Announcing the JMO Series on Attosecond and Strong Field Science

Starting with this issue, Journal of Modern Optics will be highlighting rapidly developing research areas in the general field of laser–matter interaction. To this end, the Journal is introducing a range of Series, including JMO Series on Attosecond and Strong Field Science .

Papers submitted for publication within the Series will be given priority handling during the refereeing process, with two months’ free online access offered to all Series papers. Regular editorial articles will review the most exciting recent results and publications within the area, including online highlights of the selected articles, enhancing their visibility in the scientific community.

This is the first such editorial, which gives me an opportunity to reminisce about ‘the good old times’ and present an admittedly biased, personal perspective of a theorist.

Research often develops in cycles. The study of intense, nonlinear light–atom and light–molecule interaction, which has led to the birth of attosecond technology and attosecond science, has its roots in the study of multi-photon ionisation, which began almost 50 years ago. At times, for example in the late 1970s, it looked like all essential puzzles in multi-photon ionisation were solved and only minor details remained. I am old enough to remember such sentiments. As often happens in research, that's exactly when surprises begin to pile up. Multi-photon ionisation offers yet another example of a research area that has almost exploded just after it was beginning to look dull.

The first sign of interesting things to come was the observation of above-threshold ionisation (ATI) by P. Agostini et al. in 1979 [1]. In the following decade, several highly nonlinear, highly non-perturbative phenomena were observed. These included surprisingly long (at least that is how they looked to us in the 1980s) energy plateaus in the ATI spectra and the formation of highly charged ions with surprisingly high probabilities. Various theories of ATI and of multiple ionisation flourished. The theoretical papers of L.V. Keldysh [2], V.S. Popov and coworkers [3], F. Faisal [4] and H. Reiss [5] were re-discovered and appreciated. Interestingly, these papers contained theoretical predictions of ATI as early as the mid-1960s. Yet, once again, at the end of the 1980s it looked like most things were reasonably well understood. The only disconcerting thing was the absence of a unifying, simple physical picture (not to mention the increasing complexity of theoretical modelling).

Just when things were again beginning to look mostly dull and tedious, several surprises came along. One of them was the observation of extremely efficient high harmonic generation (HHG) by atoms exposed to intense infrared light, with probably the most spectacular experimental results reported in [6,7]. Important theoretical analysis of HHG, including [8–10], preceded these experiments. In particular, the results of [9] suggested a remarkably simple yet general empirical law for the spectrum of high harmonics, indicating that a clear physical picture was just around the corner. That physical picture was the famous three-step model [11,12]. Its arrival in 1993 heralded the first steps of what has now become attosecond science.

The three-step model did not come out of the blue. In retrospect, we can see now that it had much in common with the ‘atomic antenna’ concept developed by M. Kuchiev in [13], the ideas of F. Brunel [10,14], and the earlier two-step models [15,16]. Nevertheless, its remarkable simplicity and elegance, combined with the ability to explain in a unified way the seemingly so very different physics underlying ATI, HHG and highly efficient double ionisation [17] (see [18]), had the effect ‘of the light at the end of a tunnel’ [19].

A clear physical picture is the fuel of progress, at least in physics. The three-step model has laid the foundation for the tremendous growth of intense-field and attosecond science over the past decade. Attosecond science involves the study, control and imaging of processes that occur on the attosecond (1 as = 10⁻¹⁸ s) to a few femtosecond (1 fs = 10⁻¹⁵ s) time-scale, the natural time-scale of the electronic motion in atoms and molecules. It is also the natural time-scale of the electronic response to the strong oscillating fields of modern laser pulses. Indeed, in a now routine femtosecond laser pulse with the intensity I ∼10¹⁴W/cm², the instantaneous electric field changes from zero to a few volts per angstrom in a fraction of a femtosecond, driving the attosecond electronic response.

The advent of the new generation light sources – the free electron lasers (FELs) – capable of generating ultra-intense fields (I > 10¹⁶W/cm²) in the VUV, XUV and X-ray range (DESY, SLAC LCLS) and inducing highly non-perturbative, attosecond response of deeply bound electrons in the K- and L-shells is opening new perspectives for attosecond science, including nonlinear attosecond processes in the XUV/X-ray domain.

This JMO Series on Attosecond and Strong Field Science will focus on the research area that is truly opening new horizons in the rich multi-disciplinary field at the intersection of atomic, molecular and optical physics, quantum and physical chemistry, surface science, laser technology and life sciences.

