Search

Vision in living mice is sensitive to femtosecond pulse shaping., Vision is usually assumed to be sensitive to the light intensity and spectrum but not to its spectral phase. However, experiments performed on retinal proteins in solution showed that the first step of vision consists in an ultrafast photoisomerization that can be coherently controlled by shaping the phase of femtosecond laser pulses, especially in the multiphoton interaction regime. The link between these experiments in solution and the biological process allowing vision was not demonstrated. Here, we measure the electric signals fired from the retina of living mice upon femtosecond multipulse and single-pulse light stimulation. Our results show that the electrophysiological signaling is sensitive to the manipulation of the light excitation on a femtosecond time scale. The mechanism relies on multiple interactions with the light pulses close to the conical intersection, like pump-dump (photoisomerization interruption) and pump-repump (reverse isomerization) processes. This interpretation is supported both experimentally and by dynamics simulations.

High intensity laser filamentation in air has recently demonstrated that, through plasma generation and its associated shockwave, fog can be cleared around the beam, leaving an optically transparent path to transmit light. However, for practical applications like free-space optical communication (FSO), channels of multi-centimeter diameters over kilometer ranges are required, which is extremely challenging for a plasma based method. Here we report a radically different approach, based on quantum control. We demonstrate that fog clearing can also be achieved by producing molecular quantum wakes in air, and that neither plasma generation nor filamentation are required. The effect is clearly associated with the rephasing time of the rotational wave packet in N2.Pump excitation provided in the form of resonant trains of 8 pulses separated by the revival time are able to transmit optical data through fog with initial extinction as much as −6 dB.

Femtosecond X-ray absorption spectroscopy (XAS) is a powerful method to investigate the dynamical behavior of a system after photoabsorption in real time. So far, the application of this technique has remained limited to large-scale facilities, such as femtosliced synchrotrons and free-electron lasers (FEL). In this work, we demonstrate femtosecond time-resolved soft-X-ray absorption spectroscopy of liquid samples by combining a sub-micrometer-thin flat liquid jet with a high-harmonic tabletop source covering the entire water-window range (284–538 eV). Our work represents the first extension of tabletop XAS to the oxygen edge of a chemical sample in the liquid phase. In the time domain, our measurements resolve the gradual appearance of absorption features below the carbon K-edge of ethanol and methanol during strong-field ionization and trace the valence-shell ionization dynamics of the liquid alcohols with a temporal resolution of ∼30 fs. This technique opens unique opportunities to study molecular dynamics of chemical systems in the liquid phase with elemental, orbital, and site sensitivity. ISSN:1948-7185

International audience; Lightning is highly destructive due to its high power density and unpredictable character. Directing lightning away would allow to protect sensitive sites from its direct and indirect impacts (electromagnetic perturbations). Up to now, lasers have been unable to guide lightning efficiently since they were not offering simultaneously terawatt peak powers and kHz repetition rates. In the framework of the Laser Lightning Rod project, we develop a laser system for lightning control, with J-range pulses of 1ps duration at 1kHz. The project aims at investigating its propagation in the multiple filamentation regime and its ability to control high-voltage discharges. In particular, a field campaign at the S ̈antis mountain will assess the laser ability to trigger upward lightning.

While chemotherapy is one of the most used treatments in oncology, the systemic administration of chemotherapeutics generally results in undesired damages to healthy tissues and cells, side effects such as severe nausea and leukopenia, and reduced efficacy due to multidrug resistance and poor target accessibility. The limitations of conventional chemotherapy formulation have prompted the development of alternative nanomaterials‐based strategies to achieve targeted and stimuli sensitive payload delivery to reach optimal local drug concentration at tumor sites. In this study, the anticancer drug chlorambucil (Clb) was conjugated to the surface of silica coated lithium niobate (LNO) harmonic nanoparticles (HNPs) using a photocaging tether based on coumarin‐4‐yl methyl derivative. Upon laser pulsed femtosecond irradiation at 790 nm, the second harmonic emission from the metal oxide core induced the efficient release of Clb, with concomitant contribution from the nonlinear absorption of the coumarin (CM)‐based moiety.

