Research newsletter

A paper describing the first demonstration of a compact synchrotron radiation source driven by a laser-plasma wakefield accelerator in experiments at Friedrich-Schiller-Universitat, Jena led by Dino Jaroszynski and involving colleagues at Jena and Stellenbosch University was published in the December 9th issue of Nature Physics (article nphys811). Follow up articles appeared in New Scientist, Physics World, Physics Today and elsewhere. The images show the set up at Strathclyde for producing x-rays.

"New laser-based technology could be opened up to health services through research, led by the University of Strathclyde, which aims to make equipment radically smaller and much less expensive. The team has been investigating how x-rays from powerful devices can be harnessed for use in medicine, particularly by making the technology accessible to hospitals in a way which has not been possible before. Synchrotron X-ray imaging techniques rely on differences in the refraction and scatter of X-rays as they pass through tissue to produce high resolution images for mammography and lung imaging.

At present, the devices, free-electron lasers and synchrotrons, are huge and expensive, often covering hundreds of square metres and costing hundreds of millions of pounds for construction alone. The Strathclyde led-research, however, could help to reduce their cost dramatically and to shrink their size so that they could fit in a room, in the same way that computers, mobile phones and synthesisers have become much more compact and affordable. As a first step towards the project's goal, experiments devised by Strathclyde and carried out in collaboration with teams from Jena in Germany and Stellenbosch in South Africa have led to a breakthrough in light pulse production driven by a laser-plasma accelerator. The laser-plasma accelerator uses an intense laser pulse fired into plasma, generating a charge density wake similar to the ripples left trailing behind a moving boat. A "wave" is created by the wake and small bunches of electrons gather energy from it, like a surfer would catch a wave and use its energy to move forward. This technique could make devices far smaller, potentially reduced to about the size of a pool table, at a fraction of the current costs.

Light pulses have then been generated by passing the electron bunches through a system of magnetic fields. The current experiments generated visible light pulses and future experiments in the pipeline will increase the electron energy in order to allow for generation of bright x-ray pulses. All in a very compact, "table-top" system. A wealth of applications all across of science could be opened up by these novel x-ray pulses. Medicine could benefit, for example, with improved cancer tumour detection and high-contrast x-ray imaging. Such x-ray pulses will enable a form of ultrafast photography to be preformed allowing scientists to capture unprecedented information about events as fundamental as a chemical bond breaking in a cell protein.

Professor Dino Jaroszynski, Director of the TOPS ( Terahertz to Optical Pulse Source) laboratory at Strathclyde, said: "If we can reduce the size of these devices by 1,000, you could have a powerful scientific tool for the price of a laser. "The costs could come well down, making them much more widely accessible. We are taking big steps towards developing this technology." The project is part of the Strathclyde-led ALPHA-X (Advanced Laser Plasma High-energy Accelerators towards X-rays) programme."

[DAJ/RWM]

Nick Lockerbie has been awarded a £578k grant from STFC to work on investigations in gravitational radiation in collaboration with the University of Glasgow's Institute of Gravitational Research. Nick will continue his research on the development of highly sensitive displacement sensors for use in the advanced LIGO gravitational wave detectors based in the United States. Further details are available here.

[RWM]

Noise is usually considered to be undesirable and something to be eliminated or minimised. In nonlinear systems however, noise can produce counter-intuitive behaviour, in some cases enhancing the coherent response of the system. Recent work by Gordon Robb and Willie Firth, to appear in the December 21st issue of Physical Review Letters, describes the effect of pump phase noise on Collective Atomic Recoil Lasing (CARL), a process in which cold atoms move under optical forces to scatter light collectively. Their results show that when the phase of the pump laser illuminating the cold atoms is noisy, the scattered light intensity can be increased compared to the case where a noise-free, coherent pump is used (see figure). The presence of pump phase noise also reduces the sensitivity of the CARL interaction to atomic temperature, potentially allowing the observation of CARL at significantly higher temperatures than have been observed to date (~ m K). It is anticipated that these theoretical predictions will be tested in experiments involving cold atoms/BEC trapped in high-Q cavities such as those at Eberhard Karls Universität Tübingen. In addition to their immediate relevance to nonlinear optics and cold atom physics, they are of wider significance as the cold atoms and optical fields constitute a non-locally coupled many-body system. The role of noise and coherence in such systems is of wide interest, relevant to plasma and beam physics, condensed matter and even neuroscience.

In addition to their immediate relevance to nonlinear optics and cold atom physics, they are of wider significance as the cold atoms and optical fields constitute a non-locally coupled many-body system. The role of noise and coherence in such systems is of wide interest, relevant to plasma and beam physics, condensed matter and even neuroscience.

Figure : Evolution of scattered light intensity for the case of (a) a noise-free, coherent pump and (b) a noisy pump. The parameter d represents the frequency difference the pump and scattered fields.

[GRMR]

Spontaneous pattern formation is one of the most common and interesting phenomena in nonlinear physics. Theory predicts that some systems should form patterns suddenly, jumping from smooth to fully-patterned when a parameter is changed. Such a sudden transition is rarely seen in experiments, however. Often the system develops isolated spots, the number increasing as the drive is turned up. Only under strong excitation, if at all, do the spots aggregate to form a regular pattern. Fig. 1 (below) shows an example in a semiconductor laser amplifier.

Phase separation of two coexisting species is one of the most studied phenomena. It occurs in a variety of scientific disciplines from solid state physics to chemical reactions, from fluid mechanics to photonics. In such systems, a rapid parameter change can create conditions in which separate domains of the two phases of the system compete with each other. When the two phases are perfectly equivalent, there is a simple equation for the radius of a circular domain of one phase embedded in the other and of initial radius R0 : R(t)=(R0^2-2gt)^½ . There are two major obstacles to a clear experimental test of this ("Allen-Cahn") relation: it is difficult to experimentally realize perfect circular domains and it is difficult to operate at the exact point (called the Maxwell point) where the two phases are the mirror image of each other. These difficulties have been recently overcome in a collaboration between our Optics Division and the Angewandte Physik institute at the University of Muenster (Germany) [Physical Review Letters 99, 153902 (2007)].

Figure 1 : Snapshots of a shrinking phase domain and time evolution of its radius squared. The full (empty) squares (circles) correspond to experimental (simulation) results.

Figure 2 : Shrinking of a circular phase domain close to a pattern forming threshold. The two snapshots show the unstable and stable localized peak that stops the phase front right in its tracks.

The novelties of the paper are not however limited to the realization of the Allen-Cahn theory. By changing a control parameter (the input laser power), a threshold for pattern formation of each phases can be approached. Close to this threshold, localized peaks of one phase embedded in the other have been predicted to exist. In Figure 2, the shrinking of the circular domain is now first slowed down close to one such unstable state and then totally stopped by the presence of a second, smaller one. Excellent agreement between theory, simulations and experiments has been found here also.

The project also shows the relevance of cross disciplinary research since the results apply to a wide variety of scientific and engineering disciplines. For more details about research activities at Strathclyde in self-organizing systems see the web page of the Institute of Complex Systems at Strathclyde

[GLO / TA / WJF]