What is this all about? Yesterday saw the publication in Nature of the controversial mutant bird flu paper. Bird flu (H5N1) is highly lethal, more than 50% of those known to be infected have died from it, although such figures need to be treated with caution. There might be plenty of more benign cases that went undetected so this is more of an upper limit. Still, this is scary. The good news – so far at least – is that H5N1 in the wild doesn’t spread easily between humans and unlike other forms of flu does require physical contact.

Researchers in the US/Japan and separately in the Netherlands have now studied whether H5N1 can mutate to become highly contagious, which would be a real nightmare scenario: a lethal virus that transmits easily. And as these two papers show, using ferrets this requires only a few genetic mutations. What at least the US/Japanese group has done (the other paper has not been published yet, see below), is to use genetic variants from the highly contagious swine flu virus (H1N1) to modify the H5N1 virus accordingly. These genetic modifications, however, have not been successful on their own. It is interesting what happened then: after only two further rounds of infections of ferrets the virus mutated by itself to become highly contagious! Ed Yong has more details of this on his blog. But I like to emphasize that the contagious H5N1 variant as published now appears much less lethal than the original virus.

The publication of both papers has been withheld for months out of fear such knowledge could be used by terrorists  or other mad individuals to create a deadly pandemic. Given also a reversal of opinion from a US biosecurity board,  the US/Japan paper has now appeared in Nature, and the Dutch paper is expected to appear in Science shortly. Eventually, the decision was in favour of publication, because knowledge of the mutations and their effect on the biology of the virus are so crucial to combat this disease and to possibly develop vaccines. It is not terrorists we need to be afraid of, such mutations can easily happen in nature any moment. In an interview with the BBC, Nature‘s Editor-in-Chief Philip Campbell further rationalises the decision to publish.

Other than not publishing such research at all, two further options were debated: redacting the papers, or to make them available to selected trustworthy scientists only. In an editorial, Nature has now declined such possibilities out of principle and announced this important publishing policy:

Some lessons have emerged that point to actions and policies for the future. First, it was worth deliberating at length on the possibility of redacting the key findings of the paper instead of simply rejecting it. (Rejection has long been an option if Nature is advised by security experts that the risks of publication exceed the benefits.) There was also the option that the full paper might be distributed by some third party, to selected recipients only. Having now considered these matters in depth, the editors of this journal have decided that we will not consider either alternative for papers in Nature in the foreseeable future. A paper that omits key results or methods disables subsequent research and peer review. Furthermore, after much internal and external deliberation, we cannot imagine any mechanism or criterion by which to sensibly judge who should or should not be allowed to see the work. Nor do we believe that any restricted information distributed to university laboratories would stay confidential for long.

Of course, this doesn’t mean that Nature won’t be irresponsibly publishing high-risk research. Declining publication always remains an option. And, as the editorial continues:

We are aware that the lack of an option for restricted publication has its own risks in a discipline in which results might be both beneficial to the public benefit and a threat to security. We will willingly explore ways out of this dilemma.

With this being a physical sciences blog, it is important to keep in mind that such policies also apply to the physical sciences. Nuclear research for example, as discussed in Friedrich Dürrenmatt‘s 1962 play The Physicists. Given the background of the cold war, the play makes an important point. There, a brilliant physicist discovers a formula that could destroy the world. To keep his secret, he hides himself in an asylum, only to be pursued by two physicist government agents who also feign mental illness in order to obtain the formula. Eventually, the three physicists reveal their disguise to each other, although the discoverer of the world formula can convince the other two that his knowledge is so dangerous it needs to be kept secret. All three then vow to stay in the asylum forever. In a dramatic twist at the end, the director of the institution then reveals that she secretly copied all the research and will now make use of it.

The lesson here is that once a discovery has been made it will be impossible in the long-term to keep it secret. Sure, as a publisher it is certainly possible to decline publication of papers for security reasons to prevent a too fast spreading of dangerous knowledge. And this certainly makes sense. But let’s face it, if one person can make a discovery, so can others. If someone develops something in secret, it can be obtained through espionage. Everything has its price. Look how the development of the atomic bomb spread across the world. In the long-term, the spread of high-risk research can be delayed, but eventually it will always come out. This is all the more true for alternatives such as publishing redacted papers or to circulate them to a limited audience only. It would seem impossible that such initiatives really achieve what they set out to do. Therefore, I find the editorial policy expressed in Nature yesterday so remarkable and important.