We present space and time resolved measurements of the air hydrodynamics induced by femtosecond laser pulse excitation of the air gap between two electrodes at high potential difference. We explore both plasma-based and plasma-free gap excitation. The former uses the plasma left in the wake of femtosecond filamentation, while the latter exploits air heating by multiple-pulse resonant excitation of quantum molecular wavepackets. We find that the cumulative electrode-driven air density depression channel plays the dominant role in the gap evolution leading to breakdown. Femtosecond laser heating serves mainly to initiate the depression channel; the presence of filament plasma only augments the early heating.

We introduce a nonlinear all-optical theranostics protocol based on the excitation wavelength decoupling between imaging and photoinduced damage of human cancer cells labeled by bismuth ferrite (BFO) harmonic nanoparticles (HNPs). To characterize the damage process, we rely on a scheme for in situ temperature monitoring based on upconversion nanoparticles: by spectrally resolving the emission of silica coated NaGdF4:Yb3+/Er3+ nanoparticles in close vicinity of a BFO HNP, we show that the photointeraction upon NIR-I excitation at high irradiance is associated with a temperature increase >100 °C. The observed laser–cell interaction implies a permanent change of the BFO nonlinear optical properties, which can be used as a proxy to read out the outcome of a theranostics procedure combining imaging at 980 nm and selective cell damage at 830 nm. The approach has potential applications to monitor and treat lesions within NIR light penetration depth in tissues.

We couple the full 3D ab initio quantum evolution of the light pulse polarization in interaction with an atom with a propagation model to simulate the propagation of ultrashort laser pulses over macroscopic dimensions, in the presence of self-generated harmonics up to order 11. We evidence a clear feedback of the generated harmonics on propagation, with an influence on the ionization probability as well as the yield of the harmonic generation itself., Comment: 9 pages, 4 figures

Science, 355 (6322), ISSN:0036-8075, ISSN:1095-9203

International audience; We study the use of frequency upconversion schemes of near-IR picosecond laser pulses and compare their ability to guide and trigger electric discharges through filamentation in air. Upconversion, such as Second Harmonic Generation, is favorable for triggering electric discharges for given amount of available laser energy, even taking into account the losses inherent to frequency conversion. We focus on the practical question of optimizing the use of energy from a given available laser system and the potential advantage to use frequency conversion schemes.

In this paper we examine the spectral changes in a white light laser filament due to different pulse shapes generated by a pulse shaping setup. We particularly explore how the properties of the filament spectra can be controlled by parametrically tailored white light pulses. The experiments are carried out in a gas cell with up to 9 bar of argon. Plasma generation and self-phase modulation strongly affect the pulse in the spectral and temporal domain. By exploiting these effects we show that the pulse spectrum can be modified in a desired way by either using second order parametric chirp functions to shift the filament spectrum to higher or lower wavelengths, or by optimizing pulse shapes with a genetic algorithm to generate more complex filament spectra. This paper is one of the first examples of the application of complex, parametrically shaped white light pulses., 14 pages, 7 figures

We experimentally and numerically characterize multiple filamentation of laser pulses with incident intensities of a few TW/cm2. Propagating 100 TW laser pulses over 42 m in air, we observe a new propagation regime where the filament density saturates. As also evidenced by numerical simulations in the same intensity range, the total number of filaments is governed by geometric constraints and mutual interactions among filaments rather than by the available power in the beam.

We demonstrate the simultaneous generation of second, third, and fourth harmonic from a single dielectric Bismuth Ferrite nanoparticle excited by a telecom fiber laser at 1560 nm. We first characterize the signals associated with different nonlinear orders in terms of spectrum, excitation intensity dependence, and relative signal strengths. Successively, on the basis of the polarization-resolved emission curves of the three harmonics, we discuss the interplay of susceptibility tensor components at the different orders and we show how polarization can be used as an optical handle to control the relative frequency conversion properties., Comment: include S.I. section at the end of the main pdf file

Optics Express, 26 (9), ISSN:1094-4087

We present high flux table-top 220eV HHG source (>109 photons/s/1% bandwidth) driven directly by a

