Doppler effect as a tool for ultrashort electric field reconstruction.

We present a new, to the best of our knowledge, variant of the spectral-shearing interferometry method for characterizing ultrashort laser pulses. This original approach, called Doppler effect e-field replication (DEER), exploits the rotational Doppler effect for producing frequency shear and provides spectral shearing in the absence of frequency conversion, enabling operation in the ultraviolet spectral range. Evaluation of the DEER-spectral phase interferometry for direct electric field reconstruction setup reveals a phase reconstruction of great reliability.

Observing collisions beyond the secular approximation limit.

Quantum coherence plays an essential role in diverse natural phenomena and technological applications. The unavoidable coupling of the quantum system to an uncontrolled environment incurs dissipation that is often described using the secular approximation. Here we probe the limit of this approximation in the rotational relaxation of molecules due to thermal collisions by using the laser-kicked molecular rotor as a model system.

Rotational Echoes as a Tool for Investigating Ultrafast Collisional Dynamics of Molecules.

We show that recently discovered rotational echoes of molecules provide an efficient tool for studying collisional molecular dynamics in high-pressure gases. Our study demonstrates that rotational echoes enable the observation of extremely fast collisional dissipation, at timescales of the order of a few picoseconds, and possibly shorter. The decay of the rotational alignment echoes in CO_{2} gas and CO_{2}-He mixture up to 50 bar was studied experimentally, delivering collision rates that are in good agreement with the theoretical expectations.

A generalized vibronic-coupling Hamiltonian for molecules without symmetry: Application to the photoisomerization of benzopyran.

We present a model for the lowest two potential energy surfaces (PESs) that describe the photoinduced ring-opening reaction of benzopyran taken as a model compound to study the photochromic ring-opening reaction of indolinobenzospiropyran and its evolution toward its open-chain analog. The PESs are expressed in terms of three effective rectilinear coordinates. One corresponds to the direction between the equilibrium geometry in the electronic ground state, referred to as the Franck-Condon geometry, and the minimum of conical intersection (CI), while the other two span the two-dimensional branching space at the CI.

Polarization-based tachometer for measuring spinning rotors.

We report on the polarization analysis of shortpulse ultraviolet radiation produced by third-harmonic generation in a gas of coherently spinning molecules. A pulse of twisted linear polarization imprints a unidirectional rotational motion to the molecules leading to an orientation of their rotational angular momenta. A second pulse, time-delayed with respect to the first one, circularly polarized in the plane of rotation of the molecules, acts as a driving field for third-harmonic generation.

Collisional dissipation of the laser-induced alignment of ethane gas: Energy corrected sudden quantum model.

We present the first quantum mechanical model of the collisional dissipation of the alignment of a gas of symmetric-top molecules (ethane) impulsively induced by a linearly polarized non-resonant laser field. The approach is based on use of the Bloch model and of the Markov and secular approximations in which the effects of collisions are taken into account through the state-to-state rates associated with exchanges among the various rotational states. These rates are constructed using the Energy Corrected Sudden (ECS) approximation with (a few) input parameters obtained independently from fits of the pressure-broadening coefficients of ethane absorption lines.

Collisional dissipation of the laser-induced alignment of ethane gas: A requantized classical model.

We present the first theoretical study of collisional dissipation of the alignment of a symmetric-top molecule (ethane gas) impulsively induced by a linearly polarized non-resonant laser field. For this, Classical Molecular Dynamics Simulations (CMDSs) are carried out for an ensemble of CH molecules based on knowledge of the laser-pulse characteristics and on an input intermolecular potential. These provide, for a given gas pressure and initial temperature, the orientations of all molecules at all times from which the alignment factor is directly obtained.

Dissipation dynamics of field-free molecular alignment for symmetric-top molecules: Ethane (CH).

