The March 2009 research newsletter
Editorial
Welcome to the March 2009 research newsletter. In this issue some good stories about papers in PRL and Nature Physics. Also £1.4M of newly announced research funding. We have calculated that the department earned £7.2M in funding last year, which is something to be chuffed about.
[KW, JJ]
New lecturers
The department has recently appointed two new lecturers in the Optics division. Most of you already know Daniel Oi who will remain a SUPA Advanced Fellow until 1 October 2010 and then become a lecturer.
Shashank Virmani has just joined us from the University of Hertfordshire, where he was most recently a lecturer. He did his PhD at Imperial College, and then held postdoctoral positions at the University of Pavia, the University of Hertfordshire, and Imperial College. His research interests lie in quantum information theory and related fields, and to date have included the study of entanglement, quantum state estimation, and the effects of noise on quantum information processing. He looks forward to meeting people in the department and learning new things.
Recent results in high intensity laser-plasma physics
Researchers in the department are amongst a team of scientists who have recently demonstrated a new technique for guiding fast (MeV) electrons in solid targets irradiated by ultra intense (>1020 Wcm-2) laser pulses. Strong magnetic fields created at the interface of two different metals are used to guide the transport of fast electrons through dense plasma. The research, published in Physical Review Letters 102, 055001 (2009) could have substantial impact on laser driven sources of high energy ions and on laser fusion energy.
Very intense pulses of laser radiation produce beams of multi-MeV electrons when irradiating dense targets. If the electron beam can be collimated and guided then it can be used to heat up plasma to fusion temperatures. This is the basis of the fast ignition approach to inertial confinement fusion. The department is a partner in the European HiPER High Power laser Energy Research project, which aims to demonstrate the feasibility of the fast ignition approach to laser driven fusion as a future energy source.
In fast ignition inertial fusion, if the laser-generated fast electron beam is too divergent, then the energy needed to heat the hot spot and ignite the fusion fuel could become unfeasible to implement in a practical laser facility. The new result enables control of the deposition of laser energy in dense plasma, thus substantially increasing design flexibility and efficient energy delivery in laser-fusion sources. A gradient in resistivity, produced at the interface of two metals of differing Z, is used to produce a magnetic field to collimate the fast electron propagation within the target. This uses the second term in Faraday’s law for magnetic field (B) growth
where j is the fast electron current and η is the plasma resistivity.
The figure illustrates the concept with simulations results from the original theoretical paper by Robinson and Sherlock, Physics of Plasmas 14, 083105 (2007). The figures on the left show the target initial configuration. The laser is incident from the left at Y = 55 μm. The figures on the right show the corresponding fast electron transport. A magnetic field is generated at the resistivity boundary between the two materials of differing Z, which acts to collimate the propagating electron beam.
The work was carried out using the Vulcan petawatt laser facility at the STFC-Central Laser Facility.
In a separate experiment, led by a group from QUB and involving researchers from the department, diffraction limited focusing of high harmonics generated from a relativistic plasma is reported in Nature Physics 5, 146 - 152 (2009). An ultraintense laser pulse focused onto a solid target produces a relativistic oscillating plasma surface. When a pulse of light reflects from this relativistic mirror, it is Doppler-shifted to a substantially higher frequency and compressed to a much shorter duration. The latest results show that denting of the target surface structure by the ablation pressure of the ultraintense laser pulse enables production of high quality X-ray pulses focused down to the diffraction limit. This is a promising route to the production of extremely intense x-ray fields.
Contact Paul McKenna for further information.
Small stuff
New Grants
Professor Alan Phelps, 'HiPED Principal Programme - Technology Watch Consultancy', MBDA UK Ltd - Industry and Commerce, 01/09/08 - 14/06/09, £17k
Dr Aidan Arnold, 'UK Network for Research at the Interface Between Cold-Atom and Condensed Matter Physics', EP/G029695/1, 01/01/09 - 31/12/10, Collaboration: University of Nottingham, FEC: £8k, RC: £6k
Professor Steve Barnett, 'Full-field Coherent Quantum Imaging', EP/G011567/1, 01/04/09 -31 March 2012, FEC: £500k, RC: £400k.
Professor Hugh Summers, 'ADAS-EU: ADAS for Fusion in Europe', European Commission, 01/01/09 - 31/12/13, FEC: £1,483k, RC: £899k
Professor Ken Ledingham, 'Laser Production of X-Rays for Detection of Explosive', DSTL - UK Government, 01/12/08 - 20/03/09, FEC: £21k, RC: £17k
Dr Helen Fraser, 'PATT Linked Grant: Observational Astrochemistry @ Strathclyde', STFC, 01/04/09 - 31/03/11, FEC: 26k, RC: £21k
Professor Summers, "Atomic Physics Research and Development at UKAEA", UKAEA, 01/01/09 - 31/12/09, FEC Award - £82k, RC Award = £58k
Professor Summers, "Case Studentship for Laura Gibson", UKAEA, 01/11/2008 - 31/10/2011, RC Award = £23k
Aidan compresses the rainbow
Aidan Arnold was interviewed by Roland Pease on the BBC world service radio discovery channel about laser cooling. Listen to Aidan in Episode 2.
OSA fellow
Stephen Mark Barnett has been elected a fellow of the Optical Society of America for profound contributions to quantum optics, including the introduction of the Hermitian 'Pegg-Barnett' phase operator, a consistent theory of dielectric quantum electrodynamics, and an information measure of entanglement.
Making light of optical momentum
A recent paper by Hinds (Imperial College) and Barnett has proposed a resolution of the century-old problem of the form of optical momentum inside a medium [Phys. Rev. Lett. 102, 050403 (2009)].
We learn in our earliest mechanics classes that momentum is the product of mass with velocity, but what about the momentum of light. Poynting showed that the density of optical momentum is simply 
, or should that be 
? This is the Abraham-Minkowski dilemma. Minkowski would tell us that the momentum of a photon inside a dielectric medium is its free-space value multiplied by the refractive index, while Abraham would insist that it takes the free-space value divided by the refractive index. Theory shows that the Abraham form arises as a direct consequence of Newton’s first law of motion, while the Minkowski form arises as an equally convincing consequence of diffraction and the uncertainty principle. Experiments are divided, with convincing evidence, especially in the recent literature, for both the Minkowski and the Abraham momenta.
Hinds and Barnett have stripped the problem to its simplest level in which the dielectric medium consists of just a single atom. In quantum mechanics, charges have not one momentum but two: a kinetic momentum, which is the product of mass and velocity and also a canonical momentum, which is Planck’s constant divided by the wavelength. We can understand this as a consequence of the problem of separating matter from the electromagnetic field; do we include in the properties of an electron its own electric field, or is that part of the field? Hinds and Barnett have shown that the inevitable existence of these two momenta for a single atom leads directly to the requirement of two momenta for the electromagnetic field. The total momentum is fixed, and this leads to the conclusion that the kinetic momentum of light in a medium is of the Abraham form, but its canonical momentum is of the Minkowski form. When the phenomenon is clearly mechanical, as with Newton’s first law, then it is the Abraham momentum that appears. When the phenomenon is more wave-like, as in diffraction, we expect the Minkowski momentum to manifest itself.
So, is this the end of the Abraham-Minkowski dilemma? With long-standing physics problems, it is rarely that simple.
Contact Steve Barnett for further information.
