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Metamaterials: invisibility cloaks and bending light

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Invisibility has long been employed in works of science fiction and fantasy, from 'cloaking devices' on spaceships in the various Star Trek series to Harry Potter’s magic cloak. But physicists are beginning to think they can actually make devices with just these properties.

To achieve the feat of 'cloaking' an object, they have developed what are known as metamaterials, some of which can bend electromagnetic radiation, such as light, around an object, giving the appearance that it isn’t there at all.


The first examples only worked with long-wavelength radiation such as microwaves.

One small device that made small objects invisible to near-infrared radiation and worked in three dimensions was unveiled by physicists from the UK and Germany earlier this year.

Its creators claimed there was nothing stopping them from scaling their invention up to hide larger objects from visible light – although others had pointed out a flaw in their design.

Now, researchers at Boston University and Tufts University claim that they have come up with an invisibility cloak that works within the terahertz band – the radiation between infrared and radio wavelengths – but could be modified to work with visible light. Intriguingly, it is made out of silk.


Such invisibility cloaks rely on metamaterials, which are a class of material engineered to produce properties that don’t occur naturally.

Light is electromagnetic radiation, made up of perpendicular vibrations of electric and magnetic fields. Natural materials usually only affect the electric component – this is what is behind the optics that we’re all familiar with such as ordinary refraction.

But metamaterials can affect the magnetic component too, expanding the range of interactions that are possible.

The metamaterials used in attempts to make invisibility cloaks are made up of a lattice with the spacing between elements less than the wavelength of the light we wish to ‘bend’.]The silk-based cloak recently announced uses'split-ring resonators' – concentric pairs of rings with splits at opposite ends. 10,000 gold resonators were initially attached to a one-centimetre-square piece of silk.

As silk is not rejected by the human body, it is thought that they could be used to coat internal organs so that surgeons can easily see what lies behind them.


Another use for metamaterials, potentially with greater scientific applications, is in building a superlens.

Ordinary lenses are restricted by their “diffraction limit”. As David R Smith of the University of California, San Diego explained in Physics World, this means that “the best resolution that is possible corresponds to about half of the incident wavelength of the light that is used to produce the image”.

In 2000, Sir John Pendry of Imperial College London suggested that a metamaterial with a negative refractive index might get around problems such as wave decay and allow imaging of objects only nanometers in size.

Among the first practical applications would likely be using metamaterial lenses to view live viruses and maybe even bits of DNA. In 2005, a thin slab of silver was used to image objects just 60nm across – just over one hundredth the size of a red blood cell.


Natural materials all have a positive refractive index, and this dictates how light waves interact with them. Refractivity stems in part from chemical composition, but internal structure plays an even more important role. If we alter the structure of a material on a small enough scale, we can change the way they refract incoming waves -- even forcing a switch from positive to negative refraction.

Remember, images reach us via light waves. Sounds reaches us via sound waves. If you can channel these waves around an object, you can effectively hide it from view or sound. Imagine a small stream. If you stick a teabag full of red dye into the flowing water, its presence would be apparent downstream, thanks to the way it altered the water's hue, taste and smell. But what if you could divert the water around the teabag?

In 2006, Duke University's David Smith took an earlier theory posed by English theoretical physicist John Pendry and used it to create a metamaterial capable of distorting the flow of microwaves. Smith's metamaterial fabric consisted of concentric rings containing electronic microwave distorters. When activated, they steer frequency-specific microwaves around the central portion of the material.

Obviously humans don't see in the microwave spectrum, but the technology demonstrated that energy waves could be routed around an object. Imagine a cloak that can divert a third grader's straw-fired spitball, move it around the wearer and allow it to continue on the other side as if its trajectory had taken it, unopposed, straight through the person in the cloak. Now how much more of a stretch would it be to divert a rock? A bullet?


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