Advances in Strong Field and Attosecond Physics

University College London
23-25 June 2010

Organisers:
Carla Faria (University College London)
Paul Durham (STFC Daresbury Laboratory)
Misha Ivanov (Imperial College London)
Wolfgang Sandner (Max Born Institut)
Jens Biegert and Maciej Lewenstein (Institute for Photonic Sciences Barcelona)

This workshop, sponsored by the Hartree Node of CECAM, aims to bring leaders in the fields of quantum chemistry and condensed matter physics together with pioneers in strong field laser physics to assess where we are with the various theoretical and computational approaches to short pulse problems and discuss the best ways to progress from atoms to the more complex systems of current interest.

For full details of the workshop and an online registration form please visit the workshop web pages.

Background

The recent upsurge of interest in the electronic processes occurring in the extremely strong fields produced by short pulse lasers has many facets: the possibility of generating extremely short (attosecond) pulses approaching the atomic unit of time, the prospect of investigating fundamental many-electron processes directly in the time domain, the information contained in high harmonic generation, manifestations of quantum coherence and entanglement, and so on. This is a consequence of the fact that the physical mechanisms behind strong-field optical phenomena take place within a fraction of the laser field, which, for experimentally relevant parameters, comprises hundreds of attoseconds [1].

Many fascinating effects have been seen in simple atomic systems, and attention is now turning towards more complex systems: molecules, clusters, surfaces, even nanostructures. The interaction of strong fields with such systems may have a high-impact on several areas, such as physics, chemistry, biology, or even go beyond fundamental science. Traditionally, however, strong-field laser theory places considerably more emphasis on the external laser field than on an accurate treatment of the targets. As a direct consequence, experts in this field are currently facing a great challenge, from the conceptual and methodological viewpoints, in order to describe such systems appropriately. In particular the treatment of collective, multi-electron effects, and of spatially extended systems have raised a great deal of debate. For such systems, long established theories, such as the strong-field approximation, break down, or, in the most optimistic scenario, yield contradictory results. In order to tackle this challenge, it is necessary to bring approaches and tools employed in other research areas, such as quantum chemistry and condensed-matter physics to strong-field laser physics. Hence, a strong interaction with scientists of such areas is becoming increasingly important.

For the above-stated reasons, worldwide, there is currently a major effort in order to bring scientists belonging to the strong-field, solid-state, quantum-chemistry and molecular physics communities together. An example is, for instance, the workshop "Quantum Control of Light and Matter"(Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, 20 April -- 17 July, 2009). This interaction is not only beneficial for intense-field laser physicists, but also for quantum chemists and condensed-matter physicists. In fact, it provides a fascinating opportunity of investigating phenonema occurring in ultra-fast, attosecond or subfemtosecond time scales, which may reveal completely new aspects of matter.

Therefore, in order to strengthen these scientific connections, especially within Europe, we are organising a CECAM workshop, to be held at the University College London, from June 23 to 25, 2010.

In the past few years, considerable progress has been made in the attosecond imaging of matter and in the understanding of how complex systems interact with strong laser pulses. Concrete examples are the reconstruction of molecular orbitals [2], the probing of molecular vibration [3], or the investigation of high-order harmonic generation and ionization in clusters [4] and larger molecules [5].

There exists, however, a great deal of controversy, especially with respect to an accurate treatment of the targets in intense fields. This discussion goes beyond the current scope of strong-field laser physics and moves towards the fields of quantum chemistry and condensed-matter theory.

Particular challenges in this context are:

• An accurate treatment of the residual binding potentials and their influence on strong-field phenomena. For small systems, such as atoms or diatomic molecules, these potentials may be neglected to first approximation. Indeed, in this case one may still approximate the electron propagation in the continuum by a field-dressed plane wave. For large molecules, clusters or solids, however, this distinction is less clear and it is not clear to which extent the residual potentials must be included in an adequate modelling of the electron propagation.
• An appropriate treatment of electron-electron correlation in complex systems. It has been recently suggested that multielectron effects may be a major obstacle to attosecond molecular imaging using high-order harmonics, even for relatively simple molecules [6]. Such effects are also expected to play an important role for more complex targets. To the present date, already the interaction of a two-electron system with an intense field has posed a great challenge to theorists, and only recently has a completely ab initio computation been performed [7]. For more complex systems, however, this approach seems a hopeless task. Relatively recent work using time-dependent density functional theory (TDDFT) looks very promising, if computationally non-trivial, for this problem [4].

[1] See, e.g., F. Krausz, M. Ivanov, Rev. Mod. Phys. 81, 163 (2009) for a review on the subject.

[2] J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pepin, J. C. Kieffer, P. B. Corkum and D. M. Villeneuve, Nature 432, 867 (2004).

[3] S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirila, M.Lein, J. W. G. Tisch, J. P. Marangos, Science 312, 424 (2006).

[4] See, e.g., M. Ruggenthaler, S.V. Popruzhenko and D. Bauer, Phys Rev. A 78, 033201 (2008); A. Mikaberidze, U. Saalman and J. M. Rost, Phys. Rev. Lett. 102, 128102 (2009).

[5] R. Torres, N. Kajumba, J. G. Underwood, J.S. Robinson, S. Baker, J.W. Tisch, R. De Nalda, W. A Bryan, R. Velotta, C. Altucci, I.C. Turcu, and J. P. Marangos, Phys. Rev . Lett. 98, 203007 (2007).

[6] O. Smirnova, S. Patchkovskii, Y. Mairesse, N. Dudovich, D. Villeneuve, P. Corkum, and M. Yu. Ivanov, Phys. Rev. Lett. 102, 063601 (2009).

[7] J. S. Parker, B. J. S. Doherty, K. T. Taylor, K. D. Schultz, C. I. Blaga, and L. F. DiMauro, Phys. Rev. Lett. 96, 133001 (2006).

Computational Aspects

In a sense, this workshop covers one problem in quantum dynamics, one where perturbation theory of any sort is out of the question and accurate computations are essential but difficult. There is a well-established set of codes to handle time-dependent density functional theory although many focus on linear response rather than strongly driven systems. Other approaches may have difficulty in handling many-electron effects in realistic complex systems. Thus there is a real computational issue facing the field. The workshop will address the question of whether a collaborative effort to generate the required new codes would be useful and if so how should it be done.

Training and Collaboration

We also intend that this workshop should provide training for young researchers. Intense-field laser physics is very strong in some European countries, notably Germany, and is an emerging field in many others. The workshop would provide young researchers from all over Europe with the unique opportunity of being immersed in the international strong-field community and interacting with leaders in the field. The international community, on the other hand, will profit from the UK’s long-standing expertise in high-performance parallel computing, especially in the context of condensed-matter physics and quantum chemistry