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Technology Toward Greater Transparency

The fictional world of Harry Potter has inspired everything from film adaptations to criticisms of promotion of the occult but it may have sparked the imaginations of scientists who are working on a practical ‘invisibility cloak,’ like the one the characters use to sneak through the halls of Hogwarts School of Witchcraft and Wizardry.

A team of Duke researchers initially unveiled technology that could reroute electromagnetic waves of microwave-level frequency in 2006, but the results of experiments with a newer type of cloaking material were published in the Jan. 16 issue of the journal ‘Science.’

‘It is day and night,’ Ruopeng Liu, a graduate student at Duke University and author of the report, said in an email. ‘The new cloak works much more accurately than 2006 prototype, which was a reduced or an approximated version.’

Structures created from these materials, called metamaterials, can guide electromagnetic waves around a cloaked object. The waves then emerge on the other side as if they had travelled through empty space.

The new types of metamaterials use a new series of advanced mathematical formulas, called algorithms, to improve the design of composite materials with characteristics not found in nature.

Research into metamaterial technology began as an exploration of how to create something called negative refraction indexes.

When light moves from one material to another, from air to water for example, it is bent in some way. This bending, called refraction, is the principle behind lenses that purposefully steer or focus light. A familiar example of refraction is an object in water appearing closer to the surface than it actually is from the perspective of a person standing on land.

All electromagnetic radiation , whether it is television signals or visible light, travels in waves. The electromagnetic spectrum is the range of all possible electromagnetic frequencies, arranged in order of wavelength with radio waves on one end with some of the longest and gamma rays at the shorter end.

Visible light falls roughly in the middle of the spectrum and continues travelling in the same direction when it enters a new material. All known naturally occurring materials have a positive refractive index, meaning the material reduces the speed of light by some amount from the speed of light in a vacuum.’

The idea of metamaterials, which have a negative refractivity index, is that visible light waves will travel at a negative speed, moving backwards once they hit the material.

The possibility of a material with a negative refractive index was first explored theoretically by a Russian physicist named Victor Veselago in 1967, according to Uday Chettiar, a graduate researcher at Purdue University.

Veselago was the first to develop microtechnology to produce a complete negative index, which is the foundation for the invisibility cloak research at Duke.

Even though prototypes have been demonstrated and metamaterial research has been going on in some capacity for 40 years, widespread availability of invisibility technology is still some time down the road.

‘The major hurdle in realizing an invisibility cloak lies in the difficulties associated with fabrication,’ Chettiar said. ‘We are still several years away from an experimental demonstration of an optical cloak. In short, we are not limited by the theory or the designs, but by the limitations in fabrication.’

Metamaterial researchers are exploring other uses besides ‘invisibility.’ Two scientists recently determined that sound waves may be rerouted using metamaterials, at least in theory. Although earlier researchers had deemed the idea of using metamaterials for sound impossible, a report published in Physical Review Letters’ on Jan. 11 by Steven Cummer, a professor at Duke’s Pratt School of Engineering, and David Schurig, formerly at Duke but now at North Carolina State University begs to differ.

Although the progress of sound cloaking technology is behind that for electromagnetic waves, some hope advances in sound could lead to advances for other types of waves, like tidal or seismic.

But advances in metamaterial technology could prove much more important than providing science-fiction-grade goods; metamaterial development could help provide viable green energy in the near future. Metamaterials could serve as highly efficient solar cells, which collect light particles, called photons, at high aspect ratios or wide fields of view and couple unprecedented amounts of photons for energy conversion, James L. Stewart, a graduate student at Purdue University said in an email.

‘With the current state of the economy and the energy crunch, I believe metamaterials could aid in propelling photon collecting and solar cell technology into common use,’ Stewart said.

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