L. Plaja

Attosecond extreme ultraviolet vortices from high-order harmonic generation.

We present a theoretical study of high-order harmonic generation (HHG) and propagation driven by an infrared field carrying orbital angular momentum (OAM). Our calculations unveil the following relevant phenomena: extreme-ultraviolet harmonic vortices are generated and survive to the propagation effects, vortices transport high-OAM multiples of the corresponding OAM of the driving field and, finally, the different harmonic vortices are emitted with similar divergence. We also show the possibility of combining OAM and HHG phase locking to produce attosecond pulses with helical pulse structure.

Harmonic generation beyond the Strong-Field Approximation: the physics behind the short-wave-infrared scaling laws

The physics of laser-mater interactions beyond the perturbative limit configures the field of extreme non-linear optics. Although most experiments have been done in the near infrared ( lambda <or= 1 microm), the situation is changing nowadays with the development of sources at longer wavelengths (<5 microm), opening new perspectives in the synthesis of shorter XUV attosecond pulses and higher frequencies. The theory of intense-field interactions is based either on the exact numerical integration of the time-dependent Schrödinger equation or in the development of models, mostly based on the strong-field approximation. Recent studies in the short-wave infrared show a divergence between the predictions of these models and the exact results. In this paper we will show that this discrepancy reveals the incompleteness of our present understanding of high-order harmonic generation. We discuss the physical grounds, provide a theoretical framework beyond the standard approximations and develop a compact approach that accounts for the correct scaling of the harmonic yield.

Observation of spontaneous self-channeling of light in air below the collapse threshold

We report the observation of the self-guided propagation of 120 fs, 0.56 mJ infrared radiation in air for distances greater than 1 m. In contrast with the known case of filamentation, in the present experiment the laser power is lower than the collapse threshold. Therefore the counterbalance between Kerr self-focusing and ionization induced defocusing as the stabilizing mechanism is ruled out. Instead, we find evidence of a process in which the transversal beam distribution reshapes into a form similar to a Townes soliton, with the particularity of a very high stability. We include numerical support for this conclusion.

Quantum-path signatures in attosecond helical beams driven by optical vortices

High-order harmonic generation (HHG) driven by beams carrying orbital angular momentum has been recently demonstrated as a unique process to generate spatio-temporal coherent extreme ultraviolet (XUV)/x-ray radiation with attosecond helical structure. We explore the details of the mapping of the driving vortex to its harmonic spectrum. In particular we show that the geometry of the harmonic vortices is complex, arising from the superposition of the contribution from the short and long quantum paths responsible of HHG. Transversal phase-matching and quantum path interferences provide an explanation of the dramatic changes in the XUV vortex structure generated at different relative positions of the target respect to the laser beam focus. Finally, we show how to take advantage of transversal phase-matching to select helical attosecond beams generated from short or long quantum paths, exhibiting positive or negative temporal chirp respectively.

High-Order Harmonic Generation by Electron-Proton Recombination

Classical models have been proven to successfully explain some general features of the harmonic generation in the tunnelling limit. In particular, it can be shown that the energies of the produced harmonics are bounded by an upper limit given by IP + 3.17UP. In this paper we show that this limit can be exceeded by considering the special case in which the harmonics are generated when the ionized electrons are captured not by the parent ion but by some neighbouring ion located at a given distance.

High-order harmonic generation after photodissociation

High-order harmonic generation spectra obtained from a photodissociating molecule extend to photon energies up to eight times the ponderomotive energy when the electron is ionized from one of the fragments and recombines with the other. A detailed simulation is presented for the hydrogen molecular ion that describes the photodissociation and high-order harmonic generation using two laser pulses with a convenient delay.

Extension of the cut-off in high-harmonic generation using two delayed pulses of the same colour

We demonstrate a novel technique for an extension of the cut-off frequency in high-harmonic generation using a shaped field, consisting of two delayed pulses, identical in all parameters except a carrier envelope phase shift of π. We obtain high harmonics up to an energy of 5.5Up + Ip, surpassing the conventional limit of 3.17Up + Ip. For high intensities approaching the threshold for barrier suppression ionization, beyond which no harmonics are generated, this mechanism acquires practical importance as a means to overcome the limit imposed by saturation. We analyse the underlying mechanism using both classical and quantum approaches, and highlight the role of the shaped field in modifying the electron trajectories in the continuum to enhance the collection of kinetic energy from the driving field. Of greatest importance are the trajectories describing electrons ionized across the first peak of the laser field which receive the enhanced kinetic energies responsible for the cut-off extension above the conventional limit. Interestingly, the most energetic trajectories do not contribute to the harmonic cut-off in the single pulse case. We conclude with a discussion of the sub-cycle structure of the emitted harmonic radiation and possible applications of the technique.

Group velocity matching in high-order harmonic generation driven by mid-infrared lasers

Author(s): Hernandez-Garcia, C; Popmintchev, T; Murnane, MM; Kapteyn, HC; Plaja, L; Becker, A; Jaron-Becker, A | Abstract:

A quantitative S-Matrix approach to high-order harmonic generation from multiphoton to tunneling regimes.

We present a S-matrix description of the process of high order harmonic generation during the interaction of atoms with strong electromagnetic fields. In contrast with the state-of-the-art approaches, our model does not employ the stationary phase approximation and accounts as well for the continuum-continuum transitions. Therefore we are able to reproduce quantitatively the higher frequency part of the spectrum for arbitrary pulse shapes, and for intensities corresponding to multiphoton, tunnel and soft over-the barrier ionization regimes. In addition this model can be implemented very efficiently in a Personal Computer to calculate the harmonic generation for the atom interacting with an eight-cycle pulse at lambda =800 nm in, roughly, ten minutes (a reduction of two orders of magnitude from the typical time requirements of the exact integration).

Off-axis compensation of attosecond pulse chirp

By performing calculations on the generation of attosecond pulses by intense laser fields, we demonstrate that the pulse temporal width is sensitive to the angle of emission. We show that this effect results from the interference of the short and long electronic trajectory contributions, responsible for high-order harmonic generation. In particular, we find that the shortest pulses are emitted off-axis, which are substantially shorter than those emitted on-axis, where single trajectory contributions dominate.