MAKING OBJECTS INVISIBLE
Biman Basu
Email: bimanbasu@gmail.com
From DREAM 2047, October 2008, p.26-27.
Fans of Harry Potter may be familiar with the concept of an invisibility cloak. As its name suggests, the invisibility cloak renders Harry invisible. Of course, Harry is a fictional character and so is his invisibility cloak. But it may not be too long before the idea becomes a reality. The first steps towards achieving that goal have already been taken. Materials that could one day make objects invisible to visible light have been devised by scientists at the University of California at Berkeley, USA.
It is well known that when light passes from one medium into another it bends. While passing from a lighter medium to a denser medium a ray of light normally bends towards the normal. The refractive index describes the way the light waves bend when they enter and leave the material and the speed at which they propagate. The refractive index of normal materials is always positive, e.g., 1.0003 in air, about 1.5 in ordinary glass, 2.1 in zircon, and 2.4 in diamond.
In the mid-1990s, some scientists realised that it could be possible to construct artificial materials in which the refractive index could be negative. The trick was to assemble an array of components that resonate with the electric and magnetic fields of the light waves as they pass through. It was known that these materials would be unlike any conventional substance; hence they were dubbed "metamaterials". Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials.
Although discovered only six years ago, materials with negative refractive index have been the target of intense study, drawing researchers from physics, engineering, materials science, optics, and chemistry and two breakthroughs in the development of metamaterials were reported separately in the online of Nature (13 August 2008; doi:10.1038/nature07247), and in Science(15 August 2008).
In Nature, Xiang Zhang and colleagues of the University of California, Berkeley, report creation of a 3D optical metamaterial made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. The materials can reverse the natural direction of visible and near-infrared light and could help form the basis for higher resolution optical imaging, nanocircuits for high-powered computers, and, to the delight of science-fiction and fantasy buffs, cloaking devices that could render objects invisible to the human eye.
The metamaterial described in the Science paper by the same authors takes another approach to the goal of bending light backwards. It is composed of silver nanowires grown inside porous aluminium oxide. Although the structure is about 10 times thinner than a piece of paper, it is considered a bulk metamaterial because it is more than 10 times the size of a wavelength of light. The authors of the Science paper observed negative refraction from red light wavelengths as short as 660 nanometres. It is the first demonstration of a bulk media bending visible light backwards.
For most of the applications routed for metamaterials, such as nanoscale optical imaging or cloaking devices, both the nanowire and fishnet metamaterials can potentially play a key role. While the researchers welcome these new developments in metamaterials at optical wavelengths, they also caution that they are still far off from invisibility cloaks and other applications that may capture the imagination. For instance, unlike the cloak made famous in the Harry Potter novels, the metamaterials described here are made of metal and are fragile. According to the researchers, developing a way to manufacture these materials on a large scale scale will also be a challenge.
Grateful thanks Mr.Biman Basu and Dream 2047.
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