All That Matters

Metals appear colourful by depositing an ultrathin layer of a semiconductor on top. Shown here is the example of thin germanium films on top of gold. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials (2012). doi:10.1038/nmat3443

Ocean waves are pretty relentless when hitting on a beach, and it is not always easy to protect beaches from erosion. For example, if you were to put pillars of a few centimetres in diameter into the water it won’t stop the waves or alter their behaviour. The waves will continue to hit the beach as ever. In optics, the situation is pretty similar. Structures much smaller than the wavelength will have no dramatic effect on a light beam. But as Federico Capasso and his group at Harvard University have shown in a number of papers over the past year, tiny metal structures on the surface of a material can completely alter the way in which light passes through the device. The structures are ultrathin – a few tens of nanometers in height are enough to make exotic types of lenses or to achieve an efficient absorption of the light. Indeed, their approach in designing optical interfaces represents a completely new way of thinking about surfaces, and to me is one of the most exciting developments in photonics this year. [...]

The temporal cloak. A light beam of a single colour is directed at a split-time lens (STL) that converts it into different colours. As the beam then propagates through an optical fibre, the blue light components travel faster than the red ones (the vertical axis shows time), so that eventually a brief gap is formed in the travelling beam during which there is no light present. Therefore, any even taking place during that temporal gap will be concealed from the beam. Afterwards, a reverse process restores the original beam so that an observer does not notice the cloaking device. Reprinted by permission from Macmillan Publishers Ltd. Nature (2012). doi:10.1038/nature10695

Devices that conceal objects from an observer are called cloaks. Conceptually, the idea of cloaking devices has its roots in science fiction, but such devices have indeed been demonstrated in the past few years. These cloaks are based on tiny structures that are able to bend light on predetermined paths as it passes through the structure. This is like a lens, but consisting of manmade materials, and much more versatile and powerful.

A very different type of cloak has now been published in Nature by Alexander Gaeta and colleagues from Cornell University. Following earlier theoretical proposals, they have now demonstrated the first temporal cloak where events are hidden in time, not in space so that an event is concealed from a light beam travelling through the same space for a certain amount of time. To understand the difference of a temporal to a spatial cloak, Robert Boyd and Zhimin Shi from the University of Rochester make a very good comparison in their News and Views article on the paper: [...]

Credit: Yushin Kim, Korea Advanced Institute of Science and Technology

How does a lens work? Well, as the light arrives at the lens it gets bent towards the focal point of the lens. The denser the lens material is in comparison to the surrounding air, the more it is deflected. The materials property that quantifies this effect is the refractive index.

For lenses, the general rule is that a larger refractive index is better. That’s because the maximum resolution of a lens gets better as the refractive index increases. This is of crucial importance for applications where resolution matters, for example in the fabrication of semiconductor transistors, says Xiang Zhang, a physicist from the University of California in Berkeley. “A large index is very useful for high-resolution imaging and lithography. That’s what the billion dollar semiconductor industry critically needs and have investigated in heavily.”Typically, the refractive index varies anywhere between 1 (air) and 3. That of window glass is about 1.5.

Bumki Min from the Korea Advanced Institute of Science and Technology (KAIST) along with colleagues from other institutions now have demonstrated an artificial material whose refractive index is a staggering 38.6. Their paper is published in this week’s issue of Nature.

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(c) Olympiaregion Seefeld

A key part of my job as an editor of a scientific journal is to attend to conferences. To scout new and exciting developments, to network with scientists, and to take a look at new research that I get shown in confidence. There is a lot to be gained by attending scientific meetings, and I am convinced that I couldn’t do my job as well without going to meetings.

This year, my conferences start early, I am presently attending the Nanometa conference, which coincidentally I helped to organise as a member of the technical programme committee. The conference takes place in the beautiful ski resort Seefeld in Austria.

Despite the beauty of Seefeld’s scenery, what attracted me to attend this conference is the topic — nanophotonics. Nanophotonics means the interaction of light with matter on extremely small length scales. Indeed, most nanophotonic devices are even smaller than the wavelength of light that they interact with. For example, the visible spectrum of light ranges from about 390 nanometers to about 800 nanometers wavelength, and these devices typically are on the same length scale or smaller — hence the name ‘nanophotonics’.