Frequency-dependence and magnitude of second harmonic generation (SHG) from ~4 × 105 nm2 molybdenum disulfide (MoS2) monolayers was examined in presence of single 150 nm plasmonic gold@silica shell@core nanoantenna monomer and dimers. Quantitative agreement between discrete dipole approximation-calculated fields and measured SHG enhancements was found. SHG from MoS2 was enhanced up to 1.88 × upon deposition of a plasmonic nanoantenna-dimer with 170 nm gap, reaching maximal normalized SHG conversion efficiency of 0.0250%/W. Pump losses attributable to plasmonic damping, e.g., scattering and/or hot-electron injection into MoS2, were apparent. Linear and nonlinear optical activity of MoS2 and nanoantenna controls were compared with literature values.

Light can be used to modify and control properties of media, as in the case of electromagnetically induced transparency or, more recently, for the generation of slow light or bright coherent XUV and X-ray radiation. Particularly unusual states of matter can be created by light fields with strengths comparable to the Coulomb field that binds valence electrons in atoms, leading to nearly-free electrons oscillating in the laser field and yet still loosely bound to the core [1,2]. These are known as Kramers-Henneberger states [3], a specific example of laser-dressed states [2]. Here, we demonstrate that these states arise not only in isolated atoms [4,5], but also in rare gases, at and above atmospheric pressure, where they can act as a gain medium during laser filamentation. Using shaped laser pulses, gain in these states is achieved within just a few cycles of the guided field. The corresponding lasing emission is a signature of population inversion in these states and of their stability against ionization. Our work demonstrates that these unusual states of neutral atoms can be exploited to create a general ultrafast gain mechanism during laser filamentation., Comment: 9 pages, 4 figures, 10 pages supplementary material, 8 supplementary figures

Atmospheric clearness is a key issue for free space optical communications (FSO). We present the first active method to achieve FSO through clouds and fog, using ultrashort high intensity laser filaments. The laser filaments opto-mechanically expel the droplets out of the beam and create a cleared channel for transmitting high bit rate telecom data at 1.55 microns. The low energy required for the process allows considering applications to Earth-satellite FSO and secure ground based optical communication, with classical or quantum protocols., 4 pages + 2 pages supplementary text and movies

We present laser-induced condensation as a method for detecting of ultra-low concentrations of volatile organic compounds featuring a detection threshold well below 1 ppb. The method was tested in lab scale experiments and compared with quantitative photo-ionization detection measurements. The method was shown to be sensitive to the laser excitation wavelength between 1.6’2.0 um range.

Filamentation of ultra-short TW-class lasers recently opened new perspectives in atmospheric research. Laser filaments are self-sustained light structures of 0.1-1 mm in diameter, spanning over hundreds of meters in length, and producing a low density plasma (1015-1017 cm-3) along their path. They stem from the dynamic balance between Kerr self-focusing and defocusing by the self-generated plasma and/or non-linear polarization saturation. While non-linearly propagating in air, these filamentary structures produce a coherent supercontinuum (from 230 nm to 4 µm, for a 800 nm laser wavelength) by self-phase modulation (SPM), which can be used for remote 3D-monitoring of atmospheric components by Lidar (Light Detection and Ranging). However, due to their high intensity (1013-1014 W cm-2), they also modify the chemical composition of the air via photo-ionization and photo-dissociation of the molecules and aerosols present in the laser path. These unique properties were recently exploited for investigating the capability of modulating some key atmospheric processes, like lightning from thunderclouds, water vapor condensation, fog formation and dissipation, and light scattering (albedo) from high altitude clouds for radiative forcing management. Here we review recent spectacular advances in this context, achieved both in the laboratory and in the field, reveal their underlying mechanisms, and discuss the applicability of using these new non-linear photonic catalysts for real scale weather control.

