All That Matters

Metamaterials are very exciting structures, one of the most exciting areas in photonics, I think. That’s because they allow an almost arbitrary modification of light (or acoustic) waves propagating through the material. Sadly, however, the highly promising potential of metamaterials gets often completely overblown by news reporting on fantastic effects. Cloaking devices are the prime example. Here I try to come up with a few points that might help to sort science from fiction.

Metamaterials are small metallic structures, typically rings or wires, that locally change the materials properties. These structures are much smaller than the wavelength of light. To a light wave, it is as if the structure is not made of tiny rings and wires, but looks like a homogeneous material. Hence their name ‘metamaterials’. Meta is Greek and means beyond. The first metamaterials all used the same small units of wires and rings, repeated over and over. With this, you can achieve a negative index of refraction, which enables superlenses – lenses with perfect resolution.

The original metamaterial designs consisted of electromagnetic resonators made of rings and wires. These devices are for THz and GHz radiofrequencies. Credit: NASA, via wikimedia

The next key advance was that metamaterials needn’t only consist of uniform assemblies of rings and wires. If you change the properties of each unit of a metamaterial, you can create a material that to light looks as if it changes its properties. This way it is possible to modify the propagation of light as it goes through the metamaterial. You can make it go round corners, turn it around. In theory, the possibilities are nearly endless, that much is clear.

The prime example to demonstrate the possibilities of metamaterials is the optical cloak. The term is borrowed from the science fiction series Star Trek. And naturally, it is these kind of visions that let our fantasy go wild when thinking about metamaterials cloaking. Images of Star Trek, or ‘Harry Potter cloaks’ and the ‘invisible man’ are often conjured when journalists, university press offices and even scientists try to explain metamaterials to the public. Sadly, in relation to what metamaterials can do, this is nonsense.

So here are a few things that metamaterials can and cannot do.

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A refreshable holographic image of an F-4 fighter jet. Credit: gargaszphotos.com/College of Optical Sciences, The University of Arizona

Holograms may seem like an original invention from some science fiction films. A famous scene often mentioned in this context is that from Star Wars where Princess Leia records an important holographic message, ending with the words “Help me, Obi-Wan Kenobi“.

Such visions of holograms aren’t fiction. In a paper published in Nature, Nasser Peyghambarian, Pierre-Alexandre Blanche and colleagues from the College of Optical Sciences at The University of Arizona demonstrate a holographic system that is capable of displaying holograms at speeds approaching almost that of video capability. (and sure enough, they do mention Star Wars in the abstract of the paper…)

Holograms have been invented in 1947 by Dennis Gabor. They are made by shining a laser beam on an object and then recording the laser light reflected by the object on a photographic film. Simultaneously, a reference beam of the same laser is directly guided to the photographic film, where it causes an interference of the two beams. The interference pattern stored in the photographic film not only contains information on the light intensity (as in conventional photos) but also the phase difference between the two laser beams. The phase difference is a measure of the three-dimensional shape of the object. Together, intensity and phase contain the complete information of a light beam.

To recover the holographic image, the original laser needs to be used. Therefore, more practical ways of writing holograms have been develop and that do not require the original laser for viewing. Regular white light can be used instead. Although image quality for these holograms is not as good, they are widely used, for example on credit cards. Holograms can also be artificially created, without the use of an actual object, but by using a computer to calculate the necessary holographic interference pattern. Or information from a camera is digitally scanned and used to create a hologram elsewhere. “Holographic telepresence means we can record a three-dimensional image in one location and show it in another location, in real-time, anywhere in the world,” says Peyghambarian.

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Stretchable electronic arrays. The LED sheets can be twisted by 720 degrees and considerably stretched. The wavy metal wires are visible in the image on the on the right. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials (2010). doi:10.1038/nmat2879

Take a piece of silicon, try to bend it and it will break. Stretch a thin film of gold and it will rupture. Conventional metals and semiconductors are brittle and not elastic at all. But these are properties that you need when you want to use electronic devices in unusual places and for unusual applications. In biomedicine for example, if you want to put a diagnostic sensor on top of a muscle. In electronics, when you want to put a large-scale solar cell on the curved top surface of a car.

Sure, you can make a thin film and warp it around a cylinder, and if you do this with electronic circuits it is called flexible electronics. Organic electronics and very thin metal films on plastic can do this. But you cannot fit a two-dimensional sheet on a sphere without stretching it. For such applications you need what is called stretchable electronics, which is different to the flexible electronics that has been around for a while.

The latest milestone has been achieved by John Rogers and colleagues from the University of Illinois in Urbana-Champaign. They demonstrate (disclaimer: in my journal, Nature Materials) a fully biocompatible and implantable stretchable structure containing large arrays of light-emitting diodes and photodetectors. The sheets are stretchable and can be twisted by more than 720 degrees without damage, and can be brought into almost any desirable shape or configuration, says Rogers. “This advance suggests a technology that can complement features available with organic light emitting diodes, where peak brightness and lifetime are limited, and conventional inorganic LEDs, where relatively thick, brittle supports restrict the way that they can be integrated together and the substrates that can be used.”

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