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. Possible improvements, benefits, and worthwhile prospects of the method are discussed.

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. The suggested measurement protocol may be used in other high-density media, and potentially in liquids.

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. Based on knowledge of the laser pulse characteristics and on these rates, the time-dependent equation driving the evolution of the density matrix during and after the laser pulse is solved and the time dependence of the so-called "alignment factor" is computed. Comparisons with measurements, free of any adjusted parameter, show that the proposed approach leads to good agreement with measurements. The analysis of the ECS state-to-state collisional rates demonstrates that, as in the case of linear molecules, collision-induced changes of the rotational angular momentum orientation are slower than those of its magnitude. This explains why the collisional decay of the permanent component of the alignment is significantly slower than that of the amplitudes of the transient revivals in both experimental and computed results. It is also shown that, since intermolecular forces within C2H6 colliding pairs weakly depend on rotations of the molecules around their C-C bond, the dissipation mechanism of the alignment in pure ethane is close to that involved in linear molecules.

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 C2H6 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. Comparisons with measurements show that these CMDSs well predict the permanent alignment induced by the laser pulse and its decay with time but, as expected, fail in generating alignment revivals. However, it is shown that introducing a simple requantization procedure in the CMDS "creates" these revivals and that their predicted dissipation decay agrees very well with measured values. The calculations also confirm that, as for linear molecules, the permanent alignment of ethane decays more slowly than the transient revivals. The influence of the intermolecular potential is studied as well as that of the degree of freedom associated with the molecular rotation around the symmetry axis. This reveals that ethane practically behaves as a linear molecule because the intermolecular potential is only weakly sensitive to rotation around the C-C axis.

Dissipation dynamics of field-free molecular alignment for symmetric-top molecules: Ethane (C2H6).

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. It is observed that the permanent alignment always decays slower compared to the transient alignment within the measured pressure range. This demonstrates that the propensity of molecules to conserve the orientation of angular momentum during collisions, previously observed for linear species, is also applicable to symmetric-top molecules. The results of this work provide valuable information for further theoretical understanding of collisional relaxation within nonlinear polyatomic molecules, which are expected to present interesting and nontrivial features due to an extra rotational degree of freedom.

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. The shaped pulse is then frequency doubled through the module which transfers the applied spectral shaping to the second harmonic field in a predictable manner. The relevance of the method is demonstrated by synthesizing and characterizing shaped pulses at a central wavelength of 400 nm. The results demonstrate a full control over the spectral phase and amplitude of the harmonic field. The experimental setup is simple and features interesting prospects for the polarization shaping of ultraviolet pulses and the production of shaped ultraviolet pulses requested for the seeding of free-electron lasers.

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. The physical observable is the frequency shift imparted to the probe beam as a manifestation of the angular Doppler effect. Our experimental results are confirmed by theoretical computations, which allow one to gain a deep physical insight into the laser-molecule interaction.

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. Two of the approximations used in the previous studies have been corrected, allowing the consideration of line-mixing effects without use of any adjusted parameters. The comparisons between calculated and experimental spectra under various conditions of pressure and temperature demonstrate the quality of the theoretical model. This opens promising perspectives for first principle ab initio predictions of line-mixing effects in absorption and scattering spectra of various systems involving linear molecules.

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. The results demonstrate that the model accurately reproduces the decays with time of both the transient revivals and "permanent" component of the alignment. The significant differences observed between the behaviors resulting from CO2-CO2 and CO2-He collisions are also well predicted by the model.

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. It provides an interpretation of the higher-order Kerr effect recently observed in various gases.

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 (1.2 ps), the plasma substantially contributes to filamentation, while this contribution vanishes for short pulses (70 fs). These results confirm the occurrence, in adequate conditions, of filamentation driven by the HOKE rather than by plasma.

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. In the second approach, the speed dependent Voigt profile is used. In the last approach, a model of the temporal CARS signal is developed, which takes into account the velocity changes induced by collisions and the speed dependence of the collisional parameters. The velocity changes are modeled using the Keilson and Storer memory function; the radiator speed dependences of the collisional parameters are determined from their temperature dependences. The results obtained are consistent with previous studies in the frequency domain, showing that the changes of the velocity have important effects for the H(2)/N(2) system in the Dicke narrowing density regime.

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. At higher intensities, a strong negative contribution with a higher nonlinearity appears, which leads to an overall negative nonlinear Kerr refractive index in air above 26 TW/cm(2).

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. The resources needed for providing the inputs and reading out are discussed, as well as the conditions for achieving robustness in both the intuitive and counterintuitive pump-dump sequences. The fidelity of the scheme is analyzed with respect to experimental noise and two kinds of perturbations: The coupling to the neighboring rovibrational states and a finite rotational temperature that leads to a mixture for the initial state. It is shown that the logic processing of a full addition cycle can be realistically experimentally implemented on a picosecond time scale while the readout takes a few nanoseconds.

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.u-bourgogne.fr/LPUB/shTDS.html) dedicated to X2Y4 molecules with D2h symmetry. The effective Hamiltonian was expanded up to order 2, allowing the determination of five parameters with an rms of 0.017 cm(-1). Special care was taken in the precise modeling of intensities, taking into account all instrumental effects. Relative intensities were fitted (with an rms of 7.2%) and two polarizability operators were determined.

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. The observed broadening increases strongly in an inhomogeneous way with respect to the perturber concentration. This is of considerable importance for temperature measurements. In a second part, we show that in the collision regime, this inhomogeneous broadening is due to the speed dependence of the collisional parameters and the memory effects of the radiator speed. A new modelization of the time-resolved CARS response taking into account the speed memory effects is presented and applied to the temperature diagnostic in H2-N2 mixtures. The numerical results are in good agreement with experiments.

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.