Two-electron ionization in strong laser fields below intensity threshold: signatures of attosecond timing in correlated spectra

We develop an analytical model of correlated two-electron ionization in strong infrared laser fields. The model includes all relevant interactions between the electrons, the laser field, and the ionic core nonperturbatively. We focus on the deeply quantum regime, where the energy of the active electron driven by the laser field is insufficient to collisionally ionize the parent ion, and the assistance of the laser field is required to create a doubly charged ion. In this regime, the electron-electron and the electron-ion interactions leave distinct footprints in the correlated two-electron spectra, recording the mutual dynamics of the escaping electrons.Comment: 25 pages and 4 figure

A practical scheme for generating isolated elliptically polarized attosecond pulses using bi-chromatic counter rotating circularly polarized laser fields

Spectra of circularly polarized harmonics is calculated by numerically solving the Time-Dependent Schr\"{o}dinger Equation for a 2D model of Ne atom using circularly polarized fundamental with counter-rotating second harmonic laser fields. We demonstrate strong asymmetry between left- and right- circularly polarized harmonics when a ground state with p-type symmetry is used. It arises due to the circular polarization of individual attosecond pulses in the generated pulse train. Reducing the length of the counter-rotating drivers and introducing a small time-shift between them allows to generate a single elliptically polarized attosecond pulse.Comment: 12 pages, 3 figure

Strong-field assisted extreme-ultraviolet lasing in atoms and molecules

Using ab-initio simulations, we demonstrate amplification of extreme-ultraviolet (XUV) radiation during transient absorption in a high-harmonic generation type process using the example of the hydrogen atom. The strong IR driving field rapidly depletes the initial ground state while populating excited electronic states through frustrated tunnelling, thereby creating a population inversion. Concomitant XUV lasing is demonstrated by explicit inclusion of the XUV seed in our simulations, allowing a thorough analysis in terms of this transient absorption setup. Possibilities for increasing this gain, e.g. through preexcitation of excited states, change of the atomic gain medium or through multi-center effects in molecules, are demonstrated. Our findings should lead to a reinterpretation of recent experiments

Strong-field assisted extreme-ultraviolet lasing in atoms and molecules

Using ab-initio simulations, we demonstrate amplification of extreme-ultraviolet (XUV) radiation during transient absorption in a high-harmonic generation type process using the example of the hydrogen atom. The strong IR driving field rapidly depletes the initial ground state while populating excited electronic states through frustrated tunnelling, thereby creating a population inversion. Concomitant XUV lasing is demonstrated by explicit inclusion of the XUV seed in our simulations, allowing a thorough analysis in terms of this transient absorption setup. Possibilities for increasing this gain, e.g. through preexcitation of excited states, change of the atomic gain medium or through multi-center effects in molecules, are demonstrated. Our findings should lead to a reinterpretation of recent experiments.Peer Reviewe

Physics of correlated double ionization of atoms in intense laser fields : Quasistatic tunneling limit

We revisit the recollision picture of correlated multiphoton double ionization of atoms in strong laser fields and develop consistent semiclassical model in the tunneling limit. We illustrate the model by applying it to helium and obtain quantitative agreement with recent experiments [B. Walker, E. Mevel, Baorui Yang, P. Breger, J. P. Chambaret, A. Antonetti, L. F. DiMauro, and P. Agostini, Phys. Rev. A 48, R894 (1993); B. Walker, B. Sheehy, L. F. DiMauro, P. Agostini, K. J. Schafer, and K. C. Kulander, Phys. Rev. Lett. 73, 1227 (1994)]. Developing the model, we address several problems of general interest, such as the reduction of intense field-assisted electron-ion collision to the field-free one and the total-cross-sections that include all inelastic channels. We describe a set of important physical effects responsible for the surprisingly high yield of doubly charged ions of noble gas atoms. All effects originate from the key role of the Coulomb potential and its interplay with the laser field. In addition to the Coulomb focusing of the oscillating trajectories onto the parent ion, other effects include transient trapping of electrons after tunneling in the vicinity of the parent ion, the creation of high-velocity electrons at all phases of the laser field, and the dominant role of collisional excitation of the parent ion followed by laser-assisted ionization.Peer reviewed: YesNRC publication: Ye

Control of Spin Polarization through Recollisions

Using only linearly polarized light, we study the possibility of generating spin-polarized photoelectrons from xenon atoms. No net spin polarization is possible, since the xenon ground state is spin-less, but when the photoelectron are measured in coincidence with the residual ion, spin polarization emerges. Furthermore, we show that ultrafast dynamics of the recolliding photoelectrons contribute to an apparent flipping of the spin of the photoelectron, a process that has been completely neglected so far in all analyses of recollision-based processes. We link this phenomenon to the ``spin--orbit clock'' of the remaining ion. These effects arise already in dipole approximation