All That Matters

How do you measure mass with high precision? This is not an easy question, as it is very difficult to measure the weight of something with the same ultra-high precision with which atomic clocks measure time. To this day, the kilogram is defined by a piece of metal made of platinum and iridium that is stored in Paris. If you want to know with absolute precision the weight of something, you would have to compare it to this particular piece of metal. This does not only seem very imprecise and old-fashioned, it also leads to a range of issues. Only last week there have been news reports of the official kilogram piece and its various official copies all over the world slowly gaining weight from dirt on their surface.

It comes as no surprise that physicists are searching for more precise ways to measure weight, and the method now published in Science by Holger Müller and colleagues from Berkeley is one of the most elegant and beautiful ones that I have seen in a long time. It is based on a quantity that we know very well how to measure with very high precision – time. The question is how to measure the mass of something by telling the time. [...]

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. [...]

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. [...]

A beautifully looking graphics, isn’t it? But there is a major caveat. As its creators would agree, this image is only a very crude depiction of reality and shouldn’t be used for any scientific purpose… (c) LANES, EPFL

Nanotechnology is a wonderful science that has pushed functional devices to sizes not far away from the size of atoms. So small that if you want to image such structures, even a conventional electron microscope wouldn’t get you far. There is no way to directly see what is going on. This is a common problem. Take condensed matter physics – it is impossible to directly visualize the various interactions and events taking place inside a crystal. Or photonics, where complex light fields interact with tiny nanostructures in ways that can be really difficult to visualize, especially in real-time.

So, no wonder that artificial graphics often serve to illustrate a scientific concept or a certain device. And with the prevalence of advanced computer graphics programs such illustrations are becoming more and more fancy. In my opinion, this is a dangerous trend, because such graphics can distort the underlying science they try to depict. [...]

A coaxial cable plug. The coaxial nanolaser is more than 15,000 times smaller. Photo by mikemol via flickr.

When Oliver Heaviside invented the coaxial cable in 1880 he could not have foreseen the implications of his idea on modern nanotechnology. His coaxial cables consist of three layers: an inner metallic core, surrounded by an insulator, surrounded by a metallic layer on the outside. The benefit of this design is that the outer metallic layer shields the electrical signal through the cable from outside interference. This makes coaxial cables very useful for information transfer, and coax cables are used for TV antenna cables or some computer network cables. Mercedeh Khajavikhan, Yeshaiahu Fainman and colleagues from the University of California, San Diego now present a completely new application: they have fabricated coaxial lasers on the nanoscale that turn on without the usual minimum threshold power of usual lasers. To do this they had to shrink the coaxial cables first. These lasers are more than 15,000 times smaller than typical coaxial cables.

The nanoscale coaxial laser. Similar to coaxial cables it consists of an inner metal pillar and an outer metal shield. The structure is also protected from interference from the top. Inside is a semiconductor light emitter (red; insulated from the top metal through a SiO2 plug). The laser light exits through the hole in the substrate. Figure by Mercedeh Khajavikhan and Aleksandar Simic.

The benefit of a coaxial cable is that between the core and the outer metal layer well-defined and controlled electromagnetic waves can propagate shielded from any outside influence. Furthermore, shrinking such a device to the nanoscale – to length scales comparable to the light used – means that only the smallest optical beam pattern for the wavelength of light, known as the fundamental mode, fits into the small space between the metal structures. The other modes would be too large. [...]

We are all familiar with the basic ways in which light interacts with matter, when light absorption  causes atoms to move and creates heat, or when light gets absorbed by the outer electrons of atoms so that they move into energetically excited states, which is how electricity in solar cells is created. Common to both examples is that light is mainly used as an energy source, and it is easy to visualize. When scientists draw such light interactions into the energy diagram of say a molecule, they often draw little wavy arrows from one energy state to another.

But that’s the boring stuff. Far more interesting is that light can also strongly couple to matter, but without getting absorbed. The example I am discussing here is when the interaction between light and a molecule is so strong that it profoundly alters the molecule’s energy states themselves, and not merely lifts electrons from one state to another. In particular, what Thomas Ebbesen, Tal Schwartz, James Hutchison and colleagues at the University of Strasbourg have now shown is that such interactions could find exciting new applications: to control energy levels of molecules, and in this way to influence the kinetics of  chemical reactions in a new way that creates many new possibilities.

Strong coupling of light and matter. Light confined between two mirrors can strongly interact between matter that is also between the mirrors and has a matching energy level. The strong light-matter coupling then causes a splitting of the matching energy level into two separate states.

To see how this looks in practice it is necessary to understand what the strong coupling between light and molecules means. First of all, to achieve the necessary strong coupling, it is necessary to create a strong feedback mechanism between light and matter. This can be done by squeezing the light field between two closely spaced mirrors, with the desired molecules in-between. In addition, the energy levels of the light field between the mirrors and one of the energy levels of the molecule need to match up. If all these conditions are fulfilled, then the energy state in question is split into two separated states (see figure). This is called Rabi splitting. The stronger the coupling, the larger the energy separation between the two states. Because of the beauty of quantum mechanics this doesn’t even require light to be present, the mirrors are enough. [...]