This series has already been very active in the Journal. In addition to the Special Issue, Advances in Strong Field and Attosecond Physics , that follows this editorial, the reader should look at the recent review of attosecond dynamic imaging [20], which covers an exciting area of imaging molecular structures and electronic dynamics with the combination of sub-femtosecond temporal and angstrom-scale spatial resolution.

Improving the generation of attosecond pulses, finding the new schemes of generating single attosecond pulses using relatively long IR driving pulses remains an intriguing and important problem, both for theory and experiment. Papers [21–26] address this problem, including the very rich with opportunities topic of using IR and visible driving fields with several phase-locked light frequencies, and/or changing the fundamental wavelength from the usual 800 nm towards longer wavelengths. With remarkable progress in laser technology, trying such schemes is no longer a wild dream of a theorist. From the theoretical perspective, the problem is not only exciting but also challenging, as it involves the combination of calculating single-molecule or single-atom response and solving the Maxwell equations for the propagation of the generated light. Analytical analysis that could simplify either of these two steps is of huge value, and [22,23] make good steps in this direction.

One of the most vibrant areas of research in attosecond science is HHG, including HHG spectroscopy. In this approach, amplitudes, polarisations and phases of the harmonic light are analysed to gain insight into the multi-electron dynamics that underlies HHG in molecules. However, most molecules have relatively low ionisation potentials, and the generation of sufficiently broad harmonic spectra requires application of IR radiation with wavelengths longer than 800 nm of our favourite workhorse – the Ti:sapphire laser. Experimental results of [26], which use 1.5 µm light, are also very important from this perspective.

Strong-field dynamics in general, including the generation of attosecond pulses, is almost always linked to ionisation. Developing efficient theoretical models of strong-field ionisation is both a long-standing and hot problem. In addition to the paper by Long et al. [27] in the Special Issue that follows this editorial, a recent paper [28] has discussed an interesting possibility to extend the analytical tunnelling theory developed for static electric fields to oscillating electric fields. The key idea is to approximate such oscillating fields as piece-wise constant. The paper by M. Ciappina and W. Cravero [29] discusses the possibility of improving the usual strong-field approximation with the help of the Coulomb–Volkov anzats, and applying it to diatomic molecules.

One of the hottest topics, where the surprises are expected in droves, is the study of nonlinear attosecond processes in the XUV/X-ray domain. Low absorption cross-sections in this frequency range are an obstacle for experiments, but the new generation light sources make observation of such processes feasible. A recent topical review by N. Berrah et al. [30] gives a perspective on the exciting results and the experiments underway at the SCSS Test Accelerator (Japan), FLASH (Hamburg) and LCLS (Stanford). In the same general direction, a numerical experiment described in [31] shows that even in a system as simple as an hydrogen atom, pump-probe experiments in the XUV domain can lead to interesting conclusions about the competition of direct and sequential channels in the two-photon absorption, and a very rich dynamics of Rydberg wavepackets.

The attosecond domain is the natural playground for electron–electron correlations. The dynamics of electron–electron correlation and its role in multiple ionisation of atoms and molecules in strong laser fields are reviewed by Faria and Liu in the Special Issue that follows [32], whereas [33] discusses the kinematic constraints for the dynamically very rich ‘recollision–excitation–tunnelling’ mechanism operating in correlated double ionisation driven by intense IR pulses.

In conclusion, I expect that attosecond and strong field science will continue to offer us new challenges and new puzzles. It does not look like this area has any risk of becoming boring and dull in a foreseeable future. As experimental technology and theoretical machinery develop, we will continue to discover new horizons in this rich multi-disciplinary field. The possibilities to shape optical light fields almost at will, to generate and control attosecond pulses locked to a desired phase of the oscillations of an optical light field open exciting possibilities to control the motion of electrons in atoms and molecules, to probe fundamental electronic processes on surfaces and in bulk, to visualise the motion of the electrons and holes, image core rearrangement upon the electron escape via one-photon ionisation or via tunnelling, and many others. The JMO Series on Attosecond and Strong Field Science will cover these exciting developments.

To submit your research to JMO Series on Attosecond and Strong Field Science , please go to http://mc.manuscriptcentral.com/tmop. General Instructions for Authors can be found at www.tandfonline.com/TMOP.