The field-free molecular alignment of symmetric-top molecules, ethane, induced by intense non-resonant linearly polarized femtosecond laser pulses is investigated experimentally in the presence of collisional relaxation. The dissipation dynamics of field-free molecular alignment are measured by the balanced detection of ultrafast molecular birefringence of ethane gas samples at high pressures. By separating the molecular alignment into the permanent alignment and the transient alignment, the decay time-constants of both components are quantified at the same pressure.

Shaping of ultraviolet femtosecond laser pulses by Fourier domain harmonic generation.

We present a method to finely tailor ultraviolet femtosecond laser pulses using a pulse shaper with ability in the infrared/visible spectral range. We have developed to that end a frequency doubling module in which the up-conversion mechanism is carried out in the Fourier plane of a 4 f -line. The pulse shaper is used to imprint a spectral phase and/or amplitude onto the fundamental pulse.

Orientation and alignment echoes.

We present one of the simplest classical systems featuring the echo phenomenon-a collection of randomly oriented free rotors with dispersed rotational velocities. Following excitation by a pair of time-delayed impulsive kicks, the mean orientation or alignment of the ensemble exhibits multiple echoes and fractional echoes. We elucidate the mechanism of the echo formation by the kick-induced filamentation of phase space, and provide the first experimental demonstration of classical alignment echoes in a thermal gas of CO_{2} molecules excited by a pair of femtosecond laser pulses.

Polarization shaping for unidirectional rotational motion of molecules.

Control of the orientation of the angular momentum of linear molecules is demonstrated by means of laser polarization shaping. For this purpose, we combine two orthogonally polarized and partially time-overlapped femtosecond laser pulses so as to produce a spinning linear polarization which in turn induces unidirectional rotation of N2 molecules. The evolution of the rotational response is probed by a third laser beam that can be either linearly or circularly polarized.

Dissipation of alignment in CO2 gas: A comparison between ab initio predictions and experiments.

We present comparisons between measurements and ab initio calculations of the dissipation of the nonadiabatic laser-induced alignment in pure CO2 and CO2-He gas mixtures. The experiments were made for pressures between 2 and 20 bars at 295 K by using short non-resonant linearly polarized laser pulses for alignment and probe. The calculations are carried, free of any adjusted parameter, using refined intermolecular potentials and a requantized Classical Molecular Dynamics Simulations approach presented previously but not yet confronted to experiments.

Molecular dynamics simulations for CO2 spectra. IV. Collisional line-mixing in infrared and Raman bands.

Ab initio calculations of the shapes of pure CO2 infrared and Raman bands under (pressure) conditions for which line-mixing effects are important have been performed using requantized classical molecular dynamics simulations. This approach provides the autocorrelation functions of the dipole vector and isotropic polarizability whose Fourier-Laplace transforms yield the corresponding spectra. For that, the classical equations of dynamics are solved for each molecule among several millions treated as linear rigid rotors and interacting through an anisotropic intermolecular potential.

High-field quantum calculation reveals time-dependent negative Kerr contribution.

The exact quantum time-dependent optical response of hydrogen under strong-field near-infrared excitation is investigated and compared to the perturbative model widely used for describing the effective atomic polarization induced by intense laser fields. By solving the full 3D time-dependent Schrödinger equation, we exhibit a supplementary, quasi-instantaneous defocusing contribution missing in the weak-field model of polarization. We show that this effect is far from being negligible, in particular when closures of ionization channels occur and stems from the interaction of electrons with their parent ions.

Transition from plasma-driven to Kerr-driven laser filamentation.

While filaments are generally interpreted as a dynamic balance between Kerr focusing and plasma defocusing, the role of the higher-order Kerr effect (HOKE) is actively debated as a potentially dominant defocusing contribution to filament stabilization. In a pump-probe experiment supported by numerical simulations, we demonstrate the transition between two distinct filamentation regimes at 800 nm. For long pulses (1.

From higher-order Kerr nonlinearities to quantitative modeling of third and fifth harmonic generation in argon.