Real-time monitoring of individual particles from atmospheric aerosols was performed by means of a specifically developed single-particle fluorescence spectrometer (SPFS). The observed fluorescence was assigned to particles bearing polycyclic aromatic hydrocarbons (PAH). This assignment was supported by an intercomparison with classical speciation on filters followed by gas chromatography-mass spectrometry (GC-MS) analysis. As compared with daily-averaged data, our time resolved approach provided information about the physicochemical dynamics of the particles. In particular, distinctions were made between background emissions related to heating, and traffic peaks during rush hours. Also, the evolution of the peak fluorescence wavelength provided an indication of the aging of the particles during the day., 8 pages, 6 figures

Light is used to modify or control properties in many quantum systems, leading to phenomena such as electromagnetically induced transparency, the generation of slow light or bright coherent XUV radiation. Using light fields, with strengths comparable to the Coulomb field which binds valence electrons in atoms, particularly unusual quantum states can be created which describe a nearly free electron oscillating in the laser field yet still loosely bound to the core1, 2. We demonstrate that such states can arise not only in isolated atoms3, but also in gases at a few atmospheric pressures, guiding intense laser pulses, where they can act as a gain medium. This gain is created within just a few cycles of the guided field. Using shaped pulses4 with 5–10 fs risetimes, we create pulse conditions, within a laser filament, where these states can persist, and we observe in the corresponding emission, the signatures of these states. This work demonstrates a general, new, ultrafast gain mechanism during filamentation, in which any driven atom can achieve inversion intra-pulse, using pulse shaping techniques.

Time-resolved spectroscopy at element-specific absorption edges provides a powerful tool to study ultrafast phenomena of matter on the atomic scale in the gas, solid or liquid phase. Combining element sensitivity of core-to-valence transitions due to the very localized initial core state and femtosecond/attosecond time resolution of new soft X-ray sources based on high-harmonic generation from long-wavelength drivers, accurate electronic and structural information is now accessible.

We demonstrate an approach for a real-time, consumable-free optical system operating on a liquid jet which can be easily derived from the water distribution infrastructure. We apply a pump-probe scheme based on the acquisition and nanosecond manipulation of UV-excited fluorescence to increase the selective identification of bacterial against organic pollutants in water.

The quantum concepts of entanglement and interaction-free measurements are applied to spectroscopy to successfully sense carbon dioxide in air.

We characterized the white-light supercontinuum emission by a sub-petawatt laser system in the atmosphere via light detection and ranging measurements. As much as 1 J of supercontinuum is generated in the atmosphere, corresponding to a conversion efficiency of 30 %. This generation occurs at altitudes below 100 m. The high initial beam intensity results in the saturation of the number of self-guided filaments. Therefore, the “photon bath” surrounding the filaments strongly contributes to the white-light generation. These finding is well reproduced by numerical simulations based on the experimental parameters.

While propagating in transparent media, near-infrared multiterawatt (TW) laser beams break up in a multitude of filaments of typically 100-200 um diameter with peak intensities as high as 10 to 100 TW/cm^{2}. We observe a phase transition at incident beam intensities of 0.4 TW/cm^{2}, where the interaction between filaments induce solidlike two-dimensional crystals with a 2.7 mm lattice constant, independent of the initial beam diameter. Below 0.4 TW/cm^{2}, we evidence a mixed phase state in which some filaments are closely packed in localized clusters, nucleated on inhomogeneities (seeds) in the transverse intensity profile of the beam, and other are sparse with almost no interaction with their neighbors, similar to a gas. This analogy with a thermodynamic gas-solid phase transition is confirmed by calculating the interaction Hamiltonian between neighboring filaments, which takes into account the effect of diffraction, Kerr self-focusing, and plasma generation. The shape of the effective potential is close to a Morse potential with an equilibrium bond length close to the observed value.

We evaluate the linearity of three electric measurement techniques of the initial electron density in laser filaments by comparing their results for a pair of filaments and for the sum of each individual filament. The conductivity measured between two plane electrodes in a longitudinal configuration is linear within 2% provided the electric field is kept below 100 kV/m. Furthermore, simulations show that the signal behaves like the amount of generated free electrons. The slow ionic current measured with plane electrodes in a parallel configuration is representative of the ionic charge available in the filament, after several $\mu$s, when the free electrons have recombined. It is linear within 2% with the amount of ions and is insensitive to misalignment. Finally, the fast polarization signal in the same configuration deviates from linearity by up to 80% and can only be considered as a semi-qualitative indication of the presence of charges, e.g., to characterize the filament length., Comment: 17 pages, 7 figures

