Filamentation Regime Research Articles

Effect of focusing element-induced aberrations on filamentation and supercontinuum emission in ambient air.

Femtosecond laser pulse induced filamentation in atmosphere is susceptible to a number of input laser, focusing optics and medium characteristics. Filamentation of fs pulses in atmosphere is an intense propagation regime where the focusing geometry used to focus the fs laser pulses play an important role influencing the filament intensity and the associated supercontinuum. We identified different optical elements used for focusing the fs laser pulses leading to filamentation in air and classified them according to the induced aberrations. To clearly identify the role of aberrations, all the optical elements were taken to have same focal length. The subsequent filament structure and emissions from the filament were correlated with the aberrations induced by optical element revealed stark differences. The onset of the filamentation, its longitudinal intensity and the associated supercontinuum emission (SCE) have varied drastically with the aberrations induced by the focusing optics. A systematic study directed to choose and identify suitable optical elements according to the usage of the fs pulses for a specific filamentation regime is presented.

Micro glass welding with millimeter‐long filaments

AbstractThe welding of glass with ultrashort‐pulsed lasers (USP) is usually conducted with microscope objectives. Their short working distance limits the thickness of the work piece, the usable laser power and the feed rate of the process. To increase the possible dimensions of the welding partners and the process speed to industrial levels, we present USP welding with a galvoscanner and a common F‐theta lens. Differences between the well‐known breakdown regime and the filamentation regime are discussed and tensile measurements are shown.

Ultrafast Laser Welding: Taming Ultrafast Laser Filaments for Optimized Semiconductor–Metal Welding (Laser Photonics Rev. 15(2)/2021)

In article number 2000433, Maxime Chambonneau, Qingfeng Li, Stefan Nolte, and co-workers demonstrate semiconductor–metal ultrafast laser welding—an additive manufacturing technique limited to other material families so far. This achievement is made possible by the determination and the precompensation of the nonlinear focal shift in the filamentation regime. This work offers new insights for the fabrication of microelectronic devices.

The laser lightning rod project

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 1 ps duration at 1 kHz. 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 Säntis mountain will assess the laser ability to trigger upward lightning.

Generation of Supercontinuum in Filamentation Regime in a Water Droplet Containing Silver Nanoparticles at Low Temperature

Generation of supercontinuum (SC) was studied experimentally both in water droplets containing silver nanoparticles (NP) in the temperature range from 2 to 22°C and in droplet ice floes frozen to –15°C. It is established that intensity of SC emission exponentially decays along the droplet diameter following excitation by a train of femtosecond laser pulses and linearly increases with increase in NP concentration. Spectrum of SC emission generated in a water droplet containing NPs is investigated in the presence of localized plasmons producing fluorescence in the vicinity of 430 nm. Propagation of a thermal wave along the diameter (d = 1.0 mm) of a small frozen droplet at a speed of 190 mm/s accompanied by exponentially decaying SC emission is discovered. Mathematical model of heat-transfer processes in an ice floe upon thermal-wave formation is proposed.

Electronic and vibrational processes in absorbing liquids in femtosecond laser sub- and filamentation regimes: ultrasonic and optical characterization

The ultrasonic characterization of the propagation of femtosecond laser pulses in pure transparent water and colored phenol-red dye solutions at variable concentrations indicates their thermoacoustic pressure generation due to intrinsic multi-photon absorption in the low-power sub-filamentation regime and sub-linear pressure response of sub-critical plasma via free-carrier absorption in the filamentation regime. Multi-photon absorption appears to be concentration dependent for water and dye solutions, while plasma absorption demonstrates a universal trend for all these liquids. In the filamentation regime, through the non-linear laser-matter interactions, not only new optical frequencies (intense white-light supercontinua) were generated, but supercontinuum-based ultrafast broadband spectroscopy in the filaments also revealed many additional transient absorption bands of the dye molecules, induced via power-dependent and concentration-independent stimulated Raman scattering.

Superfilamentation in air reconstructed by transversal interferometry

Multiple filamentation of terawatt femtosecond Ti:sapphire laser beams in air is experimentally investigated for intermediate external focusing conditions with the numerical apertures $\mathrm{NA}={10}^{\ensuremath{-}2}$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The electron density distribution in a multifilament plasma channel is measured by means of transversal interferometry. The results confirm the existence of superdense filaments (superfilaments). It is also found that the maximum electron density in filaments differs by an order of magnitude ($\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and $\ensuremath{\sim}9\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$) for the two values of NA. This demonstrates high sensitivity of the multiple filamentation regime to focusing conditions. The interferometric results are compared with the spatially resolved measurements of the luminescence intensity from the multifilament plasma channel to verify the accuracy of the widely used luminescence technique.

