Camilo Ruiz

Coherent control of electron wave packets in dissociating H+2

Coherent control of electron localization in the dissociation of a hydrogen molecular ion exposed to an attosecond pulse train and a time-delayed near-infrared laser pulse are studied by solving numerically the time-dependent Schrodinger equation. The attosecond pulses in the train generate a train of electron wave packets in the dissociating molecular ion, which are steered by the near-infrared laser field between the two nuclei. Our results show that a large asymmetry in the total electron localization can be achieved if the attosecond pulses are separated by a full cycle of the near-infrared pulse, while the asymmetry and the degree of control are much smaller for a pulse separation of half a cycle. The analysis of results reveals an efficient control mechanism on a timescale of few femtoseconds via the time-delay and the carrier-to-envelope phase of the near-infrared pulse.

Single attosecond pulse generation with intense mid-infrared elliptically polarized laser pulses.

We have studied theoretically high-harmonic-order and single attosecond pulse generation with elliptically polarized laser pulses at wavelengths ranging from the visible to the mid-infrared. Results of ab initio simulations of the time-dependent Schrödinger equation show that the ellipticity dependence of the high-harmonic signal intensifies with increasing wavelength of the driving pulse and saturates in the mid-infrared. The isolation of single attosecond pulses using the polarization gating method in the mid-infrared is due to an effective suppression of side pulses as compared with an operation at Ti:sapphire wavelengths.

Double ionization of helium by intense near-infrared and VUV laser pulses

We investigate the dynamics of double ionization of He atom by an intense near-infrared and an attosecond vacuum ultraviolet (VUV) laser pulse, which are either applied in sequence or at the same time. To this end we solve the time-dependent Schroedinger equation for a two-electron model atom interacting with the two fields. We compare the double-ionization yields and probability density distributions, with and without the application of the attosecond pulse, for the different scenarios. The results of our numerical simulations show how ionization or excitation of the neutral atom by a preceding or simultaneously applied VUV pulse affects the double-ionization dynamics driven by the near-infrared laser pulse. The findings provide insights regarding the question if attosecond technology can be used to temporally resolve mechanisms of correlated emission of electrons in a strong laser field.

Attosecond probing of instantaneous ac Stark shifts in helium atoms

Based on the numerical solutions of the time-dependent Schr?dinger equation within the single-active-electron approximation, we propose a method for observing instantaneous atomic level shifts in an oscillating strong infrared (IR) field with sub-IR-cycle time resolution, by using a single tunable attosecond (SA) pulse to probe excited states of the perturbed atom. The ionization probability in the combined fields depends on both the frequency of the attosecond pulse and the time delay between both pulses, since the IR field periodically shifts SA-pulse-excited energy levels into and out of resonance.

Attosecond light-pulse-induced photoassociation

We explore stimulated photo-association in the context of attosecond pump-probe schemes of atomic matter. An attosecond pulse -- the probe -- is used to induce photo-association of an electronic wave packet which had been created before, typically with an attosecond pump pulse at an atomic center different from the one of photo-association. We will show that the electron absorption is maximal for a certain delay between the pulses. Two ways of enhancing and controlling stimulated photo-association are proposed, namely using an additional infrared pulse to steer the electronic wave packet and using a train of attosecond pulses instead of a single pair. A direct application of ultrafast stimulated photo-association is the measurement of atomic distances.

Selection rules in the few-photon double ionization of the helium atom

We analyse selection rules for the emission of two electrons from the helium atom following the absorption of a few photons in an intense laser field. The rules arise, as generalization of the well-studied one-photon case, due to the symmetries of the accessible final states in the two-electron continuum. We show, in particular, that an increase in the number of absorbed photons leads to alternating suppression and non-suppression of the back-to-back emission of the two electrons. Results of numerical simulations using a model of the helium atom are in agreement with the theoretical predictions.

Nonlinear effects in the propagation of short laser pulses in air

Non linear propagation of ultra short pulses in air is studied. By preparing an initial field distribution by an amplitude mask we can obtain a Townes soliton[1] (self similar channel of coherent radiation) in air. Experimental observation can be described accurately by the numerical integration of the Non Linear Schroedinger Equation (NLSE) and allow us to explain the origin of the remarkable stability of this soliton as a balance between diffraction and Kerr effect. We further explore on the role of coherence by revisiting the two slit Youngs experiment but now in the non linear regime.

Single and Double Ionization of the Hydrogen Molecule in an Intense Few-Cycle Laser Pulse

In this paper, we present ab initio two-electron model calculations of laser-induced single and double ionization of the hydrogen molecule in a linearly polarized laser field with static nuclei located along the polarization axis. Within the model, the center-of-mass motion of the two electrons is restricted along the polarization axis of the field, while the relative electron motion is unrestricted. The results of numerical simulations allow us to identify and characterize the mechanisms leading to single and double ionization in an intense few-cycle laser pulse. The role of the rescattering mechanism on the ionization processes is analyzed in particular.

Observation of channels of radiation during the propagation in air of short pulses below the collapse threshold

We report the observation of self-guided propagation of 120 fs, 0.56 mJ infrared pulse in air for distances greater than a meter (more than thirty Rayleigh Lengths). The numerical simulations demonstrates the this localized structure corresponds to a Townes soliton, specially stable under these conditions.

Observation and Control of Electron Dynamics in Molecules

The technological development of ultrashort laser pulses makes it possible to monitor and control the dynamics of the electrons in atoms and molecules. In this Chapter we first review recent experimental and theoretical progress towards tracking and understanding of electron motion in nature’s simplest molecule, the hydrogen molecular ion, on its natural time scale. A complex and counterintuitive dynamics appears due to a strong coupling between different electronic states and of the electron with the external field. Different approaches for the observation of these single-active electron effects in the hydrogen molecular ion as well as of electron rearrangement in the valence shell of more complex molecules are presented. Based on these new insights we then turn to a discussion of recently proposed strategies to control electron localization in molecules with carrier-envelope phase locked pulses, attosecond pump-probe set-ups as well as circularly polarized laser pulses. In particular, results of experiments, in which the asymmetry of localization probabilities at the protons in the hydrogen molecular ion is observed, are complemented with theoretical results and analysis from ab-initio numerical simulations.