Phase-matching-free ultrashort laser pulse characterization from a transient plasma lens.

A phase-matching-free ultrashort pulse retrieval based on the defocusing of a laser-induced plasma is presented. In this technique, a pump pulse ionizes a rare gas providing a plasma lens whose creation time is ultrafast. A probe pulse propagating through this gas lens experiences a switch of its divergence. The spectrum of the diverging part, isolated by a coronograph, is measured as a function of the pump-probe delay, providing a spectrogram that allows for a comprehensive characterization of the temporal properties of the probe pulse. The method, called PI-FROSt for "plasma-induced frequency-resolved optical switching," is simple, is free of phase-matching constraints, and can operate in both self- and cross-referenced configurations at ultrahigh repetition rate in the whole transparency range of the gas. The assessment of the method demonstrates laser pulse reconstructions of high reliability in both near-infrared (NIR) and ultraviolet (UV) spectral ranges.

Persistent Ground-State Planar Alignment of Iodine Molecule through Resonant Excitation.

We demonstrate the generation of a persistent planar molecular alignment by subjecting a relatively warm gas sample to a resonant femtosecond laser pulse. By optically probing I_{2} molecules in their vibronic ground states, we observe a persistent delocalization of their axes near the plane orthogonal to the field direction. This phenomenon is attributed to the one-photon resonant excitation, primarily removing molecules from the thermal ground-state distribution that are initially aligned along the field, i.e., those with small projection of their rotational angular momentum along the field.

Screening for precancerous anal lesions linked to human papillomaviruses: French recommendations for clinical practice.

In France, about 2000 new cases of anal cancer are diagnosed annually. Squamous cell carcinoma is the most common histological type, mostly occurring secondary to persistent HPV16 infection. Invasive cancer is preceded by precancerous lesions. In addition to patients with a personal history of precancerous lesions and anal cancer, three groups are at very high risk of anal cancer: (i) men who have sex with men and are living with HIV, (ii) women with a history of high-grade squamous intraepithelial lesions (HSILs) or vulvar HPV cancer, and (iii) women who received a solid organ transplant more than 10 years ago. The purpose of screening is to detect HSILs so that they can be treated, thereby reducing the risk of progression to cancer. All patients with symptoms should undergo a proctological examination including standard anoscopy. For asymptomatic patients at risk, an initial HPV16 test makes it possible to target patients at risk of HSILs likely to progress to cancer. Anal cytology is a sensitive test for HSIL detection. Its sensitivity is greater than 80% and exceeds that of proctological examination with standard anoscopy. It is indicated in the event of a positive HPV16 test. In the presence of cytological abnormalities and/or lesions and a suspicion of dysplasia on clinical examination, high-resolution anoscopy is indicated. Performance is superior to that of proctological examination with standard anoscopy. However, this technique is not widely available, which limits its use. If high-resolution anoscopy is not possible, screening by a standard proctological examination is an alternative. There is a need to develop high-resolution anoscopy and triage tests and to evaluate screening strategies.

Quantum modeling, beyond secularity, of the collisional dissipation of molecular alignment using the energy-corrected sudden approximation.

We propose a Markovian quantum model for the time dependence of the pressure-induced decoherence of rotational wave packets of gas-phase molecules beyond the secular approximation. It is based on a collisional relaxation matrix constructed using the energy-corrected sudden approximation, which improves the previously proposed infinite order sudden one by taking the molecule rotation during collisions into account. The model is tested by comparisons with time-domain measurements of the pressure-induced decays of molecular-axis alignment features (revivals and echoes) for HCl and CO2 gases, pure and diluted in He. For the Markovian systems HCl-He and CO2-He, the comparisons between computed and measured data demonstrate the robustness of our approach, even when the secular approximation largely breaks down. In contrast, significant differences are obtained in the cases of pure HCl and CO2, for which the model underestimates the decay rate of the alignment at short times. This result is attributed to the non-Markovianity of HCl-HCl and CO2-CO2 interactions and the important contribution of those collisions that are ongoing at the time when the system is excited by the aligning laser pulse.

Dramatic decline in new HIV diagnoses in persons born in France in a large nationwide HIV cohort.

OBJECTIVES: As trends in new HIV diagnoses represent a measure of the HIV epidemic, we conducted a 6-year longitudinal study to evaluate the change in rates of new HIV diagnosis, stratified by birthplace, HIV risk groups and CD4 cell count at diagnosis in a large French multicentre cohort. STUDY DESIGN: We performed a retrospective cohort study using data from the mainland French Dat'AIDS cohort. METHODS: Data were obtained for subjects with a new HIV diagnosis date between 2013 and 2018. HIV diagnosis date was defined as the date of the first known positive HIV serology. RESULTS: Between 2013 and 2018, a total of 68,376 people living with HIV (PLHIV) were followed in the Dat'AIDS cohort; 9543 persons were newly diagnosed with HIV. The annual number of new HIV diagnoses decreased from 1856 in 2013, to 1149 in 2018 (-38.1%), P = 0.01; it was more pronounced among subjects born in France, from 858 to 484 (-43.6%), P < 0.01, than in those born abroad (-23.8%, from 821 to 626, P = 0.13). Among subjects born in France, the decrease over the period was -46.7% among men who have sex with men (MSM), -43.5% for heterosexual women and -33.3% for heterosexual men. CONCLUSION: Our findings show changes in HIV epidemiology in PLHIV born in France, with a decline around 40% in new HIV diagnoses, and a more pronounced decrease among MSM and heterosexual women. Our results support the long-term effectiveness of the antiretroviral therapy as a prevention strategy among the various tools for HIV prevention.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Tracking autoionizing-wave-packet dynamics at the 1-fs temporal scale.

We present time-resolved studies and Fourier transform spectroscopy of inner-shell excited states undergoing Auger decay and doubly excited autoionizing states, utilizing coherent extreme-ultraviolet (XUV) radiation continua. Series of states spanning a range of ∼4 eV are excited simultaneously. An XUV probe pulse tracks the oscillatory and decaying evolution of the formed wave packet. The Fourier transform of the measured trace reproduces the spectrum of the series. The present work paves the way for ultrabroadband XUV spectroscopy and studies of ultrafast dynamics in all states of matter.