Matter interacting with strong laser fields constitutes a very good example of how physical systems behave under extreme conditions. ‘Extreme’ relates not only to the high intensities involved, which are above 1013 W cm−2 and may extend beyond 1020 W cm−2, but also to the timescales, of hundreds of attoseconds (10−18 s), associated with strong-field phenomena. Such phenomena are, for instance, high-order harmonic generation, i.e. the emission of radiation in the extreme ultraviolet (XUV) regime from an infra-red (IR) input pulse, or the production of high-energy photoelectrons in above-threshold ionisation or laser-induced nonsequential double ionisation. The physical mechanisms behind them, namely the recombination or rescattering of an electron with its parent ion, take place within such time scales.
Hundreds of attoseconds are also the time it takes for light to move through atomic distances. Hence, the above-stated phenomena are powerful tools for controlling dynamic processes in matter in the subÅ, subfs regime. This may bring answers to unsolved, yet fundamental questions, such as how an electron migrates in a photosynthetic molecule, or how electrons and holes propagate in real time, in a condensed-matter system. The answers to such questions may have a major impact in several areas of knowledge, such as physics, chemistry, biology, or even applied science.
In the past few years, a great deal of progress has been made in the understanding of the dynamics of atoms and small molecules in intense fields. Concrete examples are the reconstruction of molecular orbitals, the coherent control of molecular wavepackets, the probing of molecular vibration, and attosecond charge migration in di- and triatomic molecules. Even these very simple systems revealed an unbelievably rich dynamics in the attosecond regime. They are only the starting point for the investigation of more complex systems, such as organic molecules, clusters and nanostructures. The interaction of extended physical systems with intense fields, however, poses an unprecedented challenge for strong-field experts, both from the conceptual and methodological viewpoints. This is due to the fact that, traditionally, the modelling of strong-field phenomena places far more emphasis on the external laser field than on the structure of the target. Hence, in order to tackle this challenge, one must abandon more conventional ways of thinking and bring ideas and tools from other research areas, such as quantum chemistry and condensed-matter physics to strong-field laser physics. It is also vital for the strong-field community to evaluate where it stands, what its objectives are and what steps must be taken if one is willing to study increasingly complex targets.
With that purpose in mind, we brought together world leaders of the strong-field community, molecular physicists, quantum chemists and condensed-matter physicists in the Workshop ‘Advances in Strong Field and Attosecond Physics’, held at University College London from 23 to 25 June 2010. The workshop involved more than 100 participants, including leaders in the field and many young scientists, and served as a platform for a lively discussion of the state of the art, and of how to tackle the challenges that lie ahead. This special issue is centred around this workshop. All contributions have been peer-reviewed, and contain significant new material. Original contributions from non-participants have also been invited.
The present issue covers a wide range of topics, which are all related with the promise of unveiling the rich dynamics of matter in the attosecond regime. It brings, for instance, a comprehensive Topical Review by Faria and Liu on strong-field electron–electron correlation. The Topical Review is focused on laser-induced nonsequential double and multiple ionisation, but also discusses other phenomena in extended, multielectron systems. This review is aimed both at the non-specialist in the field and at more experienced researchers.
There are also several articles on improved and novel strong-field approaches. An example is the R-Matrix approach with time dependence, developed by Laura Moore et al., which is a novel numerical approach suitable for multielectron systems in intense fields. Other contributions address improved versions of the strong-field approximation (SFA), which is the most widespread analytic theory employed in this field. These include a high-order asymptotic expansion for SFA-like ionisation transition amplitudes, by Long and Liu, the low-frequency approximation in which exact rescattering amplitudes are incorporated in analytic expressions, and the quantitative rescattering theory (QRS). The latter two approaches are discussed in the contributions by Fetić et al. and Le and Lin, respectively. The low-frequency approximation is applied to resonant enhancements in above-threshold ionisation, and the QRS is employed in the study of the ellipticity of high-order harmonics in diatomic molecules from mid-IR driving fields.
Other ever-present topics are the attosecond imaging of molecular oribtals, and the attosecond dynamics of single atoms. Molecular orbitals are discussed in the contributions by Hijano et al. and Augstein and Faria. The former article discusses how to avoid the coupling of multiple orbitals and the consequent obscuration of ionisation maps, and the latter article investigates multielectron effects in molecular high-order harmonic generation. Single atoms are addressed by Lein, who considers streaking by a half-cycle pulse to study ionisation of atoms, and by Popov et al. who investigate for which field parameter ranges the above-mentioned rescattering picture is valid. Finally, macroscopic effects related to the propagation of XUV high-harmonic radiation and an IR pulse are investigated in the article by Nemeth and Bryan.
We hope this special issue will be useful to the newcomer as well as to the expert in the subject.
Acknowledgements
We would like to thank the Editors of Journal of Modern Optics (in particular Misha Ivanov and Jon Marangos) for giving us the opportunity of publishing this Special Issue, and all those involved in the organization of the ‘Advances in Strong Field and Attosecond Physics’ workshop. We also acknowledge financial support from the Centre Européen de Calcul Atomique e Moléculaire (CECAM), the UK Collaborative Computational Project 2 – Quantum Dynamics in Atomic, Molecular and Optical Physics (CCP2), the UK Science and Technology Facilities Council (STFC) and the University College London.