanced the transmission of light in a "left-handed" manner.

The simulations used the discrete-dipole approximation to verify that the plasmon polariton modes in tiny parallel wires were dependent on the incident-light wavelength and the direction of propagation. Verification of the existence of localized plasmon modes and their strong local-field enhancement when fabricated into composites convinced the researchers that left-handed materials in the near-infrared and visible could be built.

"Using these plasmonic nanomaterials, we hope to directly manipulate light, guide it around corners with no losses and basically do all the fundamental operations we do with electronic circuits today, but with photons instead," said Shalaev.

Shalaev's group experimented with many different nano-antenna shapes, from spheres and wires to more complex geometric configurations based on repeated fractal-like patterns. Each metallic pattern was analyzed for its ability to enhance light using SPPs responding to selected incident wavelengths. The winning designs are now being fabricated into prototypes, due out by early 2003.

- R. Colin Johnson

EE Times





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