Light at the end of the tunnel: Two- and three-step models in intense-field laser-atom physics

Recent experimental progress has led to an improved understanding of the role of quasi-free electron propagation in intense-field laser-atom physics. A brief review is given of crucial experiments, such as high-harmonic generation, high-energy effects in above-threshold ionization (plateau and anomalous angular distributions) and nonsequential double ionization, and their interpretation in terms of two- and three-step models is discussed. These models consider the initial 'creation' of the electron in the continuum, the subsequent propagation in the field, and the eventual return of the electron to the ionic core as more or less separate steps. The propagation of the electron in the laser field is treated in terms of the path-integral representation of the propagator. This fully quantum mechanical formulation pinpoints the decisive role of particular classical closed orbits, those that start from and return to the centre of the atomic binding potential, in the above-mentioned phenomena and allows for setting up an exact framework for the formal derivation of the multi-step models. High-harmonic generation and the rescattering effects in above-threshold ionization are given special attention, along with the significance of the polarization of the irradiating laser field.