The recent measurement of negative higher-order Kerr effect (HOKE) terms in gases has given rise to a controversial debate, fed by its impact on short laser pulse propagation. By comparing the experimentally measured yield of the third and fifth harmonics, with both an analytical and a full comprehensive numerical propagation model, we confirm the absolute and relative values of the reported HOKE indices.

Higher-order Kerr terms allow ionization-free filamentation in gases.

We show that higher-order nonlinear indices (n(4), n(6), n(8), n(10)) provide the main defocusing contribution to self-channeling of ultrashort laser pulses in air and argon at 800 nm, in contrast with the previously accepted mechanism of filamentation where plasma was considered as the dominant defocusing process. Their consideration allows us to reproduce experimentally observed intensities and plasma densities in self-guided filaments.

Femtosecond time resolved coherent anti-Stokes Raman spectroscopy of H(2)-N(2) mixtures in the Dicke regime: Experiments and modeling of velocity effects.

In this paper, we present measurements and modeling of femtosecond time resolved coherent anti-Stokes Raman spectroscopy (CARS) signal in H(2)-N(2) mixtures at low densities. Three approaches have been used to model the CARS response. The first is the usual sum of Voigt profiles.

Measurement of high order Kerr refractive index of major air components.

We measure the instantaneous electronic nonlinear refractive index of N(2), O(2) and Ar at room temperature for a 90 fs and 800 nm laser pulse. Measurements are calibrated by post-pulse molecular alignment through a polarization technique. At low intensity, quadratic coefficients n(2) are determined.

Computational investigation and experimental considerations for the classical implementation of a full adder on SO2 by optical pump-probe schemes.

Following the scheme recently proposed by Remacle and Levine [Phys. Rev. A 73, 033820 (2006)], we investigate the concrete implementation of a classical full adder on two electronic states (X 1A1 and C 1B2) of the SO2 molecule by optical pump-probe laser pulses using intuitive and counterintuitive (stimulated Raman adiabatic passage) excitation schemes.

Strong-field molecular ionization: determination of ionization probabilities calibrated with field-free alignment.

We report an original optical method providing the probability of molecular ionization induced by femtosecond laser pulses. The approach consists of exploiting molecular alignment to extract reliable information about ionization. The cross defocusing technique implemented for this purpose reveals a sensitivity with respect to postpulse alignment, as well as to the free electron density induced by the ultrashort laser pulse.

Rotational Raman spectroscopy of ethylene using a femtosecond time-resolved pump-probe technique.

Femtosecond Raman-induced polarization spectroscopy (RIPS) was conducted at low pressure (250 mb at 295 K and 400 mb at 373 K) in ethylene. The temporal signal, resulting from the beating between pure rotational coherences, was measured with a heterodyne detection. The temporal traces were converted to the frequency domain using a Fourier transformation and then analyzed thanks to the D2hTDS software (http://www.

Femtosecond time resolved coherent anti-Stokes Raman spectroscopy: experiment and modelization of speed memory effects on H2-N2 mixtures in the collision regime.

With the aim of temperature diagnostic, femtosecond time-resolved CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in H2-N2 mixtures. In a first part, a Lorentzian profile is used to model the femtosecond CARS response. A difference between the experimental broadening and the expected one is observed in the collision regime.

Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses.

We show that a linear molecule subjected to a short specific elliptically polarized laser field yields post-pulse revivals exhibiting alignment alternatively located along the orthogonal axis and the major axis of the ellipse. The effect is experimentally demonstrated by measuring the optical Kerr effect along two different axes. The conditions ensuring an optimal field-free alternation of high alignments along both directions are derived.

Measurement of laser-induced alignment of molecules by cross defocusing.

The field-free alignment of CO2 produced in response to the excitation of a molecule by a high-intensity femtosecond pump pulse is measured with a simple coronography-like technique. The technique is based on the defocusing of a time-delayed probe pulse produced by the spatial distribution of aligned molecules. In the intensity regime explored here, the technique is shown to give valuable information about dynamic alignment.