We investigate the influence of ultrashort laser filaments on high-voltage discharges and spark-free unloading at various repetition rates and wind conditions. For electric fields well below, close to and above the threshold for discharges, we respectively observe remote spark-free unloading, discharge suppression, and discharge guiding. These effects rely on an indirect consequence of the thermal deposition, namely the fast dilution of the ions by the shockwave triggered by the filament at each laser shot. This dilution drastically limits recombination and increases the plasma channel conductivity that can still be non-negligible after tens or hundreds of milliseconds. As a result, the charge flow per pulse is higher at low repetition rates., Comment: 8 pages, 4 figures

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third- generation photovoltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research “Molecular Ultrafast Science and Technology,” a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mechanisms of photoinduced charge separation in dye- sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here.

We investigate the triggering and guiding of DC high-voltage discharges over a distance of 37 cm by filaments produced by ultraviolet (266 nm) laser pulses of 200 ps duration. The latter reduce the breakdown electric field by half and allow up to 80% discharge probability in an electric field of 920 kV/m. This high efficiency is not further increased by adding nanosecond pulses in the Joule range at 532 nm and 1064 nm. However, the latter statistically increases the guiding length, thereby accelerating the discharge by a factor of 2. This effect is due both to photodetachment and to the heating of the plasma channel, that increases the efficiency of avalanche ionization and reduces electron attachment and recombination., Comment: 12 pages, 6 figures

In the present review, laser fields are so strong that they become part of the electronic potential, and sometimes even dominate the Coulomb contribution. This manipulation of atomic potentials and of the associated states and bands finds fascinating applications in gases and solids, both in the bulk and at the surface. We present some recent spectacular examples obtained within the NCCR MUST in Switzerland., Structural Dynamics, 4 (6), ISSN:2329-7778

Ultrashort intense lasers are able to efficiently condensate water vapor in air into droplets, even with mid-IR laser pulses. In addition, existing droplets can be expelled from the beam to transmit information through fog.

The influence of aerosols on climate is highly dependent on the particle size distribution, concentration, and composition. In particular, the latter influences their ability to act as cloud condensation nuclei, whereby they impact cloud coverage and precipitation. Here, we simultaneously measured the concentration of aerosols from sea spray over the North Atlantic on board the exhaust-free solar-powered vessel “PlanetSolar”, and the sea surface physico-chemical parameters. We identified organic-bearing particles based on individual particle fluorescence spectra. Organic-bearing aerosols display specific spatio-temporal distributions as compared to total aerosols. We propose an empirical parameterization of the organic-bearing particle concentration, with a dependence on water salinity and sea-surface temperature only. We also show that a very rich mixture of organic aerosols is emitted from the sea surface. Such data will certainly contribute to providing further insight into the influence of aerosols on cloud formation, and be used as input for the improved modeling of aerosols and their role in global climate processes.

The electric conductivity, HV discharge triggering and guiding capabilities of filaments at 3.9 micrometer in air are investigated in the perspective of lightning control applications, and compared to near-IR filaments in identical conditions

We investigate the pulse partitioning of a 6.3 mJ, 450 fs pulse at 1030 nm to produce plasma channels. At such moderate energies, splitting the energy into several sub-pulses reduces the ionization efficiency and thus does not extend the plasma lifetime. We numerically show that when sufficient energy to produce multifilamentation is available, splitting the pulse temporally in a pulse train increases the gas temperature compared to a filament bundle of the same energy. This could improve the mean free path of the free electrons, therefore enhancing the efficiency of discharge triggering., Correcting typo in Eq. (12) and unit of $\beta_{ep}$ in Table I

The optical identification of bioaerosols in the atmosphere and its discrimination against combustion related particles is a major issue for real-time, field compatible instruments. In the present paper, we show that by embedding advanced pump-probe depletion spectroscopy schemes in a portable instrument, it is possible to discriminate amino acid containing airborne particles (bacteria, humic particles, etc.) from poly-cyclic aromatic hydrocarbon containing combustion particles (Diesel droplets, soot, vehicle exhausts) with high selectivity. Our real-time, multi-modal device provides, in addition to the pump-probe depletion information, fluorescence spectra (over 32 channels), fluorescence lifetime and Mie scattering patterns of each individually flowing particle in the probed air.