Diffraction-ray tube analysis of ultrashort high-intensity laser pulse filamentation in air

The results of theoretical simulation of femtosecond Ti:Sapphire laser pulse propagation in air in the self-focusing and filamentation regimes are presented. The self-focusing of pulsed radiation was analyzed based on the diffraction-ray tracing method, within which the beam power propagates within specific light structures, known as the diffraction-ray tubes. These tubes do not intersect in space and do not exchange their energy, but the changes in their shape and cross-section reflect the physical effects occurring with the radiation during its propagation through the medium. This allows discovering the formation of specific light structures in a laser beam during its self-focusing. One of these structures is the energy-replenishing diffraction-ray tube (ERT), which provides the filamentation domain with the necessary light energy and exists also in the form of a high-intensity light channel during the post-filamentation propagation of a pulse. The dependences of the radius and power of this energy-replenishing tube on the initial beam radius and peak radiation power at a fixed pulse length are derived. It is revealed that the radiation energy expenditure for filamentation decreases as the beam radius increases. The peak power in ERT does not exceed the critical self-focusing power for a Gaussian beam during the post-filamentation propagation of a pulse and weakly depends on the initial pulse parameters.

Energy, spectral, and angular properties of post-filamentation channels during propagation in air and condensed media

High-power femtosecond laser pulses undergo large-scale transformations during filamentation in a nonlinear medium. Spatial transformations result in the formation of the post-filamentation channels—intense spatially localized light structures observed in the laser beam at the plasma-free stage of pulse propagation. We present the experimental results on the main characteristics of post-filamentation channels formed by the filamentation of Ti:sapphire laser pulses (744 nm, 90 fs) in air. Angular divergence, spectral composition, power, and temporal parameters of post-filamentation channels at different initial pulse energy are investigated. We study a specific cascade pulse filamentation regime when the post-filamentation channel is initially formed in air and then experiences recurrent filamentation in a condensed medium (fused silica).

Optimizing the third harmonic generated from air plasma filaments pumped by femtosecond laser pulses

Third-harmonic (TH) generation inside air plasma filaments provides a simple method to convert near-infrared laser pulses in the UV range. Here, we report on the optimization of the third-harmonic generation during filamentation in the parameter space of focusing geometry, gas pressure, and incident laser pulse energy. It is observed that, in the nonlinear filamentation regime, the TH yield is largely dominated by the geometrical focusing condition and gas pressure. Using ambient air as the medium for filamentation, an optimal numerical aperture (NA) = 0.07 was found for a laser pulse with a peak power close to and above the critical power for self-focusing. For more loose focusing geometry (NA<0.047), reduction of the air pressure can improve the conversion efficiency. In this case, the spatial mode of the third harmonic was also found to be more uniform than in ambient air. With the scanning pinhole and crossing-filaments methods, it is revealed that in the situation of optimized focusing geometry (NA = 0.07) the TH accumulates constructively along the filament and exits at its end. In contrast, for the loose focusing geometry, the destructive interference effect of the TH generated at the leading and trailing edges of the filaments decreases the TH yield efficiency. This study provides a practical guidance for efficient conversion of near-infrared femtosecond pulses to the UV domain.

A Method of Laser Coloration in Experiments on Filamentation of Individual Impulses and the Formation of a Light Bullet in a Homogeneous Transparent Dielectrics

The advantages of using the method of laser coloration for investigation of specific features of filamentation in homogeneous transparent dielectrics are described. The results of several experiments conducted with a lithium fluoride crystal by this method are presented. The possibility is shown of obtaining data on the structure of the light field in a filament formed by a single laser pulse and processing them at a substantially later time after the experiment, which is not available by using other methods, allowed observing a plasma-free regime of filamentation, detecting single-cycle light bullets, and demonstrating their robustness.

Two-stage filamentation of 10 μm pulses as a broadband infrared backlighter in the atmosphere.

We identify a two-stage filamentation regime for high-power 10μm multipicosecond pulses propagating in the atmosphere. The first low-intensity stage is mainly regularized by ionization through excitation induced dephasing, which can lead to strong pulse shortening downstream. This shortening in turn causes a significant reduction of the many-body induced plasma, which changes the dynamics drastically. As a result, a distinct second stage is predicted where peak intensities are clamped at 1 order of magnitude higher than in the first stage. The complex dynamics found in the second stage can result in the spatial and temporal breakup of the wavepacket, reduction of ionization losses, and extreme spectral broadening.

Chirp-controlled filamentation and formation of light bullets in the mid-IR.

Formation of light bullets-tightly localized in space and time light packets, retaining their spatiotemporal shape during propagation-is, for the first time, experimentally observed and investigated in a new regime of mid-infrared filamentation in ambient air. It is suggested that the light bullets generated in ambient air by multi-mJ, positively chirped 3.9-μm pulses originate from a dynamic interplay between the anomalous dispersion in the vicinity of CO2 resonance and positive chirp, both intrinsic, carried by the driver pulse, and accumulated, originating from nonlinear propagation in air. By adjusting the initial chirp of the driving pulses, one can control the spatial beam profile, energy losses, and spectral-temporal dynamics of filamenting pulses and deliver sub-3-cycle mid-IR pulses in high-quality beam on a remote target.

