The May 2008 research newsletter

Welcome

Welcome to the May edition of the Research Newsletter of the Department of Physics, SUPA at Strathclyde. As announced previously, the newsletter will be run by the new Director of Marketing & Communication (DMC) who will be happy to consider your stories for this newsletter, the SUPA newsletter, Research Matters, the Physics website, and elsewhere. The offer previously made is still valid: the DMC is willing to take photographs of you and your labs under the condition that they may be used to promote the department. Steady progress is being made with the academic staff pages. Check it out for yourself and send your bio and research description to the DMC. Your publications and grants are pulled from the Big Brother database so make sure that your record is up to date (not the DMC’s responsibility – go here to update your publications – go to Kirsten to make sure your grants are entered correctly).

In related matters, the Strathclyde Press Office has a new press officer – Paul Gallagher (see below) – whose responsibility is the Science and Business faculties. You may have noticed quite a few more press releases from the Science faculty appearing recently. As DMC, I would like to urge all of you to consider your stories for a press release. It will not just appear on the University homepage but might get you an interview with the press. Essentially any science story mentioning the physics department is good for us. Come and talk to me if you need to know more or contact Paul Gallagher at, t: 0141- 548-2370, e: paul.gallagher strath.ac.uk [K]

Resonances, but not as you know them

Resonances appear in virtually all branches of physics and we all assume that the response of a given system to external modulations is mainly determined by the difference between its natural and forcing frequencies. This simple picture turns out to be true only when the modes of the original system are ‘orthogonal’ as are, for instance, the modes of a harmonic oscillator. The coupling between non-orthogonal modes can lead to a response to a periodic forcing orders of magnitude larger than what the separation between forcing and natural frequencies would suggest. This leads also to a quantum effect, the ‘excess noise’, whereby the level of spontaneous emission noise in a device is more than the one photon per mode expected from basic quantum considerations. When resonances are observed in nonlinear systems, two main questions arise: (1) what are the modes that we should consider? (2) Are these modes orthogonal? In a recent letter (Phys. Rev. Lett, 100, 123905 (2008)), we have shown that – in the case of lasers – the response to modulation is not determined by the modes of the optical cavity but by the modes of the linear stability operator. These modes can be orthogonal or not depending on the way in which the laser operates. As a consequence, in nonlinear systems strong response to non-resonant modulations and excess noise can affect some output states but be completely absent from others. We then proceeded to extend the phenomenon of ‘state-dependent resonance’ to general nonlinear models with relevance not only to optics but to all spatially extended systems near an oscillatory instability. In particular, we found cases (see the figure) where the response to modulations can be enhanced by an order of magnitude with obvious relevance to the field of control of complex systems.

Figure: Comparison of two systems with identical resonance frequencies and losses: the solid blue line (||R||₂²) is the maximum response of the system with non orthogonal modes; the dashed red line (dist²(Λ)) is the maximum response of the system with orthogonal modes. (click on figure for bigger version)

[F. Papoff¹, G. D’Alessandro², and G.-L. Oppo¹]

¹ SUPA, Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK
² School of Mathematics, University of Southampton, Southampton SO17 1BJ, England, UK

The science of syrup and traffic jams

An international group of scientists from the UK, Germany, and Australia has discovered the reason why syrup is so sticky by comparing it with traffic jams. When salts or sugars are added to water, the resulting mixture is very ‘sticky’ and flows with great difficulty or – to use the scientific term – it has a high viscosity. This effect is probably most familiar in mixtures of sugars and water such as in syrup and honey.

A debate has been raging for decades as to the scientific reason why things like honey are so sticky. The consensus had been that sugar and salt molecules in water would bind to the water molecules and thereby alter the structure of water. So-called structure inducers (with the beautiful scientific name ‘kosmotropes’) would make liquid water more ice-like and thus stickier... Unfortunately, work by a Dutch group threw a spanner in the works by suggesting that no such structure-inducing effect exists (DOI: 10.1126/science.1084801) leaving chemists and physicists at a loss how to explain stickiness.

