Revival of femtosecond laser plasma filaments in air by a nanosecond laser

International audienceShort lived plasma channels generated through filamentation of femtosecond laser pulses in air can be revived after several milliseconds by a delayed nanosecond pulse. Electrons initially ionized from oxygen molecules and subsequently captured by neutral oxygen molecules provide the long-lived reservoir of low affinity allowing this process. A Bessel-like nanosecond-duration laser beam can easily detach these weakly bound electrons and multiply them in an avalanche process. We have experimentally demonstrated such revivals over a channel length of 50 cm by focusing the nanosecond laser with an axicon

Universal scaling of plasmon coupling in metal nanostructures: Checking the validity for higher plasmonic modes using second harmonic generation

The universal scaling of plasmon coupling in metallic nanostructures is now a well-established feature. However, if the interaction between dipolar plasmon modes has been intensively studied, this is not the case of the coupling between higher order ones. Using Mie theory extended to second harmonic generation, we investigate the coupling between quadrupolar plasmon modes in metallic nanoshells. Like in the case of dipolar plasmon modes, a universal scaling behavior is observed in agreement with the plasmon hybridization model

Dendritic Cells from HIV Controllers Have Low Susceptibility to HIV-1 Infection In Vitro but High Capacity to Capture HIV-1 Particles

ANRS CO21 CODEX cohortInternational audienceHIV controllers (HICs), rare HIV-1 infected individuals able to control viral replication without antiretroviral therapy, are characterized by an efficient polyfunctional and cytolytic HIV-specific CD8+ T cell response. The mechanisms underlying the induction and maintenance of such response in many HICs despite controlled viremia are not clear. Dendritic cells play a crucial role in the generation and reactivation of T cell responses but scarce information is available on those cells in HICs. We found that monocyte derived dendritic cells (MDDCs) from HICs are less permissive to HIV-1 infection than cells from healthy donors. In contrast MDDCs from HICs are particularly efficient at capturing HIV-1 particles when compared to cells from healthy donors or HIV-1 patients with suppressed viral load on antiretroviral treatment. MDDCs from HICs expressed on their surface high levels of syndecan-3, DC-SIGN and MMR, which could cooperate to facilitate HIV-1 capture. The combination of low susceptibility to HIV-1 infection but enhanced capacity to capture particles might allow MDDCs from HICs to preserve their function from the deleterious effect of infection while facilitating induction of HIV-specific CD8+ T cells by cross-presentation in a context of low viremia

Dynamique femtoseconde de la liaison hydrogène

International audienceA pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5-4.4 urn spectral region, at 10 nJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the OH -O motions are "photographed" in real time. Cop. EDP Sciences

Rotation moléculaire en temps réel dans l'eau liquide [Real-time molecular rotation in water]

International audienceLa connaissance déjà acquise sur la dynamique de la liaison hydrogène dans l'eau liquide grâce au développement de laser délivrant des impulsions ultra-courtes dans l'infrarouge moyen nous permet de filmer la rotation de molécules HDO dans une solution D2O. L'expérience réalisée au laboratoire est basée sur la technique de spectroscopie pompe sonde résolue en polarisation. L'anisotropie mesurée permet de détecter en temps réel l'angle de déflexion du moment dipolaire de transition dont le point de départ est la direction du faisceau laser de pompe

Complete temperature study of the relaxation from the high-spin state to low-spin state in a strongly cooperative spin crossover compound

Using two complementary techniques, the relaxation of a photo-induced spin state transition in a strongly cooperative compound has been studied from 10 to 170 K. Up to 150 K, the relaxation rate increases with temperature..

Eau liquide : le film" à l'échelle femtoseconde"

International audienceL'eau est de loin le liquide le plus important sur terre. Il est présent à tous les échelons de l'écosystème terrestre, des océans qui couvrent les 3/5 de la surface, au corps humain dont il représente 70% du volume. Sans l'eau, les systèmes biologiques ne fonctionneraient pas et la vie telle que nous la connaissons n'existerait pas. Si l'eau joue un rôle aussi fondamental dans la nature [1], c'est grâce à ses propriétés exceptionnelles qui proviennent essentiellement de la liaison hydrogène [2, 3]. Dans ces conditions, il n'est pas surprenant que D'immenses efforts aient été portés par le passé, efforts qui continuent de nos jours, pour mieux comprendre ce liquide si particulier. Ses propriétés mécaniques, physicochimiques et moléculaires ont ainsi été étudiées en détails et ont été soigneusement décrites dans un grand nombre d'ouvrages [4, 5]. L'arrivée au début des années 90 de lasers délivrant des impulsions pico- puis femtoseconde va cependant bouleverser la situation en ouvrant la voie à la femtochimie de l'eau liquide. Grâce à ces impulsions incroyablement brèves, la dynamique moléculaire de l'eau peut maintenant être étudiée. Il est même possible de " filmer " les mouvements des molécules d'eau et de la liaison hydrogène en temps réel. C'est ce thème qUE NOUS ALLONS MAINTENANT TRAITER

Etude des propriétés optiques de composés à transition de spin

A l'aide de deux techniques complémentaires, nous avons étudié sur une large gamme de température la dynamique de relaxation de composés à transition d'état de spin photo-excités..

Laser spectroscopic visualization of molecular rotations in liquid water

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Dielectric characterization of [Fe(NH2–trz)3]Br2H2O thermal spin crossover compound by terahertz time domain spectroscopy

The complex optical index refraction of an iron (II) spin-crossover coordination polymer is measured in the Terahertz frequency range by Terahertz time-domain spectroscopy (THz-TDS). By scanning the temperature from 288 to 333 K, we have recorded the evolution of the THz spectrum within the low spin - high spin thermal hysteresis loop. We were able to simultaneously infer the refractive index and absorption variations. The low spin-high spin transition has a marked spectral signature in the millimeter wavelength. In the 0.1–0.6 THz frequency range, the variation of real and imaginary part of the index of refraction is 6% and 20%, respectively. A marked absorption is observed above 500 GHz. The THz-TDS provides a clear direct fingerprint of this class of materials, which are interestingly potential candidates for optical data storage and processing device