How do 'invisibility cloaks' work?

After another news story about a 'Harry Potter invisibility cloak', we take a look at the science behind metamaterials.

A metamaterial cloak bends light around an object
Science

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.

Metamaterials

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.

Superlens

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.


 

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