We study the triggering of single filaments due to turbulence in the beam path for a laser of power below the filamenting threshold. Turbulence can act as a switch between the beam not filamenting and producing single filaments. This 'positive' effect of turbulence on the filament probability, combined with our observation of off-axis filaments suggests the underlying mechanism is modulation instability caused by transverse perturbations. We hereby experimentally explore the interaction of modulation instability and turbulence, commonly associated with multiple-filaments, in the single-filament regime., 5 pages, 6 figures

We use Hyper Rayleigh Scattering and polarization resolved multiphoton microscopy to investigate simultaneously the second and third-order nonlinear response of Potassium Niobate and Bismuth Ferrite harmonic nanoparticles. We first derive the second-to-third harmonic intensity ratio for colloidal ensembles and estimate the average third-order efficiency of these two materials. Successively, we explore the orientation dependent tensorial response of individual nanoparticles fixed on a substrate. The multi-order polarization resolved emission curves are globally fitted with an analytical model to retrieve individual elements of susceptibility tensors.

Intense laser illumination of cirrus-like ice particles increases the amount of condensed water and modifies the particles’ albedo., We investigate the interaction of ultrashort laser filaments with individual 90-μm ice particles, representative of cirrus particles. The ice particles fragment under laser illumination. By monitoring the evolution of the corresponding ice/vapor system at up to 140,000 frames per second over 30 ms, we conclude that a shockwave vaporization supersaturates the neighboring region relative to ice, allowing the nucleation and growth of new ice particles, supported by laser-induced plasma photochemistry. This process constitutes the first direct observation of filament-induced secondary ice multiplication, a process that strongly modifies the particle size distribution and, thus, the albedo of typical cirrus clouds.

International audience; SiC nanoparticles by carbothermal reduction show promising properties in terms of second harmonic and multiphoton excited luminescence. In particular, we estimate a nonlinear efficiency < d > = 17 pm/V, as obtained by Hyper Rayleigh Scattering. We also present results of cell labelling to demonstrate the potential use of SiC nanoparticles for nonlinear bioimaging by simultaneous detection of second harmonic and luminescence.

We deploy a network of autonomous stations measuring meteorological and soil parameters, as well as the airborne particle size distribution with a focus on the size ofPlasmapora Viticola(PV). They provide early warning and detection of PV spore outbursts with high spatial and temporal resolution. We evidence the high spatial inhomogeneity of this pathogen, potentially allowing to limit treatments to the specific times and locations where infection risk is detected.

We report on the first study of live cell opto-poration by single temporally shaped femtosecond laser pulses. Based on an ad hoc developed cell staining protocol, we demonstrate the influence of the pulse temporal profile on the efficiency of poration and on cell viability at four hours comparing the results obtained for four different temporal pulse shapes: positive and negative temporal Airy, positively chirped, and 30 fs bandwidth limited pulses. Each pulse has been tested on a thousand cells. The most suitable pulses for opto-poration are the positive Temporal Airy Pulses (TAP+), likely because they enhance avalanche ionization compared to bandwidth-limited shorter pulses. We discuss the results in the context of previous studies, highlighting the differences between single and multi-pulse opto-poration strategies.

We studied the nonlinear optical properties of single layer graphene using high terahertz (THz) fields. With the use of a back gate and cooling down the sample to cryogenic temperatures we are able to spectrally probe the nonlinear THz properties of intrinsic to highly doped graphene. The carrier density strongly affects the nonlinear properties of graphene; in the low doping and high THz field regime, an increase of the transmission on the order of 4% is found in our experiments. At higher doping levels we observe a larger relative nonlinear response: the larger the doping in the single layer the larger the relative field induced increase in transmission becomes. In all experiments, the THz field is opposing the effect of the gate, but field effects are never larger than the doping effects. We use the thermodynamical model for a hot electron gas also used by Mics et al (2015 Nat. Commun.. 6 7655) to simulate our data and study the effects of doping on the nonlinear properties of single layer graphene. We find that the highest carrier temperatures are obtained in low doped graphene. The model shows a good qualitative agreement with our data for high doping levels. Nevertheless our results demonstrate the limitation of the model for low doping levels. Our results are a road map for further explorations for the control of nonlinear light-matter interaction and functionalization of graphene layers in active THz devices in which carrier temperature and saturable absorption play a role.

