It a dream possession that until now has been consigned to Harry Potter and Lord Of The Rings-a cloak that will render the wearer invisible. However, a recent paper published in 'Nature Materials' has moved invisibility into the realms of reality, by demonstrating the perfect cloaking of microwaves on a small scale.
In October 2006, the principle of the cloaking device was first demonstrated at microwave frequencies. A cloaking device having a diameter of five inches and a height of less than 13 mm was used in the experiment. The device had a small cylinder containing the object to be hidden at its centre. The object is covered with a shell that prevents light from passing through it. The electromagnetic waves diverged from the invisibility device were directed towards the cylinder with minimum distortion to make the object invisible. The properties of the cloak material vary at each point with respect to a particular electromagnetic interaction. As a result, a gradient is established in the cloak material properties.
However, this is not the first time that researchers have looked at developing invisibility.
In 2010, researchers from the Imperial College London invented a "space-time cloak", which manipulates light and influences the movement of photons to produce visual gaps and thus screens objects.
Another invisibly technique is to use metamaterials. Metamaterials are tiny nanostructures arranged in criss-crosses and stacks and have the capability to direct electromagnetic rays around objects and thus shield them. These structures were employed in an invisibility cloak, where they control the transmission of light to make an object invisible by directing the incident waves around the object without affecting them. The cloaking device is used for hiding an object by invisibly isolating a specific region from an electromagnetic spectrum. In addition, researchers make use of metamaterials to develop blind spots by deflecting some parts of the electromagnetic spectrum.
Theory Behind Calcite Crystals
Recently, researchers from Denmark and England used calcite crystals to develop an invisibility cloak. Calcite is nothing but the crystalline form of calcium carbonate found in seashells. These natural crystals are transparent and have some special optical properties such as birefringent and double-refraction characteristics. They divide the light entering them into two light rays having two different polarizations and pass the rays along two different paths at different speeds.
Calcite shields can make 1 – 2 mm tall objects disappear by bending laser beams in many different directions according to the orientation of the crystal. In an experiment carried out by MIT researchers, two prism-shaped pieces of calcite were combined together and the object to be cloaked is kept under this calcite crystal layer. Under visible light, the object becomes invisible on viewing from a certain angle. Objects having a size of few centimeters can be hidden by this method, and the cloaking region size is restricted to the size of the calcite crystal.
The recent developments in microwave cloaking will be hard to transfer to visible light, however it certainly has applications in telecommunications and radar.
Researchers hope that the use of calcite crystals for developing cloaking devices would expand the applications of invisible cloaks in future. The invisibility crystals can be used for developing new optical devices and enhancing optical computing and signal processing techniques. The crystals can also help in improving the design of the existing microscopes.
Using Crystals in Fish Skin
Researchers have confirmed that the skin of silvery fishes like herring and sardines exhibit a unique optical mechanism which deviates from the existing laws of reflection. The scales of these fish contain guanine crystals that prevent the polarization of reflected light. As a result, the color of the fish appears to match the light environment of the sea. Thus their skin acts as an invisibility cloak and protects themselves from predators.
The skin of these fish can be a great inspiration for scientists to design innovative optical fibres and LED lights. Most of the current optical devices employ reflectors with non-polarization effect in order to improve their efficiency. But the materials used in these reflectors do not have ideal optical properties required to achieve high reflectivity. However, the fish skin optical mechanism can be used to overcome the constraints involved in the existing design of non-polarizing reflectors and will likely pave a way for the development of a novel optical technology.
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