Supercontinuum generation of ultrashort laser pulses in air at different central wavelengths

Supercontinuum generation by femtosecond filaments in air is investigated for different laser wavelengths ranging from ultraviolet to infrared. Particular attention is paid on the role of third-harmonic generation and temporal steepening effects, which enlarge the blue part of the spectrum. A unidirectional pulse propagation model and nonlinear evolution equations are numerically integrated and their results are compared. Apart from the choice of the central wavelength, we emphasize the importance of the saturation intensity reached by self-guided pulses, together with their temporal duration and propagation length as key players acting on both supercontinuum generation of the pump wave and emergence of the third harmonics. Maximal broadening is observed for large wavelengths and long filamentation ranges.Comment: 10 pages, 11 figure

Femtosecond Laser Filamentation for Atmospheric Sensing

Powerful femtosecond laser pulses propagating in transparent materials result in the formation of self-guided structures called filaments. Such filamentation in air can be controlled to occur at a distance as far as a few kilometers, making it ideally suited for remote sensing of pollutants in the atmosphere. On the one hand, the high intensity inside the filaments can induce the fragmentation of all matters in the path of filaments, resulting in the emission of characteristic fluorescence spectra (fingerprints) from the excited fragments, which can be used for the identification of various substances including chemical and biological species. On the other hand, along with the femtosecond laser filamentation, white-light supercontinuum emission in the infrared to UV range is generated, which can be used as an ideal light source for absorption Lidar. In this paper, we present an overview of recent progress concerning remote sensing of the atmosphere using femtosecond laser filamentation

Ultrashort filaments of light in weakly-ionized, optically-transparent media

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure

Filamentation induced nonlinear optics

La filamentation du laser femtoseconde provient d'un équilibre dynamique entre l’autofocalisation Kerr et la défocalisation par le plasma autogénéré produit de l’ionisation multiphotonique/tunnel des molécules dans l'air. Ce phénomène a attiré beaucoup d’attention des scientifiques telles que la télédétection de polluants atmosphériques et l'identification moléculaire par l'alignement des molécules. Cependant, il y a une multitude de processus non linéaires lors de la filamentation. Quant à l'application, il est important d'avoir une compréhension des mécanismes physiques présents lors de la filamentation induite par l’optique non linéaire. Étant donné de nombreux de phénomènes et d’applications de la filamentation, cette thèse se concentre sur une partie de ces aspects. Ceux-ci sont la rotation de la polarisation laser dans les gaz atomiques/moléculaires, le processus d’émission laser des molécules d'eau dans l'étalonnage air, lde l'humidité à travers la spectroscopie induite par un filament, ainsi que le renforcement de la fluorescence par un réseau de diffraction de plasma. La rotation de la polarisation laser d'une sonde polarisée initialement linéaire a été étudié dans les gaz atomiques/moléculaires. Dans les gaz atomiques, la biréfringence ultrarapide induite par l’effet Kerr a été mesurée quantitativement. Dans les gaz moléculaires, la biréfringence et les états de polarisation de la production de la sonde ont été modulés à la renaissance rotationnelle de la molécule. Également, nous avons étudié expérimentalement la fluorescence induite par filament à partir des fragments dissociés dans l'air. Les émissions de fluorescence des radicaux libres OH à 308.9 nm et NH à 336.0 nm ont été observés dans l'air. La fluorescence rétrodiffusée par le groupement OH et le groupement NH présentait une augmentation exponentielle accompagnant l'augmentation de la longueur du filament qui indique l’existence de l'émission spontanée amplifiée (ASE). En plus, on étudie la spectroscopie de fluorescence induite par filament à partir du réseau de diffraction pour le plasma. Le réseau de diffraction pour le plasma a été généré par des filaments non colinéaires qui se superposés et synchronisés temporellement dans l'air. Une série de spectres des fragments excités du CN a été observée. L’intensité de fluorescence du radical CN en utilisant un réseau de diffraction par le plasma est beaucoup plus forte que celle utilisant des filaments séparés temporellement.Femtosecond laser filamentation, which originates from a dynamic equilibrium between Kerr self-focusing and defocusing by the self-generated plasma produced by multiphoton/tunnel ionization of air molecules, has attracted a lot of scientific applications such as remote sensing of atmospheric pollutants, molecular identification by the alignment of molecules, etc. However, there are many nonlinear processes taking place during filamentation. From the application point of view, it is important to have a good understanding of the detailed physics behind filamentation induced nonlinear optics. Since there are many nonlinear phenomena and applications for filamentation, the thesis only focuses on few aspects of filamentation. Those are: the polarization rotation in atomic/molecular gases, the lasing action of water molecules in air, the humidity calibration through the filament-induced spectroscopy, as well as the fluorescence enhancement by plasma grating. The polarization rotation of an initially linearly polarized probe pulse was studied in atomic/molecular gases. In atomic gases, the ultrafast birefringence induced by Kerr effect was quantitatively measured. In molecular gases, the birefringence and the polarization states of the output probe were modulated at the rotational revival of molecule. We also experimentally investigate the filament-induced fluorescence from the dissociated fragments in air. Fluorescence emissions from OH free radicals at 308.9 nm and NH free radicals at 336.0 nm were observed in air. The backscattered fluorescence from both OH and NH exhibited an exponential increase with increasing filament length, indicating amplified spontaneous emission. We have further investigated the filament-induced fluorescence spectroscopy from a plasma grating. The plasma grating was generated by non-collinearly overlapping temporally synchronized filaments in air. A series of spectral lines from the excited fragments of CN was observed. The fluorescence intensity from CN radicals in plasma grating was much stronger as compared to the case of temporally separated filaments

