Alternative S-Matrix Approaches for Matter in Strong Laser Fields

Lead Research Organisation: City University London
Department Name: Sch of Engineering and Mathematical Sci


The theoretical description of matter in strong laser fields is a rather challenging task. This is due to the fact that the external laser field is comparable to the atomic binding forces, and the usual theoretical methods considered in optical physics, such as perturbation theory with the laser field, are not applicable. In particular, it is very difficult to apply analytical or semi-analytical methods to such a physical framework. There exists, however, one such method, namely the Strong-Field Approximation. This method has served to establish the main paradigms in strong-field laser physics, and has been employed in over 500 publications in this field of research. In particular, it is very powerful for studying quantum interference effects in detail. This approximation suffers, however, from severe drawbacks, which are particularly critical for molecules and systems involving more than one electron. Such systems can not be described by such an approximation in a satisfactory way, and indicate that new, radical ideas are necessary in order to develop the theory further. In this project, we intend to bring ideas and methods from quantum-field theory and mathematical physics to strong-field laser physics to develop a new semi-analytical approach which replaces such an approximation. As a testing ground, we will use such a theory to describe molecules in strong laser fields, and, simultaneously, make a rigorous assessment of the limitations of the Strong-Field Approximation. Such systems have been chosen not only due to their critical behavior, but also due to the fact that, nowadays, there exists pioneering experiments in Britain, at the Imperial College, involving molecules, which will pave the way towards dynamic measurements of matter with a never-imagined precision. This will not only be important for the specific physical systems above, but will revolutionalize a whole area of research.


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Augstein B (2011) Influence of asymmetry and nodal structures on high-harmonic generation in heteronuclear molecules in Journal of Physics B: Atomic, Molecular and Optical Physics
De Morisson Faria C (2011) Electron-electron correlation in strong laser fields in Journal of Modern Optics
De Morisson Faria C (2009) Laser-induced nonsequential double ionization in diatomic molecules: one- and two-centre rescattering scenarios in Journal of Physics B: Atomic, Molecular and Optical Physics
De Morisson Faria C (2007) Speed-up and slow-down collisions in laser-induced non-sequential multiple ionization in Journal of Modern Optics
De Morisson Faria C (2011) Introduction to Special Issue on Advances in Strong Field and Attosecond Physics in Journal of Modern Optics

Related Projects

Project Reference Relationship Related To Start End Award Value
EP/D07309X/1 01/10/2006 31/03/2007 £340,656
EP/D07309X/2 Transfer EP/D07309X/1 01/04/2007 30/09/2011 £310,014
Description The theoretical description of matter in intense laser fields poses a great challenge due to many reasons. First, since the external fields are comparable to the atomic binding forces, perturbation theory with regard to the field is unapplicable and one must model the highly irregular
behavior of a multi-electron system under the sinultaneous influence of the laser field and the binding potential. Second, the time scales involved,
of hundreds of attoseconds(10-18s), are among the shortest in nature. In this extreme, far from equilibrium regime, matter behaves in a highly
transient way and many standard physical processes may not have time to build up. In particular electron-electron correlation, excitation, and electron migration are open questions, whose answers may unveil an unbelievably rich dynamics. In particular molecules have attracted a great deal of attention.

Since the mid-2000s, the main analytic theory employed in this research area, the strong-field approximation (SFA), started to exhibit a series of contradictions. This was strong evidence that it breaks down for molecular systems in strong fields. Concrete examples are different interference patterns in high-order harmonic or photoelectron spectra obtained for different SFA formulations. In this project, this theory has been substantially modified in order to account for the influence of multiple molecular orbitals, multielectron effects and excitation. A detailed assessment of its shortcomings have been made, and specific conditions have been identified for which interference patterns can be modeled unambiguously. A wide range of strong-field phenomena have been studied, such as high-order harmonic
generation (HHG) and laser-induced nonsequential double ionization (NSDI). Multielectron models have been constructed for HHG in molecules, and excitation and electron-electron correlation have been incorporated in NSDI, with applications in atoms and molecules. This led to the most comprehensive analytic studies so far of excitation in this phenomenon. Our theoretical predictions have been confirmed by very recent experiments in the Max Planck Institute, Munich (Berger et al, Nature Physics, submitted, 2011).