But what is the difference between nanophotonics and regular photonics? Well, conventional optical instruments, such as lenses, can only focus light down to length scales that are on the same scale as the wavelength of light. In the case of visible light this would be a few hundred nanometers. On the other hand, electronic devices such as transistors are smaller than that, on the order of tens of nanometers. So if you want to do photonics with say visible light on that kind of length scale, conventional optics simply won’t work.

One way to escape this dilemma is to use so-called surface plasmons, which are collective motions of electrons on the surface of a metal. These movements act like very powerful antennas that can create extremely strong light fields close to the surface of metal nanostructures. There are several exciting research trends based on such plasmonic devices that all are key parts of the conference here in Seefeld:

• Plasmons can focus light very efficiently into solar cells: Because they are such strong antennas, gold nanostructures on top or at the back of a solar cell can very efficiently concentrate light into the device and that way enhance the efficiency of the light collection. This is of particular use for solar cells made of thin films, because if a solar cell is very thin, some of the light would simply pass through without getting absorbed. The intense light concentration by metallic nanostructures on the other hand counters exactly that. In comparison to thicker solar cells, such new devices won’t set world-records in efficiency, but they enable lighter solar cell modules that use less material. Therefore they promise to be more cost-efficient and resource-saving. For more information, see my earlier blog post ‘Solar cells brought into shape‘.
• Plasmonic devices make efficient sensors: atoms and molecules absorb light at specific optical wavelengths that can be used to identify these molecules. The strong intensity of light around metal nanoparticles strongly amplifies the absorption, so that even a single molecule can be detected! Earlier last year I blogged about a clever way of doing that in ‘The dark side of photonics‘. There are also other ways of sensing, where the mere presence of a chemical molecule can alter the optical property of a nanophotonic device.
• Plasmonic devices can transport light: An exciting talk at the conference was that of David Miller from Stanford University. He (and other scientists as well) highlighted the benefit of using light to transmit information on an electric chip. The currents between transistors on a computer chip produce a large amount of heat. So much heat in fact that shrinking transistors further and putting more of them on a chip can only work if this heat production is reduced. One way of reducing heat generation is to use optical beams instead of electrical conductors. And the best way to connect these tiny transistors optically is to use plasmonic metal nanostructures.
• Last but not least, metamaterials for visible light use plasmons: metamaterials are devices that shape the propagation of light by being made from metallic structures that are smaller than the wavelength of light. This requires the use of plasmon effects. One of the most-quoted application of such metamaterials are invisibility cloaks, but there are plenty of others. In particular I like to re-emphasize my earlier comments on ‘What are the realistic promises of metamaterials and cloaking?‘

Obviously, this is only a small selection of the trends in the field. Many more developments emerging from this conference in Seefeld will appear in the scientific literature soon, and some of them will be covered on this blog for sure, simply because controlling light on such small length scales offers so many exciting opportunities!

The past year has been a great year for science with major advances in several areas. Too many exciting results to mention here. Instead, to reflect about the past year I have chosen a representative paper for each month of the year that I hope can serve as an example of the great science going on in a number of research fields. Of course, this is a highly subjective and personal collection, and indeed there might be others worth mentioning. But the aim was also to provide a balanced overview of the year that covers a variety of topics.

Of course, if you have an exciting paper to add, please feel free to use the comments section below to let us know!

Anyway, enough said, here are some of my highlights from the past year:

Simulations of electronic excitations in an iron-based superconductor. Image by Oak Ridge National Laboratory via flickr.

JANUARY – iron-based superconductors

Since they were discovered in 2008, iron-based superconductors, the pnictides, have been one of the hottest topics in condensed matter physics. Part of their appeal stems from the fact that they are based on iron, which is a magnetic element. Normally, magnets and superconductivity exclude each other.

The iron-based compounds have a similar crystal structure as the so-called cuprates, which are the materials with the highest superconducting temperatures known. The mechanism for these high-temperature superconductors is unknown, and studying the iron-based superconductors may also be relevant to the understanding of the cuprates.

This paper published in Science shows for the first time that the electrons in the iron-based superconductors show a periodic arrangement that is different to the periodicity of the atoms in the crystal. Similar observations have been made in the cuprates, and their understanding is considered important to the mechanism of high-temperature superconductivity.

Chuang, T., Allan, M., Lee, J., Xie, Y., Ni, N., Bud’ko, S., Boebinger, G., Canfield, P., & Davis, J. (2010). Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2 Science, 327 (5962), 181-184 DOI: 10.1126/science.1181083

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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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