baudrunner's space: Amazing properties of metamaterials
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Tuesday, January 22, 2008

Amazing properties of metamaterials

The invisibility cloak may not be so far off. The secret lies not in the composition of the surface material used to render an object undetectable but in the material's structural design properties. A new class of material has been developed with surface features designed to respond to EM wavelengths such that the waves pass into and through the material, following its contours as they pass around an object then exit in the same orientation and direction as they entered.

metamaterial sample

Thus far, metamaterials, as these unusual structures are called, have been designed to respond to microwaves and acoustic waves. 'Respond' actually refers to the material's unique property of providing a negative refractive index, whereby the EM waves that pass through it are refracted in the direction opposite that of the normal refractive behaviour of transparent materials such as glass and water. In a sense, the material acts as a three-dimensional waveguide wherein the behaviour of the EM waves passing through it is determined by the dimensions of its internal structures. A 'cloaking device' has been demonstrated which effectively prevented a copper cylinder from being detected by microwave imaging. It follows that cloaking of stealth planes from radar is the next likely application. Currently the technology does not exist to fabricate similar devices to respond to the higher frequencies of the visible wavelengths in the optical spectrum but their eventual development is probably not far off. In fact, researchers at the U.S. Department of Energy's Ames Laboratory have developed a candidate material with a negative refractive index for visible light. Wavelengths at the red end of the visible spectrum, about 780 nm, can be attenuated by the material. Many challenges still need to be met before this metamaterial can be considered practical. However, since this milestone represents the most significant improvement over the six years since the discovery of the first metamaterial, those challenges will most certainly be overcome within the short term.

Acoustic metamaterials promise serious improvements in the resolution of ultrasound images because they can be designed to function as acoustic lenses which can focus sonic waves. Furthermore, there is no shortage of application in the field of sound damping and the fabrication of baffles to reduce noise pollution from heavy machinery and loud engines. In short, acoustic metamaterials will allow engineers to control sound waves.

There are no natural materials with a negative index of refraction. They must be engineered to have this property. An excellent explanation of the wave properties of metamaterials is provided by this slideshow by Faustus Scientific Corporation, where the images of two types of metamaterials below can be found. The company manufactures Multi-Purpose Electromagnetic Field Simulation Tools (MEFiSTo™) which model the dynamics of electromagnetic fields in user-defined environments.

credit: A. K. Iyer and G.V. Eleftheriades, UoT                   credit: D. R. Smith and S. Schultz, UCSD

One important implication offered by metamaterials is that they offer the possibility of providing optical resolution below the diffraction limit. What this means is that ultimately we will be able to image objects optically that are smaller than the smallest wavelength of visible light. Images resolved by conventional optics do not include "evanescent" waves, additional detail carrying EM waves which comprise the energy losses that occur as the light passes through a conventional lens. Physicists have long known about this phenomenon. It was thought that a lens made out of a material with a negative index of refraction could in theory refocus these waves to provide detail not otherwise available. Enter the 'Superlens'. The actual development of a lens with a negative index of refraction was first proposed by the British physicist John Pendry in 2000, 30 years after Russian physicist Victor Veselago first conceived of the idea. The actual demonstration of such a metamaterial was achieved by a team headed by principal investigator Xiang Zhang, UC Berkeley associate professor of mechanical engineering, in 2005.

Consider the ultra high resolution imagery displayed by high-end flat panel LCD TV's showing a Blue-Ray DVD program and then you can readily understand the basic idea that by human hands technology can indeed enhance and supplement reality far beyond nature's intentions.

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