Politique relative à la valorisation et à la qualité de la langue française (politique n°24)

Chronique sous la direction de Mihaela AilincaiInternational audienc

Optical gratings induced by field-free alignment of molecules

We analyze the alignment of molecules generated by a pair of crossed ultra-short pump pulses of different polarizations by a technique based on the induced time-dependent gratings. Parallel polarizations yield an intensity grating, while perpendicular polarizations induce a polarization grating. We show that both configurations can be interpreted at moderate intensity as an alignment induced by a single polarized pump pulse. The advantage of the perpendicular polarizations is to give a signal of alignment that is free from the plasma contribution. Experiments on femtosecond transient gratings with aligned molecules were performed in CO2 at room temperature in a static cell and at 30 K in a molecular expansion jet.Comment: 10 pages, 5 figures, submitted to PR

Compte-rendu de lecture de Dominique Pestre, "À contre-science, politique et savoirs des sociétés contemporaines" (Paris, Ed. du Seuil, 2012, coll. " La couleur des idées ")

Recension de l'ouvrage de D.Pestr

Transition from plasma- 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.Comment: 6 pages, 4 figures. Accepted for publication in Physical Review Letter

Harmonic generation and filamentation: when secondary radiations have primary consequences

In this Letter, it is experimentally and theoretically shown that weak odd harmonics generated during the propagation of an infrared ultrashort ultra-intense pulse unexpectedly modify the nonlinear properties of the medium and lead to a strong modification of the propagation dynamics. This result is in contrast with all current state-of-the-art propagation model predictions, in which secondary radiations, such as third harmonic, are expected to have a negligible action upon the fundamental pulse propagation. By analysing full three-dimensional ab initio quantum calculations describing the microscopic atomic optical response, we have identified a fundamental mechanism resulting from interferences between a direct ionization channel and a channel involving one single ultraviolet photon. This mechanism is responsible for wide refractive index modifications in relation with significant variation of the ionization rate. This work paves the way to the full physical understanding of the filamentation mechanism and could lead to unexplored phenomena, such as coherent control of the filamentation by harmonic seeding.Comment: 7 pages, 5 figure

Revisiting interferences for measuring and optimizing optical nonlinearities

A method based on optical interferences for measuring optical nonlinearities is presented. In a proof-of-principle experiment, the technique is applied to the experimental determination of the intensity dependence of the photoionization process. It is shown that it can also be used to control and optimize the nonlinear process itself at constant input energy. The presented strategy leads to enhancements that can reach several orders of magnitude for highly nonlinear processes.Comment: 6 pages, 5 figure

Réflexions pour le développement de projets de connaissance du jardin de l’ENS de Lyon

Le jardin de l'ENS de Lyon est assurément l'un des plus beaux lieux verts de la ville et il le doit, pour beaucoup, à son créateur, Gilles Clément, dont l'école de pensée a déplacé les normes classiques des jardins « ordonnés ». Ainsi, son concept de « jardin en mouvement » fait-il du jardin de l'école une oeuvre artistique affirmée (cf. Arnould et al., 2012) qui porte des valeurs d'écologie humaniste, s'appuie sur des connaissances objectives et voit la nature en train de vivre. C'est ce mouvement, présenté dans Le juste jardin, que nous entendons poursuivre, en dépliant effectivement ses différentes dimensions : horticoles, biologiques, historiques, poétiques, littéraires, philosophiques, sociologiques, géographiques, etc. Le jardin, ce jardin, est en quelque sorte le cinquième « bâtiment » de l'ENS de Lyon – avec les bâtiments Monod, Diderot (Bibliothèque), Descartes et Buisson –, dont il en est un trait d'union. Nous soutenons qu'il est un lieu de culture et de savoir, qu'il est un artisan et un symbole des missions de l'école et de la pensée du Juste jardin. Car il est bien, en effet, juste sous nos yeux, objet autant que sujet de connaissance et constitue ce faisant un terrain original d'interrogation (son organisation, son architecture, sa mare, ses moutons, etc.) et un outil qui ne l'est pas moins, de médiation des savoir(s) d'un bien commun reçu en partage

Orientation and Alignment Echoes

We present what is probably the simplest classical system 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/alignment of the ensemble exhibits multiple echoes and fractional echoes. We elucidate the mechanism of the echo formation by 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