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International audience; Photoionization of atoms and molecules by intense femtosecond laser pulses is a fundamental process of strong-field physics. Using a three-color femtosecond laser scheme with attosecond phase control precision, we demonstrate coherently controlled ionization of nitrogen molecules with a modulation level up to 20% by varying the phase shifts between the fundamental laser frequency at 800 nm and its second and third harmonics. Furthermore, the phase dependence of the ionization degree qualitatively changes with the laser intensity ratios between the three colors. The observations are interpreted as a manifestation of the competition between different parametric channels contributing to the ionization process. Such coherent control of ionization opens new ways to finely tune and optimize various phenomena accompanying laser-material interactions: high-order harmonic and attosecond generation, nanofabrication, remote ablation of samples, and even guidance of discharge and control of lightning by lasers.

We report a straightforward method to control main spatio-temporal couplings in a CPA laser chain system using a specially designed chromatic doublet in a divergent beam configuration. The centering of the doublet allows for the control of the spatial chirp of the CPA laser chain, while its longitudinal position in the divergent beam enables the control of the amount of longitudinal chromatism in a wide dynamic range. The performance of this technique is evaluated by measuring main spatio-temporal couplings with a simple method, based on an ultrafast pulse shaper, which allows for a selection of narrow windows of the spectrum., Comment: 9 pages, 5 figures 1 supplemental document : 2 pages, 2 figures

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic comb at 1.03 µm, delivering sub-picosecond signal pulses across 1.5–1.7 µm from a MgO-doped periodically poled L i N b O 3 crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 µm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach–Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from one to 14, allowing the OPO to operate with a flexible repetition rate from 1 to 14.2 GHz. A strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.

Pulses at 744 nm with 90 fs duration, 6 mJ energy, and a weakly divergent wavefront propagate for more than 100 m and generate a filament followed by an unprecedently long high intensity ( ≥ 1 T W / c m 2 ) light channel. Over a 20 m long sub-section of this channel, the pulse energy is transferred continuously to the infrared wing, forming spectral humps that extend up to 850 nm. From 3D+time carrier-resolved simulations of 100 m pulse propagation, we show that spectral humps indicate the formation of a train of femtosecond pulses appearing at a predictable position in the propagation path.

International audience; Postfilamentation channel resulting from filamentation of freely propagating 744-nm, 5-mJ, 110-fs pulse in the corridor air is examined experimentally and in simulations. The longitudinal extension of postfilament was determined to be 55–95 m from the compressor output. Using single-shot angle-wavelength spectra measurements, we observed a series of red-shifted maxima in the spectrum, localized on the beam axis with the divergence below 0.5 mrad. In the range 55–70 m, the number of maxima and their red-shift increase with the distance reaching 1 μm, while the pulse duration measured by the autocorrelation technique is approximately constant. Further on, for distances larger than 70 m and up to 95 m, the propagation is characterized by the suppressed beam divergence and unchanged pulse spectrum. The pulse duration increases due to the normal air dispersion.