A new study published in the Journal of Chemical Physics puts an end to all the confusion (DOI: 10.1063/1.2906132). The team consists of researchers from the University of Regensburg, Murdoch University, and the University of Strathclyde. Leader of the team, Professor Klaas Wynne explains that ‘the German, Australian, and British teams use complimentary techniques making it possible for the first time to understand the motions of water molecules in solution on time scales of about one-trillionth of a second’ (that is: 1/1,000,000,000,000 seconds).

The team studied solutions of metal salts rather than those of sugars because the salt ions are simpler appearing like smooth spheres rather than knobbly molecules. He continues ‘The metal ions we have studied hold on tightly to a few neighbouring water molecules forming tacky spheres that randomly jam together at high enough concentration like cars in a traffic jam.’ His colleague Dr David Turton explains: ‘You can think of a crystal as oranges in a box at the supermarket, neatly arranged in rows and layers. If you instead throw the oranges into the box, they jam into a random jumble – what physicists call – a glass. We find that ions and sugar molecules in water jam like oranges while the water is free to spin in the pockets between the “oranges”.’

This type of jamming has now been seen in the most unlikely collection of situations such as the movement of dunes in the desert, the flow of grains in silos, and – yes – traffic jams. So, next time you are stuck in traffic, think about how you are simply following the laws of physics like sugar in the cup of coffee you had for breakfast that morning.

Some more information is at our page The science of syrup and traffic jams on the BCP site

[David Turton, Klaas Wynne]

Attosecond pulses

Part-time PhD student Neil Thompson and supervisor Brian McNeil have invented a method of generating attosecond pulse trains from Free Electron Laser amplifiers. Their work, to be published in Phys. Rev. Lett. at the end of May,  borrows techniques from conventional mode-locked lasers. These conventional lasers use phase-locking of the longitudinal cavity modes to great effect, generating few femtosecond pulses, e.g. the ubiquitous Ti:Sapphire laser. Thompson & McNeil have predicted that a cavity-like longitudinal mode structure can be synthesised in a FEL amplifier. These modes may be locked and in the x-ray, at wavelength 1.5 Å, to generate attosecond pulse trains of duration 23as separated by 150as with peak powers of a few GW – see figure.  Such pulses could, for the first time, enable the spatiotemporal imaging of atoms and their interactions.   

(See: http://arxiv.org/abs/0802.4159 for pre-print)

European Research Council Funds SPRITES!

SPRITES (Structure changes in Protein Reactions via Infrared Time Evolution Spectroscopy) a € 1M ERC Starting Grant Scheme proposal submitted by Neil Hunt has been approved for funding. The project, one of only ~300 funded projects from more than 9000 Europe-wide applications, will apply a derivative of the new ultrafast laser technique of transient 2D-IR spectroscopy¹ to study enzymatic and protein reactions in real time.

2D-IR spectroscopy uses a train of mid-infrared pulses to observe interactions between vibrational modes of a molecule.² In a similar way to multidimensional NMR spectroscopy, 2D-IR spreads the infrared spectrum of a molecule over two frequency axes; peaks corresponding to the normal linear spectrum appear on the diagonal, while off-diagonal peaks show interactions between the modes. These interactions can take the form of direct coupling or population transfer from one mode to another – effectively observing energy flow through the molecule. Of particular relevance to the SPRITES project, 2D-IR spectroscopy can also detect changes in molecular structure with ultrafast time resolution and by adding a visible-light pulse to initiate a biological process such as a change in protein structure or enzymatic reaction, SPRITES will effectively create real-time molecular movies of biological processes.

Anyone wishing to learn more about 2D-IR spectroscopy is subtly directed to Neil’s recent article in Spectroscopy Europe for more information – it’s worth a look for the impressive web-browser alone!!!

(1)           Kolano, C.; Helbing, J.; Kozinski, M.; Sander, W.; Hamm, P. Nature 2006, 444, 469.
(2)           Khalil, M.; Demirdoven, N.; Tokmakoff, A. J. Phys. Chem. A 2003, 107, 5258.