International audience; Charge transfer mechanisms lay at the heart of chemistry and biochemistry. Proton coupled electron transfers (PCET) are central in biological processes such as photosynthesis and in the respiratory chain, where they mediate long-range charge transfers. These mechanisms are normally difficult to harness experimentally due to the intrinsic complexity of the associated biological systems. Metal-peptide cations experience both electron and proton transfers upon photoexcitation, proving an amenable model system to study PCET. We report on a time-resolved experiment designed to follow this dual charge transfer kinetics in [HG3W+Ag]+ (H = histidine, G = glycine, W = tryptophan) on time scales ranging from femtoseconds to milliseconds. While electron transfer completes in less than 4 ps, it triggers a proton transfer lasting over hundreds of microseconds. Molecular dynamics simulations show that conformational dynamic plays an important role in slowing down this reaction. This combined experimental and computational approach provides a view of PCET as a single phenomenon despite its very wide time-domain span.

International audience; We have investigated nanocrystals of Fe(IO3)3 by polarization-sensitive second harmonic generation (SHG) microscopy. As the nonlinear optical properties of this material were only poorly characterized, we have first determined the relative values of the elements of its second-order susceptibility tensor, by the global fitting of the polarization-resolved SHG response of an ensemble of nanocrystals. This information allows one to optically retrieve the orientation of individual particles in the sample. The high SHG efficiency measured for nanocrystals of Fe(IO3)3 and their polar nature could make them very attractive for nonlinear microscopy of biological samples.

We measured the chemical composition and the size distribution of aerosols generated by femtosecond-Terawatt laser pulses in the atmosphere using an aerosol mass spectrometer (AMS). We show that nitric acid condenses in the form of ammonium nitrate and that oxidized volatile organics also contribute to particle growth. These two components account for two thirds and one third, respectively, of the dry laser-condensed mass. They appear in two different modes centred at 380 nm and 150 nm. The number concentration of particles between 25 and 300 nm increases by a factor of 15. Pre-existing water droplets strongly increase the oxidative properties of the laser-activated atmosphere, substantially enhancing the condensation of organics under laser illumination.

International audience; We report new experimental and numerical results on supercontinuum generation at ultraviolet/visible wavelengths produced by the propagation of infrared femtosecond laser pulses in air. Spectral broadening is shown to similarly affect single filaments over laboratory distance scales, as well as broad beams over long-range propagation distances. Numerical simulations display evidence of the crucial role of third harmonic generation in the build-up of UV-visible wavelengths, by comparison with current single-envelope models including chromatic dispersion and self-steepening.

A correction in the transit time of electrons between the filaments and the electrodes leads us to reattribute the remote unloading to ions rather than to electrons. The experimental results reported in [Opt. Express23, 286407 (2015)] about remote electrical unloading and discharge suppression, as well as the analogy with the analogy with a supercorona, remain valid.

Nous avons utilise des cristaux de KTP et de KNbO 3 comme oscillateurs parametriques optique (OPO) pompes par un laser Titane:Saphir accordable. Pour l'OPO base sur un KTP «monolithique» en configuration d'accord de phase non critique de type II (θ=90° et Φ=0°), on obtient un seuil de 0,2 MW/cm 2 et une efficacite de conversion totale de 37%. L'OPO base sur le KNbO3 permet d'obtenir un signal et une complementaire variant de 0,95 a 3,8 μm lorsque l'on balaie la pompe entre 740 et 860 nm. Implemente dans notre systeme Lidar, ce cristal nous a permis de mesurer a 3,5 μm la taille et la concentration des gouttelettes d'eau lors d'un episode de brouillard, sur une distance de 1500 m. Le rayon moyen des gouttelettes d'eau est de 8,7 μm et la concentration est de 10 mg/m 3 .