All That Matters

This USB flash drive used a bulk metallic glass for its casing. Credit: Liquidmetal Technologies.

This week I am attending the 2010 Materials Research Society Fall Meeting in Boston — one of the key meetings in materials science. One of the sessions is on bulk metallic glasses and their applications, which this year is a little special. It is organised in honour of the 50 year anniversary of the first demonstration of a metallic glass by William Klement, Ronald Willens and Pol Duwez from Caltech. Their paper on gold-silicon alloys was published in Nature on September 3^rd, 1960. (In addition to Duwez and colleagues, David Turnbull must be mentioned here as one of the pioneers with several key contributions to the field. For example in 1948 he demonstrated that metals can be considerably undercooled below their crystallisation temperature.)

Metallic glasses have since become very interesting for applications that include sports products, coatings, power transformers where they reach several ten thousands of metric tons annual production, sports equipment, bioimplants and others. At the same time research researchers still try to learn more why these glasses form in the first place.

Why are metallic glasses so special?

All metals prefer to form crystals, as  the metal atoms easily form structured bonds with other atoms. So easy in fact that even when the metal is melted some of that arrangement carries over into the liquid. This makes the formation of a crystal the preferred pathway once the melt is cooled down again. Glass on the other hand is amorphous, which means that the atoms are disordered and there is no long-range periodicity. This is not something metals prefer. Unlike the window glass made of silicon oxide. In comparison, metallic glasses are a very different animal.  […]

Light is special. In our everyday experience it behaves like a wave, which gets reflected, refracted and shows interference with other light of the same wavelength. At the same time, light also consists of particles, so-called photons. This duality is quite fundamental: the Hanbury Brown and Twiss experiment for example only works because of the particle-like properties of light.

The experimental setup. A laser beam injects photons into a cavity filled with light, and a camera observes the photons coming out of the cavity. Reprinted by permission from Macmillan Publishers Ltd. Nature 468, 545-548 (2010).

This amazing and perhaps confusing duality, where light in one experiment appears to be a wave and in others it behaves like particles, is now laid bare in a paper published in Nature. There, Jan Klaers, Martin Weitz and colleagues from the University of Bonn in Germany take one of the classical properties of light waves and turn it upside down — by demonstrating a related effect that only works when considering the particle qualities of light!

The classical effect they use is that light waves can all oscillate synchronously. This is exactly what happens in a laser, and is typical behaviour for a class of particles to which photons belong to, the bosons. Bosons love to be all in the same state.

A similar synchronous behaviour can also occur for other bosons, including certain atoms, which then all assume the same quantum state. This state is called a Bose-Einstein condensate, after Satyendra Nath Bose (after whom bosons are named) and Albert Einstein, who described it first in 1924. It is a Bose-Einstein condensate of light that Weitz and colleagues have now demonstrated. […]

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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