The September 2009 research newsletter

Editorial

Welcome to the September 2009 research newsletter.

[KW, JJ]

New insights into high intensity laser-solid interactions

In recent years high power lasers have emerged as attractive drivers of ultrabright sources of high energy particles and radiation. As well as the many potential applications of these sources in science, medicine, industry and security, high power lasers can potentially be used to drive inertial fusion energy production.

Understanding the propagation of huge currents (hundreds of MA) of relativistic electrons in solid targets is of fundamental importance to virtually all areas of high power laser-solid interaction physics and is vital to the success of the Fast Ignition scheme for inertial fusion energy. When an ultraintense laser pulse is focussed onto a solid density target a relatively large fraction of the laser energy is absorbed (>80% has been measured at >10²⁰ W/cm²), and subsequently partitioned into the kinetic energy of electrons, ions, hydrodynamic expansion and the generation of fields within the target. Of the order of 50% of the absorbed energy is transferred to relativistic electrons for a picosecond laser pulse focused to ~10²⁰ W/cm².  

Measurement of the relativistic electron generation and transport in solid targets is notoriously difficult because the electron energy distribution inside the target cannot be measured directly. Instead, many techniques have been adopted to indirectly determine the current flow. In 2007 it was demonstrated that a significant fraction of the electron energy is transported laterally in thin foil targets, using spatially resolved measurements of ion emission from the target surface [1]. This phenomenon is significant because it affects the efficiency with which electron energy can be coupled to ion and radiation production. Recently, in a collaborative experiment with Queen’s University Belfast and others, the dynamics of the charge wave movement on the surface of solid targets has been directly probed using laser-accelerated protons. The protons are deflected by the transient field produced by the charge wave, which is found to propagate along the surface at close to the speed of light. This work is reported in Physical Review Letters [2].

Multi-frame proton radiography images (over ~2ps) showing the propagation of the field front, produced by an electron current, along a wire target from bottom to top. The red arrows indicate the position of the front, which has a measured velocity equal to (0.95±0.05)c [2].

In a separate experiment, a novel scheme for enhancing and spectrally controlling high harmonic emission from high intensity laser-solid interactions has been demonstrated. This work, also reported in Physical Review Letters [3], shows enhancement factors >50 in a narrow band of harmonics and illustrates the application of XUV radiation emission as a powerful diagnostic of advanced laser-plasma radiation sources.

• [1] P McKenna et al., Physical Review Letters, 98, 145001 (2007)
• [2] K Quinn et al., Physical Review Letters, 102, 194801 (2009)
• [3] B Dromey et al., Physical Review Letters, 102, 225002 (2009)

Contact Paul McKenna for further information (ext 5712)

Serving nanoparticle “soup”

A research collaboration involving groups in Regensburg, Freiburg, Perth, and Glasgow has shown [1] that the strange properties characteristic of room temperature ionic liquids (RTILs) are due to the presence of a high degree of structure. RTILs are the subject of intensive research as a promising new breed of green solvents for industry with low toxicity and lower environmental impact than traditional organic solvents. RTILs consist entirely of molecular ions and have exceptionally low volatility but also have properties that can be fine-tuned – through changes in the molecular structure – to provide more efficient routes of synthesis. However, the physical properties of RTILs are poorly understood; for example, they are extremely viscous but it is not understood why this is so.

The study found that RTILs organise themselves spontaneously into fluid nanoparticles floating in a liquid sea. This nanoparticle “soup” acquires greater viscosity much like mayonnaise is more viscous than its constituents, oil and vinegar. The physics behind this effect was developed by Einstein in the early 20th century.

The discovery was made possible by terahertz spectroscopy, which is an exciting new technology that has also been used for medical imaging and weapons' detection. Terahertz spectroscopy bridges the gap between microwaves and infrared radiation where materials absorb radiation characteristic of their structure.

The collaborating groups have combined four types of terahertz spectroscopy to produce the detailed picture necessary to reveal this structure. At Strathclyde, pulses of infrared light shorter than a picosecond were used to induce and then probe fluctuations in the refractive index that are determined by the microscopic liquid interactions. These interactions in turn determine the macroscopic properties: viscosity, density, thermal conductivity, and freezing and boiling points, that are essential to their applications.

For more information contact Klaas Wynne (klaas.wynne phys.strath.ac.uk) or Prof Richard Buchner (richard.buchner chemie.uni-regensburg.de).

[1] David A. Turton et al., J. Am. Chem. Soc. 131, 11140-11146 (2009) DOI: 10.1021/ja903315v.

FluoroFest Workshop

Our Head of the Department of Physics, David Birch, organized the three day FluoroFest Workshop in Prague at the beginning of June. Held at the Faculty of Mathematics and Physics of Charles University, in Malostranske Namesti close to the famous Charles Bridge, Prague Castle and the Old Town this festival of fluorescence was the first of its kind in the field by coordinating tiered approaches in order to achieve the exchange of information at multiple levels. As well as lectures on the fundamentals and frontier topics, presented by leaders in the field, there was extensive hands-on training in fluorescence techniques using state-of-the-art instrumentation, discussion groups and posters. By using frontier applications as exemplars it was possible to bring out the generic nature of fluorescence techniques which underpin progress across numerous disciplines. Measurement techniques covered included spectra, quantum yield, lifetimes, polarization anisotropy, energy transfer, time-correlated single-photon counting, multi-frequency phase-modulation, imaging, microscopy and data analysis. Application of these techniques was brought to a common focus through multidisciplinary sessions on nanotechnology, bioscience and sensing. Invited lecturers included Barry Lentz (University of North Carolina), Jeff Fagan (National Institute for Standards and Technology), Martin Hof (Heyrovský Institute), John Pickup (KCL School of Medicine) and David Philips (Imperial College).

The workshop proved to be suitable for both the expert and beginner alike and attracted over 50 delegates from across the globe from as far apart as Israel and Brazil. The event was generously sponsored by Horiba Scientific who provided key instrumentation lectures and demonstrations. The conference dinner was held in the traditional “Czech style” pub Baracnicka rychta. There was even time for a Workshop trip – a journey in the lift to the basement of the Faculty to see the Roman ruins!

Departmental collaborations with the Czech Technical University (CTU) Prague are developing on a number of fronts with Professor Birch holding the Visiting Chair of Applied Physics at CTU, our CNQO Group collaborating with CTU and increasing student exchange under the ERASMUS programme.