We are offering the following DPhil projects starting in 2011:

1. Laser-driven plasma accelerators

Electrons are pushed away from the front and back of an intense laser pulse as it propagates through a plasma, leading to the formation of a trailing longitudinal plasma wave. The longitudinal electric field in the plasma wave can be as high as 100 GV/m, more than three orders of magnitude larger than that found in conventional RF accelerators such as those used at CERN. One factor which limits the energy to which particles can be accelerated is the distance over which the intensity of the driving laser can be maintained. Over the last few years our group has developed a technique for channelling laser pulses with peak intensities of up to 10¹⁸ Wcm^-2 over distances which are much longer than the limit set by diffraction. In collaboration with a group at Lawrence Berkeley National Laboratory (LBNL), we have used this technique to extend the length over which acceleration can be maintained by an order of magnitude and thereby generated electrons with energies of 1 GeV. This energy is of the order of that used in many synchrotrons around the world, but the plasma accelerator is only 33 mm long instead of 150 m! For further details, see Leemans et al.

Our work on laser-driven plasma accelerators is in three areas: (i) investigation of techniques for improving the parameters of the electron beams they generate; (ii) development of techniques for staging plasma accelerators, with a view to reaching beam energies beyond 1 GeV; (iii) development of applications of laser-driven plasma accelerators, particularly their application to the generation of x-rays.

We pursue these goals by both experiment and numerical modelling. Some experiments are undertaken in our laboratories in Oxford, at facilities based at the Rutherford Appleton Laboratory (just outside Oxford), or with our collaborators in the USA and Europe.

Further reading:
1. W. P. Leemans, S. M. Hooker et al., "GeV electron beams from a centimetre-scale accelerator," Nature Physics 2 696 (2006).
2. J. Osterhoff, S. Karsch, S. M. Hooker et al., "Generation of Stable, Low-Divergence Electron Beams by Laser-Wakefield Acceleration in a Steady-State-Flow Gas Cell," Phys. Rev. Lett. 101 085002 (2008).
3. T.P. Rowlands-Rees, S. M. Hooker et al., "Laser-Driven Acceleration of Electrons in a Partially Ionized Plasma Channel," Phys. Rev. Lett. 100 105005 (2008).
4. M. Fuchs, F. Gruner, S. Karsch, S. M. Hooker et al., "Laser-driven soft-X-ray undulator source," Nature Physics 5 826 (2009).
5. T. Ibbotson, S. M. Hooker et al., "Laser-wakefield acceleration of electron beams in a low density plasma channel," Phys. Rev. ST Acc. Beams 13 031301 (2010)

2. Novel Phase-Matching Techniques for High-Harmonic Generation

It has been known since the late 1980s that radiation with frequencies equal to the odd harmonics of the frequency of the driving laser is generated when a pulse of light with an intensity of order 10¹⁵ Wcm^-2 interacts with a gas. The harmonics can be of very high order, as high as several hundred, so that the generated radiation can lie in the soft x-ray region. Further, this high-harmonic generation (HHG) produces beams of short-wavelength radiation of good spatial and temporal coherence. As such, HHG is now commonly used as a simple and reliable source of tunable short-wavelength radiation in a wide range of scientific disciplines.

However, the efficiency with which harmonics can be generated is very low: for generated photons with energies up to about 100 eV the generation efficiency is of order 10^-7; for higher energy photons it is even lower. One reason for the low generation efficiency is the difference between the phase velocities of the driving laser and the harmonic beam. This causes the intensity of each generated harmonic to oscillate with propagation distance with a characteristic period of 2L_c , where L_c is known as the coherence length. An intriguing way of overcoming this problem is to suppress harmonic generation in those regions where the generated harmonics are out of phase with the harmonic beam, thereby preventing back-conversion of harmonic radiation to the frequency of the driving laser. This technique is known as quasi-phase-matching (QPM).

We are studying two techniques for quasi-phase-matching. In multi-mode QPM, two or more modes of a waveguide are excited to produce periodic beating of the axial intensity of a guided laser pulse; this oscillation of the driving intensity strongly modulates the harmonic generation, allowing quasi-phase-matching. Inpulse-train QPM, a train of counter-propagating laser pulses is used to suppress the harmonic generation periodically. These projects are largely, but not exclusively, experimental and will use our high-intensity femtosecond laser systems in Oxford.

Further reading:
1. M. Zepf, B. Dromey, M. Landremann, P. Foster & S. M. Hooker, "Bright quasi-phase-matched soft-x-ray harmonic generation from argon ions," Phys. Rev. Lett. 99 14301 (2007).
2. K. O'Keeffe, T. Robinson & S. M. Hooker, "Generation and control of chirped, ultrafast pulse trains," J. Opt. 12 015201 (2010).
3. T. Robinson, K. O'Keeffe, M. Zepf, B. Dromey, & S.M. Hooker, "Generation and control of ultrafast pulse trains for quasi-phase-matching high-harmonic generation," J. Opt. Soc. Am. B 27 763 (2010).

For further details of the formal application procedure please see the Graduate Admissions page for the sub-Department of Atomic and Laser Physics. If you have questions on this procedure, please contact Sue Gardner.

Prof. Simon Hooker
Clarendon Laboratory
Parks Road
Oxford OX1 3PU.
Tel: +44 (0)1865 282209
Fax: +44 (0)1865 282296
Email: simon.hooker@physics.ox.ac.uk