Effect of group velocity dispersion on femtosecond filamentation of Bessel – Gaussian beams

Laser colouration technique is employed to investigate the structures of colour centres induced in an isotropic LiF crystal by Bessel – Gaussian and Gaussian beams in the single-pulse filamentation regime. Femtosecond laser pulses at radiation wavelengths corresponding to zero (1250 nm) and anomalous (1900 nm) group velocity dispersions were focused into the sample using an axicon or a spherical lens. As shown experimentally and numerically, with increase in radiation energy the length of the structure of colour centres (or the plasma channel length) increases in the domain of zero group velocity dispersion but remains invariable under conditions of anomalous dispersion, when there emerge light bullets. The path length of the light bullets amounts to about 200 μm and does not depend on the pulse energy and the way of focusing, which confirms their robustness.

Filamentation of an Ultrashort Laser Pulse in a Medium with Artificial Nonlinearity

The introduction of a small amount of subwavelength gold nanoparticles (extinction coefficient ∼0.01–1 cm−1) to water allows the efficient control of filamentation owing to nonlinear dissipative losses of a primary ultrashort pump laser pulse and secondary broadband radiation (supercontinuum), because the intrinsic extinction coefficient in the multiple filamentation regime is below 0.04 cm−1. Optical emission and photoacoustic spectroscopy studies have shown that an increase in losses in a colloidal solution sublinearly increases the power of an ultrashort laser pulse for the nonlinear generation of supercontinuum with the same intensity, and it also increases the filamentation threshold near the supercontinuum generation threshold. At the same time, supercontinuum emission is significantly enhanced in the region of plasmon resonance of nanoparticles against the background of broadband nonlinear absorption. The results demonstrate new possibilities of controlling the nonlinear optical characteristics of media for the generation and filamentation of ultrashort light pulses in various spectral ranges.

Multi-mJ mid-IR light bullets in air

We examine mid-IR light bullets generated in ambient air. 2-optical cycle pulses confined in space are generated in filamentation regime. Few-fold solitonic self-compression is achieved for strongly chirped mid-IR pulses.

Метод лазерной колорации в экспериментах по филаментации одиночных импульсов и образованию световых пуль в однородных прозрачных диэлектриках

AbstractThe advantages of using the method of laser coloration for investigation of specific features of filamentation in homogeneous transparent dielectrics are described. The results of several experiments conducted with a lithium fluoride crystal by this method are presented. The possibility is shown of obtaining data on the structure of the light field in a filament formed by a single laser pulse and processing them at a substantially later time after the experiment, which is not available by using other methods, allowed observing a plasma-free regime of filamentation, detecting single-cycle light bullets, and demonstrating their robustness.

Robust Remote Sensing of Trace-Level Heavy-Metal Contaminants in Water Using Laser Filaments.

Water is the major natural resource that enables life on our planet. Rapid detection of water pollution that occurs due to both human activity and natural cataclysms is imperative for environmental protection. Analytical chemistry–based techniques are generally not suitable for rapid monitoring because they involve collection of water samples and analysis in a laboratory. Laser‐based approaches such as laser‐induced breakdown spectroscopy (LIBS) may offer a powerful alternative, yet conventional LIBS relies on the use of tightly focused laser beams, requiring a stable air–water interface in a controlled environment. Reported here is a proof‐of‐principle, quantitative, simultaneous measurement of several representative heavy‐metal contaminants in water, at ppm‐level concentrations, using ultraintense femtosecond laser pulses propagating in air in the filamentation regime. This approach is straightforwardly extendable to kilometer‐scale standoff distances, under adverse atmospheric conditions and is insensitive to the movements of the water surface due to the topography and water waves.

Effect of Group Delay Dispersion on Femtosecond Pulse Self-shortening in Kerr Media: Investigation of Femtosecond Pulse Self-shortening Phenomenon Based on the Transient Regime of Multiple Filamentation in Kerr Media with Different Group Delay Dispersion

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Transition from linear- to nonlinear-focusing regime of laser filament plasma dynamics

Laser filament properties, including the plasma induced by the filamenting pulse in the medium, depend on the numerical aperture (NA) of the focusing optics used to create them. Recent studies of this dependence have revealed two distinct linear and non-linear filamentation regimes. High-resolution spatial and temporal electron density measurements are presented demonstrating the transition from the linear to nonlinear focusing regime. This study shows that the dominance of geometrical focusing in the linear (high NA) regime produces plasma with high peak densities and large plasma diameters, while filamentation in the nonlinear regime, equivalent to long distance filamentation, leads to low peak densities and small plasma diameters.