References:

1. Imai, M., Watanabe, T., Hatta, M., Das, S., Ozawa, M., Shinya, K., Zhong, G., Hanson, A., Katsura, H., Watanabe, S., Li, C., Kawakami, E., Yamada, S., Kiso, M., Suzuki, Y., Maher, E., Neumann, G., & Kawaoka, Y. (2012). Experimental adaptation of an influenza H5 HA confers respiratory droplet transmission to a reassortant H5 HA/H1N1 virus in ferrets Nature DOI: 10.1038/nature10831

2. Editorial (2012). Publishing risky research Nature DOI: 10.1038/485005a

One of the first question the book addresses is of course to understand why are people doing research? What drives them in addition to the obvious curiosity? What’s the economic currency that makes a career in science lucrative? Money of course, let’s face it, is one reason. Some scientists really do get rich from all the startups and patent revenues  – and Stephan provides good examples. But of course, that’s just one aspect. A stronger driver perhaps are fame and recognition. Making an important discovery can create a historic legacy that is unrivalled in comparison to other professions. We know the names of famous scientists even after centuries but not nearly as well those of successful business men.

The points that Stephan make here are all interesting and plausible. Indeed, my impression is that economics already knows a lot about the people doing science. The salaries of scientists, the economic costs of doing a PhD (basically, in most cases you lose out financially). International migration patterns. The increasing number of people studying science, and consequently the fact that fewer and fewer of the scientists we train have a long-term perspective in academia. Academia no longer educates mainly for itself, but for others. There is a lot of data on that and the people working in science, and this book gives a great summary.

Another area the book tries to tackle is that of science policy issues, although clearly there is still a lot that needs to be understood about the larger economic implications of doing science. What is the payback, can it be measured at all? How much public money to spend on science? As much as possible? Scientific research does pay back a lot in areas such as health or engineering. But surely there is a point where investing too much means a lot of irrelevant research being done. But how much investment then? We have not really a good idea of how to do science efficiently and in the best way. Take US universities. As Stephan mentions, these are like shopping malls – inviting scientists to set up shop, bring in the money for these shops in terms of research grants, and then hoping customers will come, leading to lots of students as well as technology transfer. But is such a grant-driven system really the best way of doing long-term science versus short-term grant-based science where tough selection procedures mean that often you know the outcome before you even start the research? Hardly.

The same with science eduction. The book makes it pretty clear that the PhD and postdoc system is heavily tilted to the benefit of laboratory heads. There is not much specific, institution-based information available on the career paths of PhDs and postdocs. My own PhD granting institution certainly never followed up with me on my career path. Indeed, laboratory heads usually are eager to educate as many PhD students if possible, irrespective of their future career perspectives. And as Stephan consequently asks, is it really the best economics to educate a lot of PhDs only to have them become teachers, venture capitalists, journalists (ahem!) and so on. And she has a point. Of course, too much planning of careers can be disastrous. Higher education is a long process, and the economic situation can change. The call for more engineering students in one year can lead to disappointed faces a few years down the line when too many graduates mean that employment perspectives are meagre. But generally, it might be useful to adjust the economics by making it more ‘expensive’ to use PhD or postdoc resources. Or as Stephan suggested, separating education of PhDs from research labs by creating more research institutes outside of universities.

It is generally difficult to find the right balance in science policy. Take technology transfer. Between 1989 and 1999 US universities have increased patent applications from 1,245 to 3,689 per year. But although patent revenues might be a welcome boost for the budget of universities their wider economic benefit for most universities might not be as compelling and as outlined in the book, they might stifle scientific discoveries elsewhere. The obvious alternative that would benefit businesses and not universities is to publish scientific results freely, and to ‘transfer’ technology to companies by recruiting scientists from universities. There is a great quote by J. Robert Oppenheimer in the book: “The best way to send information is to wrap it up in a person.” Start-ups out of universities could also be a great tool to stimulate economic activity and to benefit universities at the same time. A 1997 study by BankBoston also cited in the book found that by 1994 a total of 4,000 companies were founded by MIT faculty, employing a total of 1.1 million people. But which system makes the most sense on the macro-economic scale? My own opinion here is that universities should not try too hard to be corporate entities but instead should have sufficient public funding.