The investigation of ultrafast molecular dynamics is of great importance towards the understanding of a variety of natural phenomena in physical and chemical sciences. With the rapid development of femtosecond laser systems and precision detection technologies, it is possible now to visualize and steer the motion of molecules in matter as well as the ultrafast dynamics of electrons and nuclei in molecules on a microscopic timescale. When a molecule is exposed to a strong laser field, its electrons can be freed or excited, which often triggers a rapid dissociation of the system, in which the released electrons and nuclei exhibit a strong correlation, while the electronic motion on attosecond timescale is much faster than that of the nuclear ranging from femtosecond to picosecond timescale. One example is the absorbed photon energy shared between electrons and nuclei in a molecular dissociation process. In addition, a lot of physical processes such as electron tunneling ionization show a strong dependence on the spatial direction of the molecular axes with respect to the polarization of the laser field. Confining molecular axes to a specific direction is significant in unambiguously understanding and controlling the ultrafast response of the molecules to the laser fields. Impulsive alignment induced by ultrafast laser pulses with field-free revivals is considered as a powerful method to control the spatiotemporal distribution of the molecules. Field-free alignment is also recognized as an important tool for studying basic physical processes such as rotational dynamics of complex molecules and ultrafast collisional dissipation.By using advanced technologies based on intense femtosecond laser systems, multi-particle coincidence measurements of Cold target recoil ion momentum spectroscopy (COLTRIM), and high-sensitive time-resolved birefringence detections, this thesis reports a variety of studies revealing ultrafast dynamics observed in molecules exposed to strong laser fields, including dissociative Rydberg excitation, tunneling-site-sensitive ionization and dissociation, molecular alignment, rotational echo, and ultrafast collisional dissipation probed by rotational alignment echo., L'étude de dynamiques ultra-rapides dans les molécules revêt une importance particulière pour la compréhension de divers phénomènes naturels en sciences physiques et chimiques. Avec le développement rapide des lasers femtosecondes et de méthodes précises de détection, il est possible de visualiser et de contrôler la dynamique ultra-rapide des électrons et des noyaux de molécules à des échelles microscopiques. Lorsqu'une molécule est exposée à un champ laser intense, ses électrons peuvent être ionisés ou excités, ce qui dans certains cas entraine une dissociation de la molécule, processus dans lequel les mouvements des électrons et des noyaux sont fortement corrélés. Un des exemples est le partage corrélé de l'énergie des photons absorbés entre les électrons et les noyaux dans le processus de dissociation moléculaire. Les processus d’ionisation ou de photodissociation, de même que de nombreux processus d’interaction laser-matière, dépendent de la direction des axes moléculaires par rapport à la polarisation du champ laser excitateur. Les lasers ultra-courts permettent d’aligner efficacement ces axes moléculaires suivant des directions de l’espace, alignement qui perdure un certain temps après extinction du champ laser. Au-delà du fait qu’aligner des molécules permet d’optimiser l’interaction laser matière, ou encore d’analyser le système moléculaire dans son propre référentiel, il permet également d’observer la dynamique rotationnelle des molécules et leur dissipation collisionnelle à des échelles de temps ultra-courtes.En utilisant des techniques avancées basées sur les laser femtosecondes amplifiées, les mesures de coïncidence multi-particules par spectroscopie COLTRIM (Cold target recoil ion momentum) et la détection de biréfringence résolue en temps, cette thèse présente un certain nombre d’études de dynamiques ultra-rapides observées dans des molécules soumises à des champs laser intenses, incluant l’excitation Rydberg dissociative, l’ionisation tunnel, l’alignement moléculaire post-impulsionnel et les échos rotationnels, et la dissipation collisionnelle ultra-rapide sondées par les échos rotationnels.

International audience; We present an ultrafast thin-disk based multipass amplifier operating at a wavelength of 1030 nm, designed for atmospheric research in the framework of the Laser Lightning Rod project. The CPA system delivers a pulse energy of 720 mJ and a pulse duration of 920 fs at a repetition rate of 1 kHz. The 240 mJ seed pulses generated by a regenerative amplifier are amplified to the final energy in a multipass amplifier via four industrial thin-disk laser heads. The beam quality factor remains ∼ 2.1 at the output. First results on horizontal long-range filament generation are presented.

Nonlinear signals from metal and crystalline nanostructures are known to be highly polarization-dependent, due to their local symmetry properties originating from their bulk and/or surface responses. In nanostructures of sizes below the diffraction limit, polarized signals are generally analyzed by averaging polarization dependencies over the spatial extent of their diffraction-limited image spot (~300 nm). This approach misses however the possibility to exploit the spatial specificity of their polarization response. In this work, we develop a novel polarized nonlinear microscopy method that exploits sub-diffraction resolution information. Fourier analysis of the polarization modulated nonlinear signal is performed on over-sampled, drift-corrected images (50 nm pixel size). Even though the resulting nonlinear image is a diffraction-limited spot size, the information gained by polarization-induced modulation signals provides a higher level of spatial selectivity that is directly related to the local optical response of the investigated system, at a scale below the diffraction limit. The gain in spatial scale is due to the additional spatial sensitivity brought by polarization. This approach is applied to polarized second harmonic generation imaging in plasmonic nanostructures (150 nm size) of multi-branched shapes, in which the vectorial nature of the local field confinement can be retrieved with a resolution of 40 nm using polarized nonlinear microscopy. We also demonstrate the possibility to image spatial heterogeneities within crystalline ferroelectric BaTiO3 nanoparticles of 100 nm to 500 nm size, emphasizing in particular the existence of a centrosymmetric shell in small size structures. At last, KTiOPO4 nanocrystals which are ideal candidates for well-reported efficient nonlinear emission, have been used as nanoprobes of spatial local polarization states of a focused beam. We have developed a method based on phase and polarization wavefront shaping to create any desired sub-resolution vectorial pattern and studied the possibility to measure locally such polarization state exploiting the sum frequency polarized signal from such crystals. These studies show promising applications in the use of coherent vectorial probes in complex media., Les signaux non linéeaires venant de nanostructure métallique et cristallines sont connus pour être fortement dépendant à la polarisation. Ceci est du à leur propriété de symétrie locale, c'est a dire de leur réponse volumique ou surfacique. Les signaux de polarisation venant de nanostructures de taille inférieur à la limite de diffraction sont mesurés avec un spot limité par la diffraction (~300 nm) ce qui représente la moyenne du signal. Cette technique a pour défaut de perdre l'information spatiale du signal de polarisation. Nous avons développé une nouvelle méthode de microscopie à polarisation non-linéaire qui exploite l'information en dessous de la limite de diffraction. Une analyse de Fourier d'un signal non linéaire a été faite en dessous de la limite de diffraction sur une image sur-échantillonnée et corrigé par translation (taille du pixel = 50 nm). Le gain en résolution est du à la sensibilité spatiale de la polarisation. Pour ce faire, nous avons mesuré un signal polarisé de seconde harmonique de nanostructures plasmoniques de différentes formes (~150 nm). En effet la nature vectorielle du champs local confiné peut être retrouvé avec une résolution de 40 nm en utilisant la nanoscopy polarisée non linéaire. Nous avons par ailleurs montré que nous pouvons imager l'hétérogénéité spatiale de nanoparticules ferroélétriques cristallines (BaTiO3) de taille allant de 70 nm à 500 nm. Ceci prouve l'existence de coque centrosymétrique dans des petites structures. Enfin, les nanocristaux de KTiOPO4 nanostructures sont les candidats idéaux pour la générations de signaux non linéaires bien maîtrisée. Ils ont été utilisés comme nanosondes de la répartition spatiale des états de polarisation dans des faisceaux focalisés. Nous avons utilisé une méthode à base de contrôle de front d'onde en phase et polarisation pour créer un quelconque faisceau vectoriel sub-résolu, et étudié la possibilité de mesurer localement une telle polarisation en exploitant la génération de somme de fréquence de tels cristaux. Ces études peuvent donner à des applications intéressantes pour l'utilisation de sondes vectorielles cohérentes dans les milieux complexes.