We have also undertaken most of the necessary steps in order to incorporate the Coulomb binding potential, which is left out in the SFA, in analytic strong-field models. Due to its extrme difficulty, this has only been achieved by two research groupos worldwide, and we are very close to achieving this goal in a more rigorous way than performed so far. Furthermore, we brought a method used in quantum chemistry, the coupled coherent state (CCS) method, to strong-field physics, and,
together with leading CCS experts, were the first group ever to compute high-harmonic spectra employing the CCS. This has been acheived for one-electron atoms
and, in the next year, will be extended to multielectron atoms and molecules. Hence, the work performed in the course of this fellowship broke new ground
in many ways and set out some of the foundations for modeling the attosecond dynamics of multielectron systems in intense fields in the future. We intend to
address this issue in a subsequent project.
Exploitation Route Current beneficiaries:

(a) Strong-field and attosecond scientists: Within strong-field and attosecond physics, our findings, especially with regard to excitation pathways in correlated two-electron processes, have called the attention of several groups worldwide, both experimental and theoretical. Concrete examples are Prof Jing Cheng's group (Beijing University, China), Prof. Matthias Kling's group (LMU Munich, Germany) and Prof. Xiaojun Liu (Chinese Academy of Sciences, Wuhan, China) In particular Prof. Cheng took our model further in order to investigate the quantum interference of different excitation channels in NSDI, and has proposed an explanation of how this interference may lead to features that observed experimentally. These findings have also strengthened the collaboration with Prof Liu, which led to a joint review article in 2011, several academic visits and exchange, such as a self-funded postdoc from Wuhan (Dr Xuanyang Lai) in 2012.

(b) Scientists in other areas: This project has led to interdisciplinary work involving quantum chemists (Prof Dmitry Shalashilin, University of Leeds) and mathematical physicists (Prof Henning Schomerus, Lancaster University). In the framework of this collaboration, we are currently exploring novel approaches from these areas for multielectron systems. This may have a huge impact in several areas, such as biology, chemistry or material science. First results already exist as publications, and a recently completed PhD project in my group (Jie Wu, PhD thesis, UCL, September 2014, ``Novel Orbit-based Approaches for Matter in Strong Laser Fields'').

(c) beneficiaries beyond academia: Through this project, there has been substantial training of personnel in numerical methods, which are applicable to other areas such as industry and the financial sector. Some of the students working on this project (Tom Nygren, Bradley Augstein) currently work in these areas, and have profited from the techniques used therein, such as root-finding methods, the numerical solution of partial differential equations, sampling methods, and random-number generators.

Potential beneficiaries:
I expect that the results obtained in this project will continue to benefit scientists of several areas (strong-field physics, attoscience, mathematical physics, quantum chemistry, to cite a few) and the wider community in general. The foundations laid at the time of the project, as well as the software developed during this time, has been quite useful in forming personnel who are currently active both in academia and industry. In this time I have supervised around 10 students in total, at the PhD, MSc, and BSc level, using such techniques.
Sectors Digital/Communication/Information Technologies (including Software),Education,Financial Services, and Management Consultancy
Description PLEASE NOTE THAT THIS AWARD HAS BEEN TRANSFERRED TO UCL IN 2007. HENCE, THIS IS DUPLICATE INFORMATION, Our research has had impact both within and beyond academia. (a) Scientific impact: Within academia, it has called the attention of several groups across the globe, and has attracted considerable recognition in form of citations, invited talks and collaborations. In particular the work on the analytical treatment of excitation and electron-electron correlation, due to its ground-breaking nature, has attracted the attention of several research groups, such as Prof Matthias Kling's group (LMU Munich, Germany), Prof Jing Cheng's group (Beijing University, China) and Prof. Xiaojun Liu (Chinese Academy of Sciences, Wuhan), with whom collaborations have been either strengthened or established. In particular Prof Cheng has applied our theory to the interference of different ionization channels in RESI, and has provided a novel explanation for RESI experiments which rules out classical interpretations. These findings have been published in several high-impact journals. Apart from that, our development of novel strong-field approaches have led to an interdisciplinary collaboration with quantum chemists (Dmitry Shalashilin, University of Leeds) and Mathematical Physicists (Henning Schomerus, Lancaster University). This is currently contributing to the establishment of an interdisciplinary research team in the UK, in which these problems are being addressed. (b) Training of Personnel and Impact Beyond Academia: This project has led to the training of several students at all levels (PhD, MSc, BSc), which have won several prizes (Tahir Shaaran, UCL Carey Foster Prize 2010 and EPSRC Doctoral Training Prize 2010; Brad Augstein, UCL Carey Foster Prize 2011 and EPSRC Doctoral Training Prize 2011), and wither continued working in academia (Tahir Shaaran has been a very successful researcher at the ICFO Barcelona, CEA Saclay and MPI Heidelberg) or the Financial sector (Tom Nygren and Brad Augstein). In particular the skills and techniques learned by these students during the project, which involved root finding, the numerical solution of partial differential equations, sampling methods, and random numbers, turned out to be highly transferable to the financial sector. The software developed by us helped set the foundations for further, ongoing work in my group.
First Year Of Impact 2011
Sector Creative Economy,Digital/Communication/Information Technologies (including Software),Education,Financial Services, and Management Consultancy,Other
Impact Types Cultural,Societal