Such issues also play into the broader issue of how to best organize science. And the answer is that we don’t know. Issues that we have to deal with are short versus long-term policy incentives, and short versus long-term research goals. How to deal with the international connectivity of science and the fact that funding mostly happens locally? How much weight should science funding get in times of tight public budgets? We only know a few obvious things, for example that short-term ups and downs in science funding don’t do much good as science isn’t done on the tap. Otherwise, we often are short of useful data.

Either way, Paula Stephan’s book provides a great summary of what we do know so far. For anyone interested in the economic aspects of science seeking to get a good brief on where we stand, this book is essential reading.

Reference:

Stephan, Paula. How Economics Shapes Science. Harvard University Press, 2012. 384 pages. ISBN: 9780674049710. $45.00 / £33.95 / €40.50

In How We Decide Lehrer writes about the neuroscience of decision making. Whether it is an airline pilot averting disaster, a quarterback throwing passes at the Super Bowl, or simply any of us buying strawberry jam or a new sofa – our brain takes decisions all the time. Commonly we tend to think that the best decisions are the rational ones. Look at all available options, ponder over the pros and cons and then take a decision. Well, not only would this be a painstaking process (the book describes such a case), it furthermore isn’t even a good thing to shut out emotions entirely. Psychopaths that lack any emotional response can do terrible things. And as tests with people tasting strawberry jams have shown, the more rational people are trying to be about it, the worse their choice gets because then they’re trying to include factors other than taste. On the other hand, yielding solely to your emotions can lead to pretty bad choices, too. Coming back home from shopping might be one of those moments to realize the penalty of taking too emotional decisions…

What fascinated me about Lehrer’s book is that it is very good at explaining the underlying science of decision making, how the prefrontal cortex balances emotional and rational thoughts. It illustrates horrible cases where this decision-making capability has gone wrong, but also describes cases of heroic and successful decisions, by fire fighters, airline pilots, poker players and so on. It was interesting to me to read how powerful and often correct underlying emotional feelings are, and consequently how important it is to consider these in the decision-making process. And then there are the moral implications of decisions, and the inconsistency at which we often digest information. How easy it is for us to dismiss objective conflicts with existing beliefs, ignoring obvious facts. Politics is an area where this is widespread. And I suppose that’s why the first impression is so important when meeting someone new, as it shapes our emotional response.

Either way, understanding the scientific aspect of decision making is certainly a much more informative read than most literature on this topic. So if you haven’t heard about the book yet, take a look. The book has been published back in 2009 already – yes, I am that far behind in my reading – but Lehrer has soon another book coming out, Imagine, which is on the way creativity works. In my view, after reading How We Decide the decision-making process of whether to give this latest book of his a try should be an easy one…

Reference:
(Note: in the UK the book is called The Decisive Moment, and that’s the ebook version that I read)

Lehrer, Jonah. How We Decide. Houghton Mifflin Co, 2009. 302 pages. ISBN: 9780618620111

A scanning tunnelling microscope image of a single-atom transistor during fabrication. The pink colours represent the areas where a single phosphorus atom (centre) as well as phosphorus source and drain contacts will be placed. The gate contacts that control the transistor action from the side are not visible here. Credit: Martin Fuechsle

When Gordon Moore made his observation in 1965 that the number of transistors integrated on a single silicon chip is doubling roughly every two years, the only logical end point for such a trend would be a transistor made from a single atom. This point has now been reached. Writing in Nature Nanotechnology, Michelle Simmons from the University of New South Wales in Sydney and colleagues report a single-atom transistor, the world’s smallest, on a silicon chip. The transistor is based on current flowing through a single atom of phosphorus embedded in a silicon wafer.