International audience; We investigate the ultrafast absorption dynamics of fused silica irradiated by a single 500 fs laser pulse in the context of micromachining applications. A 60-fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of 0:5 ps after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction.

International audience; During last decade, laser-based synthesis of nanoparticles and nanostructures has attracted particular attention [1]. Nanoparticles demonstrate unique plasmonic and/or photoluminescent properties, as well as a capacity of field amplification. These effects are essential in many promising applications, such as imaging, sensors, photodynamic therapy, etc. Many of medical applications require the absence of any toxicity and/or of the incompatibility with biological tissues. Nanoparticle and nanostructure generation by laser ablation provides possibilities to respect such strict requirements [2]. This study is aimed at the better understanding of the mechanisms involved in nanoparticle formation by laser ablation and at the possibilities of the process optimization. For this, we carry out very detailed simulation based on combined numerical methods. In particular, we focus our attention on the role of laser parameters and ambient environment in both target decomposition and following laser plume dynamics. The obtained results are also used to explain several promising experiments where nanoparticles and nanostructures are obtained with unique properties that are impossible to reproduce by other methods.[1] D. B. Geohegan et al., Appl.Phys. Lett. 72, 2987 (1998)[2] S. Besner et al., Appl. Phys. A 93, 955-959 (2008)

Electric currents circulating between charged clouds and the earth surface during lightning discharges are responsible for considerable damages and casualties. It is therefore important to develop better protection methods in addition to the traditional Franklin rod. Here we present the first demonstration that filaments formed by short and intense laser pulses can guide lightning discharges over considerable distances. We believe that this experimental breakthrough will lead to progress in lightning protection and lightning physics. An experimental campaign was carried out on the S\"antis Mountain in Northeastern Switzerland during the Summer of 2021 with a high repetition rate terawatt laser. The guiding of an upward negative lightning leader over a distance of 50 m was recorded by two separate high-speed cameras. The guiding of negative lightning leaders by laser filaments was corroborated in three other instances by VHF interferometric measurements, and the number of X-ray bursts detected during guided lightning events was significantly increased. While this research field has been very active for more than 20 years with many research groups around the world working to achieve this goal, this result demonstrates lightning guiding by lasers, which may lead to the development of a laser lightning rod. This work paves the way for new atmospheric applications of ultrashort lasers and represents a significant step forward in the development of a laser based lightning protection for airports, launchpads or large infrastructures., Comment: Laser-guided lightning. Nat. Photon. (2023)

International audience; Single shot hybrid fs/ps-CARS spectroscopy of N 2 is demonstrated at repetition rate up to 5 kHz using an amplified probe delivering a constant energy per pulse between 1 and 5 kHz. We performed 5 kHz CARS thermometry in a laminar CH 4 /air flame and in ambient air, with a precision under 0.5% at typical flame temperature, which is 2 times more precise and 5 times faster than previous state of the art with this technique. Temperature was measured during long acquisition times up to 100 s, making the system suitable to record signals in the 0.01-2500 Hz spectral window; in our case 10 Hz temperature oscillations were probed.