All That Matters

It’s been only a week ago that I wrote about the increasing competition for graphene. But as I said then, there are still some exciting advances based on graphene. An example is photonics, which is an area where traditionally graphene perhaps has not been as strong as in electronics. A reason for this is that being only a single atomic layer thin, graphene initially wasn’t expected to show much interaction with light. One of the more intriguing historic results in this area has been the fact that the absorption of light in graphene is determined by one of nature’s most fundamental numbers, the fine structure constant.

Plasmons in graphene can be created by illuminating the tip of an atomic force microscope (grey) with an infrared laser beam (red). Reprinted by permission from Macmillan Publishers Ltd. Fei Z. et al. Nature 487, 82–85 (2012). doi:10.1038/nature11253

But absorption of light is not where the true potential of graphene lies, namely on the nanoscale. On the same scale as electronic applications, because ultimately the aim is to achieve photonic functionality on a chip.

However, the control of light on the nanoscale typically requires surface plasmons. These are collective movements of electrons at the surface of metals. So in a sense surface plasmons function a bit like antenna that can focus light into tiny spots. [...]

Hydrogen sensing at the nanoscale. Hydrogen molecules (red) are absorbed by a palladium nanoparticle (silver) and the resulting changes in optical properties amplified by a gold antenna. (c) Mario Hentschel, Na Liu, Harald Giessen

Sensing the presence of molecules in gases and liquids is a billion dollar business. Just think about all the carbon monoxide detectors in private homes, or blood glucose sensors. In particular for many technical and scientific applications, ultrasmall and precise sensors are desired. This includes sensors to measure gases in catalytic nanoreactors and fuel cells, or the monitoring of biochemical processes.

Laura Na Liu and Ming Tang from the group of Paul Alivisatos, director of Lawrence Berkeley Lab in the USA, and Mario Hentschel from Harald Giessen‘s group at the University of Stuttgart in Germany have now developed a new class of optical nanoscale sensors that are able to measure specific molecular concentrations down to single particles. This, says Alivisatos, “should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing.” Their paper is published this week in Nature Materials (declaration of interest: I was the handling editor of this paper, although I like to stress that I don’t benefit in my day job by blogging about this work). [...]

How do you measure a field like electrical or magnetic fields? The field itself is of course not visible. But you can see the effects of a field and use that for the visualization. For example, in case of magnetic fields a nice high school type of experiment is to use iron filings sprayed around a  magnet. The filings align along the field lines (so that the force on them is minimum), and this alignment makes the field lines visible.

The electromagnetic fields of a hot spot on the surface of aluminium. The hotspot is only a few nanometres in size. (c) Nature Publishing Group. Nature 469, 385-389 (2011).

Although this kind of visualization technique works very well for larger objects, things get trickier if the fields are concentrated within the tiniest spots only a few nanometers large. There aren’t many objects that could be used to measure fields on that scale. Yet Xiang Zhang and colleagues from the University of Berkeley have achieved exactly that and developed a method capable of measuring the electromagnetic fields down to an area only 15 nanometres in size.

The fields they measure are so-called hotspots that form at the surface of metals or around metallic nanostructures such as nanoparticles or nanoscale bow tie structures. There, collective movements of the electrons – the surface plasmons – can create huge electromagnetic fields. This is very much in analogy to the way any other antenna works: oscillating electrical currents create electromagnetic radio waves. On the surface of metal nanostructures, the same happens; but because the geometry is so small the effect is much larger.

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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!

Photo by Scott Robinson (Clearly Ambiguous) via flickr.

Solar energy is a huge market and any improvement to the efficiency of solar cells has a significant impact. In 2008, worldwide photovoltaic solar energy production was about 5 gigawatts, and this is expected to rise to 15 gigawatts in 2015. To put this figure in context, a nuclear reactor produces around 1 to 1.5 gigawatts of electricity.

The overall conversion efficiency of the best solar cell devices, with complex designs that include multiple materials, reaches up to about 50%. For more common silicon-based solar cells efficiencies are around 20%, although commercial photovoltaic panels achieve even less than that. Any enhancement in solar cell efficiency, whether it is in terms of costs or in terms of efficiency, is therefore highly desirable and significant. If all photovoltaic cells that produce those 5 gigawatts of energy were only 5% more efficient, for example with 21% conversion efficiency instead of 20%, it would result in an increase in energy production equivalent to about 250 megawatts.

Another issue is cost. Bulk solar cells are better at catching more sunlight, but use more material and therefore cost a lot more than thin-film films. Albert Polman, Harry Atwater and colleagues have now developed a solar cell design that enhances the power efficiency of thin solar cells. In their study published in Optics Express they are able to enhance the efficiency of a 340 nanometer thick silicon solar cell by 27% when compared to a regular thin-film cell.

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Gold heptamer structures show strong Fano resonances. The gold nanoparticles are only about 150 nanometers in diameter. (c) Sven Hein, Na Liu, Harald Giessen, University of Stuttgart, Germany.

Photonics is all about light. Processing of light for applications ranging from holograms and displays to optical telecommunications. Thanks to a better theoretical understanding and to advances in fabrication technology, photonic devices and gadgets have become increasingly versatile and powerful.

But photonics also has a dark side. In many light-processing devices and structures there are dark modes — oscillations of the light wave that while not forbidden cannot be directly excited by a given experimental configuration. In a violin for example, the strings are best sounded by drawing the bow perpendicular to their length.

However, the dark modes in photonic devices are not lost to applications. In the past months, researchers have developed new approaches that can make use of the dark modes by using interference effects with the allowed, bright modes. The resonances created by this interference are called Fano resonances, after Ugo Fano who first described them in 1961.

“Fano resonances are cool because they are the manifestations of dark modes that cannot be excited directly. In any system there are only a few bright modes but an infinite number of dark modes. The Fano resonance is the interference between a bright mode and one of the dark modes,” explains Peter Nordlander, a physicist from Rice University in Houston, Texas.

Fano resonances are very sharp, with a spectral shape that is much narrower than what can be achieved with comparable regular oscillators and their Lorentzian lineshape. This advantage makes them attractive for sensing applications. Because the spectra of Fano resonances are so narrow, the tiniest changes to the local environment of the resonator structures lead to noticeable shifts of the resonance wavelength. Even the presence of a single molecule could be detected.
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