Polarization characterization of nonlinear laser propagation

High power laser pulses propagate nonlinearly in media transparent to their wavelength. Self-induced nonlinearities lead to strong modifications of the spatial, spectral as well as temporal characteristics of the pulse. The interaction of the high power laser pulses with the propagation medium leads to partial ionization of the latter. This highly complex and dynamic nonlinear process, induced by the high power laser pulses, with a narrow high intensity core that is maintained over long distances is known as filamentation . Filaments created from a linearly polarized electric field have been studied by different research groups. However, it is interesting to know how the properties of the filament change with polarization and most importantly whether the polarization state is maintained during the nonlinear propagation of the laser pulse. This dissertation addresses the polarization evolution of a laser beam going through a nonlinear medium which can be expanded to filamentation. It is shown that the presence of the nonlinear interaction before the starting point of the filament, leads to polarization modification of an initially elliptically polarized light and the rotation of the polarization ellipse with propagation can be used to measure the intensity. Filaments generated in different gas media are also investigated to provide a better understanding of matter-light interaction when we deal with a highly nonlinear process such as filamentation. For gases such as N2 and O2, an extra component of optical nonlinearity due to the delayed rotational response (molecular alignment) is present which has been shown to have an effect on propagation and properties of the filament. Instabilities of the polarization of the filament lead to observation of an anomalous spectral broadening phenomenon due to molecular effects which is further investigated with a theoretical model. This dissertation leads to recognition of polarization as a new dimension to characterize nonlinear laser propagation and facilitates polarization dependent applications such as THz emission and supercontinuum generation

Influence of tunnel ionization to third-harmonic generation of infrared femtosecond laser pulses in air

Here we present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5μm. At low intensities, longitudinal phase matching is dominating in the formation of 3rd harmonics, whereas at higher intensities locked X-waves are formed. We provide the arguments that the X-wave formation is governed mainly by the tunnel-like ionization dynamics rather than by the multiphoton one. Despite of this fact, the impact of the ionization-induced nonlinearity is lower than the one from bound–bound transitions at all intensities. © 2020, The Author(s)

Influence of tunnel ionization to third-harmonic generation of infrared femtosecond laser pulses in air

Here we present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5μm. At low intensities, longitudinal phase matching is dominating in the formation of 3rd harmonics, whereas at higher intensities locked X-waves are formed. We provide the arguments that the X-wave formation is governed mainly by the tunnel-like ionization dynamics rather than by the multiphoton one. Despite of this fact, the impact of the ionization-induced nonlinearity is lower than the one from bound–bound transitions at all intensities. © 2020, The Author(s)