Phosphorus is a natural choice for such a transistor, as it is relatively easy to integrate into silicon. There it acts as an electron donor because it has an additional electron compared to silicon. This additional charge can be used for conventional electronic devices such transistor, but also more complex schemes are possible. For example, the magnetic property of this single excess electron, its spin, can be used for new types of quantum computing. Indeed, I have previously blogged about such efforts from another research group in Sydney using multiple phosphorus atoms for silicon-based quantum computing.

One of the key challenges in making a single-atom transistor is to place a single phosphorus atom into silicon in a controlled fashion. Here, this is achieved by the careful placement of three phosphine (PH₃) molecules on the surface of silicon using a scanning tunneling microscsope. In a number of reaction steps these molecules dissociate and cause the ejection of a silicon atom from the surface and the incorporation of a single phosphorus atom in its place.

The single-atom transistor structure. The electric current between source (S) and drain (D) contacts, through the phosphorus atom in the centre, is controlled by the two gate contacts (G1,G2) (c) 2012 Nature Nanotechnology

Once incorporated into silicon, the energy level of the phosphorus’ outer electron states lies below that of the surrounding silicon. The precise match between silicon and phosphorus energy levels can be controlled by the electrical potential applied between two gate electrodes on either side (see figure on the right). This voltage controls the electric current between source and drain, and through the atom. If the energy levels of silicon and the phosphorus atom do match up, it is possible for electrons to pass through the phosphorous atom one by one. If the voltage between the two gates is set so that the silicon and phosphorus energy levels do not match up, there is no electric current.

There are, however, a few drawbacks in the present approach. So far this transistor only works for really low temperatures, barely above absolute zero. And in the current implementation the contacting electrodes are still several tens of nanometres apart – hardly on the single atom scale. Also, the fabrication of the structure with a scanning tunnelling microscope doesn’t allow to make billions of them in the same efficient way as commercial computer chips are fabricated.

Still, such single atom transistors could already be of interest for the quantum computing schemes investigated for phosphorus atoms, where the low temperature is less of an issue. Either way, considering that according to Gordon Moore’s law commercial silicon transistors are only expected to reach single atom scales beyond 2020, this study certainly is ahead of the curve.

Reference:

Fuechsle, M., Miwa, J., Mahapatra, S., Ryu, H., Lee, S., Warschkow, O., Hollenberg, L., Klimeck, G., & Simmons, M. (2012). A single-atom transistor Nature Nanotechnology DOI: 10.1038/nnano.2012.21

This post was chosen as an Editor’s Selection for ResearchBlogging.org

Further information:

The University of New South Wales has produced a YouTube video explaining these findings.

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.

Atom balls emit beams of light. It might have been better to keep it simple and not go down to the atomic scale. (c) National Science Foundation

Before starting to outline just how wrong such fancy images can be, I like to emphasize that I don’t want to criticize individuals over this, this is just to highlight a general trend that most of us are guilty of following. I mean, I am guilty, too, and have published papers as an editor containing overly fancy graphics. And the image shown above, by the way, made it on the cover of one of the Nature family of journals, although of course other journals show plenty of such graphics, too.

But why are such nice graphics wrong? Here are some reasons:

• Like above, they often show a mix of atoms and dense material. The atoms of the gold contacts are not shown, yet they are of similar size as those depicted. Why is this wrong? Well, such an image suggests that the gold contacts are governed by bulk material properties, whereas atomic effects are only relevant for part of the structure. Yet, on such length scales all atoms of the device influence each other and can’t be isolated in this way. The same for the sticks that are supposed to be atomic bonds in such images. They stop at the interface, which isn’t scientific reality either – atoms across such interfaces influence each other.
• Both the atoms as well as the contacts reflect light. At these length scales there can’t be such reflections because these sizes are smaller than the length scale of light. Why is this wrong? Especially for illustrations of photonic devices this is a problem because it creates a wrong thinking about light on the nanoscale. Basically, you would have to consider local electromagnetic fields. On this scale the interaction of light fields with metals for example is much more sophisticated than simple reflection. And come on, such detailed, mirror-like reflections from atoms?
• On the nanoscale, laser beams are often shown as diffuse beams of light. I am always puzzled why devices that are on the scale of the wavelength of light are shown to ‘emit’ such beams in a way that a floodlights would do. Why is this wrong?As in the previous example, light is emitted as electromagnetic fields, and has a wave as well as particle character. Such images of laser beams on this scale give the illusion that geometrical optics/ray optics still prevails at these length scales. And this is simply not the case. It would be better to simply depict the modes of light, and to be careful about mixing local and global properties.
• Sharp interfaces. Notice how clean and well-defined the various parts of devices often are in such graphics? Well, guess what, on the nanoscale interfaces are never as sharp and clean, atoms never arranged perfectly. Why is this wrong? Rough interfaces influence considerably device performance and moreover, interactions of materials across interfaces are often key to their performance. Keeping everything clinically clean can suppress key device physics and divert from this crucial aspect.
• Wrong scales. In the illustration above, the wires that connect the device to the outside world are of the same width as the atoms. (note: the creators of this particular graphic above did make explicit note that the scales are off). Why is this wrong? Wrong scaling relations can also give the wrong impression of the physics of a device, and could distort the impression of device functionality.

Finally a better example: the figure shows the interface between two materials on the atomic scale. The "balls" depict atoms but are sufficiently abstract to illustrate nothing other than the point being made - the crystal structure as well as the arrangement of atoms at the interface. (c) J. Mannhart, A. Herrnberger

To scientists, these criticisms are probably nothing new. Often, such graphics are produced not to make into actual scientific publications, but as ‘sexy’ cover art or for news stories. However, I do notice a disturbing trend of wrong graphics making their way into a paper. Just look at the table of content illustrations that some journals have. I have seen examples where the majority of illustrations in a paper were based on such science fiction and not on hard, solid data. In some cases, such graphics were even used to underline a scientific argumentation in a paper, to make a scientific point. It can’t get much more wrong than that.

My plea is to scale back the arms war in scientific illustrations. It is of course great to be able to make a device look appealing. But do atoms have to reflect light mirror-like? Why these attempts to be superrealistic when in fact the outcome is the opposite? Illustrations can still be visually attractive even if they are scientifically more accurate, for example by being more abstract. Abstract graphics can look cool, too. Illustrators should keep it simple and stick to the main purpose of the illustration. That way science wins.

Update: In the initial version of the blog post as published today I used the words ‘wrong’ and ‘misleading’ at various instances of the text. To avoid misunderstandings I have now changed it all to ‘wrong’, because it was not my intention at all to create the impression that such graphics are created (or used) with bad intentions.

Well, as with so many other facts, Wikipedia would also give you the answer to these questions. But that’s not the point. What Petroski has now done is to collect and curate interesting facts related engineering, and published them in alphabetical order as “An Engineer’s Alphabet“.

There are plenty of gems to discover in the book. Many of them I would never have thought to even look up on the internet without being prompted, and in that respect the book is inspiring. I certainly enjoyed browsing through the text. Written by an American professor, it is more American Dream than Steampunk in character, although to be fair Isambard Brunel does appear in eight different entries. Herbert Hoover thirteen times. Robert Noyce only once, in passing.

The best way to go about reading this book is simply do flip through it and to read here and there. Or, to use the indispensable index at the end. Indeed, if £18.99 or $21.99 should be a bit too much of an expense it might make sense to consider the various ebook options, with the highly useful possibility of searching the book. On the Kindle or the Nook prices are about half of the hardcover ones. It doesn’t seem to be available on iBooks. Either way, if you love engineering and are interested in broadening in particular your historic knowledge of the profession, this book might be for you.

Reference:

Petroski, Henry. An Engineer’s Alphabet. Cambridge University Press, 2011. 268 pages. ISBN: 9781107015067. $21.99 / £18.99

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.

If we now add a matching light-emitting semiconductor into the structure (see figure), light emission from this semiconductor is only possible if it emits light in the form of the fundamental mode. All other emission modes are forbidden because they are not sustained in this small space. Here, the coaxial structures have a diameter of about 300 nanometres, matched to the lasing wavelength of the InGaAsP semiconductor of about 1,400 nanometers.

The single mode has other implications, adds Khajavikhan, “by ensuring that cavity only supports one mode (the lasing mode) and all other channels for emission are absent, the emitters in the gain region only emit in the lasing mode thus resulting in thresholdless lasing.” Normally, when lasers are turned on they first need to overcome various internal losses before lasing can set it. But because there is only one emission channel that is furthermore shielded from outside losses, lasing sets in immediately.

As a proof of this principle, the authors fabricated two coaxial lasers. A large one in which several optical modes can exist, and a smaller one with only the fundamental optical mode. As expected, the larger structure only lases above a certain threshold whereas the smaller device starts lasing immediately. Xiang Zhang from the University of California, Berkeley, who works on nanoscale lasers, considers this a very interesting development, particularly for fundamental quantum physics research. “The demonstrated nanoscale coaxial laser could be an interesting platform for the investigation of plasmon-based cavity quantum electrodynamics and quantum metamaterials.”

At the same time, Zhang cautions that there are still obstacles to overcome before coaxial lasers could complement the wide-spread use of coaxial cables. Low operating temperatures as well as the fact that these lasers are optically excited and not electrically need to be addressed in future work.

However, the practical potential is certainly clear, says Khajavikhan. “In the fabrication of these lasers we have used standard nanofabrication processes. More importantly, the lasers are batch fabricable, that is many of them can be fabricated at simultaneously and in the parallel way. [...] No other nanolaser is so well adapted to the existing nano-fabrication tools.”

With their commercialization still a bit off, however, as Zhang suggests these lasers could be subject of plenty of fundamental research on quantum mechanical effects. And more possibilities will arise, adds Fainman. “We feel this is just a beginning of a new family of light emitters with superior characteristics and many advances in this new area is yet to come.”

Reference:

Khajavikhan, M., Simic, A., Katz, M., Lee, J., Slutsky, B., Mizrahi, A., Lomakin, V., & Fainman, Y. (2012). Thresholdless nanoscale coaxial lasers Nature, 482 (7384), 204-207 DOI: 10.1038/nature10840

This post was chosen as an Editor’s Selection for ResearchBlogging.org

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.

The impact of this effect on chemical reactions is now clear, because the change in energy states of a molecule also changes chemical reactions. Different energy levels enable new reaction pathways, or could accelerate or slow down chemical reactions. Of course, chemical reactions often can be influenced by light. But in many cases this is down to electrons being excited from one state into another. Here, to use the language of the authors, the entire energy landscape of the system is remodelled in a controlled fashion. In particular, both the energy position as well as the strength of the coupling can be tuned via the properties of the mirrors, so that specific energy states can be addressed.

For their experimental demonstration, Ebbesen and colleagues studied the well-known molecules spiropyran and merocyanine, which can be converted into each other by selective irradiation with light. A film containing spiropyran was placed between two silver mirrors. The energy levels of spiropyran are not matched to that of the light field between the mirrors, so this molecule doesn’t show any coupling effect. Then the sample was irradiated with UV light, which converts spiropyran into merocyanine. Because merocyanine’s excited energy state is aligned with that of the light states between the mirrors, a strong coupling is expected. And indeed, this is shown in the authors’ first paper (ref. 1), where clearly merocyanine shows a Rabi splitting, and spiropyran doesn’t. Having demonstrated the basic principle of the effect, Ebbesen and colleagues now show the implications of the effect on chemical reactions (ref. 2). by studying the reaction dynamics between spiropyran and merocyanine. They found that the conversion from spiropyran to merocyanine is slowed down in the presence of strong coupling.

Taken together, both papers provide a beautiful proof of principle how the energetics of molecular reactions can be influenced by the coupling to light fields. And given that there is complete control over selecting the right energy level and over the Rabi splitting itself,  this strong coupling approach could lead to a considerable freedom in modifying chemical reactions, possibly with profound implications for many areas of chemistry.

Of course, first there is still a lot to do. So far, a reaction involving only a single molecule has been studied, and in future it will be interesting to see how more complex reactions can be modified in this way. Also, the use of silver mirrors with only a very narrow space in-between could make more complex experiments difficult. But as the authors note, “strong coupling is not limited to the [...] configuration used here. Any photonic structure that provides a sufficiently sharp resonance can be used.” In other words, certain open systems, such as photonic crystals, could show the same effect. We’re certainly only at the beginning of this new research area.

References:

1. Schwartz, T., Hutchison, J., Genet, C., & Ebbesen, T. (2011). Reversible Switching of Ultrastrong Light-Molecule Coupling Physical Review Letters, 106 (19) DOI: 10.1103/PhysRevLett.106.196405

2. Hutchison, J., Schwartz, T., Genet, C., Devaux, E., & Ebbesen, T. (2012). Modifying Chemical Landscapes by Coupling to Vacuum Fields Angewandte Chemie International Edition DOI: 10.1002/anie.201107033

Information stored by a chain of magnetic atoms. Left: an STM tip measures the magnetic state of the iron atoms. Right: through increasing the current between tip and atoms the magnetic states can be switched. Peaks become valleys and vice versa. (c) Science Magazine

I now finally got the time to follow-up on last week’s paper in Science by Andreas Heinrich‘s group at IBM on magnetic storage elements that are only a few atoms in size. There have been a few misconceptions in some of the news reports with some being plainly wrong (‘smallest storage device ever made’), and many didn’t mention much about the scientific principles behind this study, although these are quite interesting. One of the better reports appeared in the New York Times, albeit again without going much into details. So I hope I can still add something useful with this blog post.

And actually, we’ve come across Andreas Heinrich’s previous research before, he does very innovative research with scanning tunneling microscopes (STM). In this latest Science paper he has now explored the limits of magnetic storage devices. Magnetism is of course the basis for storage such as magnetic hard drives. The problem in increasing the storage density in any magnetic storage device is that the magnetic regions begin to interfere with each other as they become smaller and are integrated closer together, because magnetic states on the order of just a couple of atoms are not very stable.

For this reason, Heinrich studies antiferromagnets. In antiferromagnets the magnetic fields of neighbouring atoms point in opposite directions (see figure above). The opposing fields of the atoms mean that on average an antiferromagnet has no overall magnetic field, which makes them far less susceptible to outside interference from magnetic fields. Unfortunately, this also makes measurement of the magnetic properties very difficult and is the reason antiferromagnets are not used in data storage at present.

The problem of measuring antiferromagnets, however, is solved when using a STM with its atomically sharp metal tip, because then antiferromagnet is not measured on a global scale, but atom by atom. This happens via a small electric current that flows between the tip and the iron atoms on the surface. The tip of the STM is magnetized, so that the current through the tip depends on the relative magnetic alignment between the tip and the atom. Low if tip and atom’s magnetism is opposite, and high if in the same direction. In the actual experiments, the current was actually held constant, but instead the height of the tip above the surface was varied, so topographic maps of magnetism are obtained, see the figure above. The alternating highs and lows of these images show that the Fe atoms indeed are antiferromagnetically ordered. Manganese atoms on the other hand do not show this kind of order – the energetics for them are unfavourable.

Anyway, the antiferromagnetism can also be controlled. To read out, the electrical voltage applied to the STM tip is less than 2 millivolts.  By increasing this to 7 millivolts the force on the magnetic orientation of the atom underneath the tip becomes too strong and it switches its orientation. The magnetic interaction of the switched atom with its neighbours means that they all swap around. This can be seen from the figure above, where the peaks and valleys are reversed. The mechanism works best when initiated at either end of the chain.

The alternative order of the peaks and valleys can be used to encode information as either 1 or 0. What is so appealing about this mechanism is that the switching as well as the reading of the information does not need a magnetic field applied to the sample (although the tip is magnetized in a magnetic field).  Switching times as short as 20 nanoseconds were reached, which would translate to a maximum of 50 million switches per second.

But the real appeal is the size of the device. Bits were encoded into chains of only 6 atoms long. Unfortunately, low operation temperatures are needed to avoid random switching of the atoms. The details depend on the size and shape of the atomic arrangement, but is typically below 5 Kelvin (degrees above absolute zero), which is -268 degrees Celsius. To keep an array stable for 17 hours even requires cooling down to 0.5 K. So clearly, such kind of memories aren’t going to take off in earnest any time soon. In comparison, Resistive Random Access Memory or Phase-change Memory both are much closer to the market and achieve impressive storage densities.

Still, scientifically this work will have a strong impact. Not only shows it a path towards a more systematic way of studying antiferromagnetic order of atoms on a surface, it also does re-emphasize new ways of utilizing magnetism on the atomic scale.

Reference:

Loth, S., Baumann, S., Lutz, C., Eigler, D., & Heinrich, A. (2012). Bistability in Atomic-Scale Antiferromagnets Science, 335 (6065), 196-199 DOI: 10.1126/science.1214131

This post was chosen as an Editor’s Selection for ResearchBlogging.org

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:

“The distinction between temporal and spatial cloaking can be understood in terms of a metaphor involving automobile traffic. A spatial cloak acts like a junction in the form of a ‘cloverleaf’ interchange or flyover, in which the traffic is guided (by slip roads) to bend around a certain region of space. After passing through the junction, the traffic continues in the same direction as if the junction did not exist. By contrast, a temporal cloak behaves like a railway crossing. Traffic is stopped when a train passes, forming a gap in the traffic. After the train has passed the crossing, the stopped cars speed up until they catch up with the traffic in front of them, and the fact that a train has crossed the intersection cannot be deduced by observing the traffic flow.”

The realization of such a cloak requires a number of steps. First of all, it isn’t easy to completely halt the propagation of light and then carry on as before in a way similar to traffic at a crossing. But what is possible is to create a gap in the traffic whilst it is flowing by making some of the cars go slower and others go faster.

In the actual device, cloaking starts when a beam of a single colour is directed at a so-called split-time lens (STL), which basically broadens the colour range of the original beam. For example, a green light beam would be converted into blue and red colours. This is a key step for the device, because it is relatively easy to make the beam components with different colours go either faster or slower – this kind of manipulation is routinely done in optical fibres. Here, in the optical fibre blue rays are made to travel faster and red ones slower. Eventually, the blue ones race far enough ahead so that the red ones can’t keep up and a gap forms in the middle of the beam where there is no light. Any event happening within this gap is going to be unnoticed by the light beam. Afterwards, once the zone of interest it passed, an inverse process first brings the different colours back together and then reinstates the original, single-colour light beam. The temporal cloaking is complete.

However, before we get too excited here (and I imagine that these cloaks will get quite some attention in the media), here are a few things that the device as demonstrated here does not do: first of all, it only works for a single colour. Also, it only works for a short moment – about 110 picoseconds (around a ten billionth of a second), although temporal gaps ten times longer should be feasible. And finally, the cloak only works for a small volume in a complicated experimental setup. This is nothing to carry around, so it won’t make anyone disappear for certain amounts of time, and it certainly isn’t a time-machine either and never will be.

With these caveats out of the way,  scientifically this is certainly an intriguing demonstration because not only complement temporal cloaks nicely the spatial ones and realize a new concept of light manipulation, but they also could find some real-world applications – even if these probably will be less head-line grabbing. For example, the in initial operation of the STL needs to be triggered by a short optical pulse. This possibility to control the operation of the cloak can be used in optical communications and data processing to selectively turn the cloak on and off so that light pulses carrying data are either processed (cloak off) or left alone (cloak on). But this is certainly something left for future research. As with any new research field, we don’t really know where these cloaks will take us, and certainly this is the exciting part.

References:

1. Fridman, M., Farsi, A., Okawachi, Y., & Gaeta, A. (2012). Demonstration of temporal cloaking Nature, 481 (7379), 62-65 DOI: 10.1038/nature10695

2. Boyd, R., & Shi, Z. (2012). Optical physics: How to hide in time Nature, 481 (7379), 35-36 DOI: 10.1038/481035a