Many atmospheric laser applications rely on the long-range propagation of laser pulses. In this work, we demonstrate a method for remotely shaping laser beams by generating stable gaseous optical elements in air. These structures emerge from controlled gas dynamics driven by filamentation of a kilohertz picosecond Yb:YAG laser. We observe a new, to the best of our knowledge, air photonic structure-a tunable defocusing lens-attributed to the cumulative effects of low-density channels. Additionally, we show that self-focusing of a collimated Laguerre-Gauss beam can form a permanent air waveguide of 6 mm extending over more than 20 meters. Numerical simulations of the gas dynamics reproduce the observed features of a continuous wave (CW) probe beam injected into these optical structures. This air photonics strategy has broad potential applications in laser power modulation and atmospheric optics.

We demonstrate a very strong laser gain in the near UV, at 391 nm, involving one- and two-photon transitions between initial and final levels of the transition. This opens the way to the realization of all-optical lasers at short wavelengths. The amplifying medium is N2+ at low density, pumped by an intense laser source at a longer wavelength, i.e., 800 nm. It is probed by measuring with femtosecond precision the absorption of XUV high harmonics through the gas, displaying the evolution of the relevant electronic levels during the amplification process. We show the role of a third level forming a "V scheme" with the initial and final levels.

Femtosecond laser filamentation in air presents significant potential for atmospheric scale applications, including remote sensing and lightning control. However, projecting a filament over large distances remains a challenge due to the complex nonlinear propagation dynamics of high-intensity femtosecond pulses. This work demonstrates the generation of an intense ring-Airy-beam femtosecond pulse in air, employing a specially designed phase plate that converts an input quasi-Gaussian beam into the Fourier transform of a ring Airy beam at the front focal plane of a Fourier lens. The high damage threshold of the phase plate enables the corresponding near field of the ring Airy beam at the rear focal plane of the lens to reach a tens-of-gigawatt peak power. This near field subsequently evolves, at a distance of 10 m, into an ionizing filament, capable of ablating an aluminum plate and performing laser-induced breakdown spectroscopy (LIBS). Extending the cm-scale propagation of a ring Airy beam in liquids or glasses, as reported in previous works, to the 10-meter scale with the generation of an ionizing filament, represents a crucial step toward real-world applications of remote LIBS with ring-Airy-beam femtosecond pulses. Experimental observations show a filament length of two meters, corroborating the numerical simulation and expanding the potential to applications that necessitate a long plasma channel. Further numerical simulations indicate the feasibility of controlling the starting point of a filament beyond 100 meters by increasing the focal length of the Fourier lens, opening avenues for fascinating applications in remote sensing and beyond.

We study experimentally and numerically the filamentation produced by various Laguerre gaussian (LG) beams in air and demonstrate their ability to guide electric discharges over longer distances than gaussian beams. (c) 2024 The Author(s)

We study the use of Laguerre-Gaussian (LG) femtosecond laser filament with multi GW peak power to guide electric sparks in the atmosphere. We demonstrate that an LG beam with a vortex phase or with 6 azimuthal phase steps generates a filamentation regime, where a longer and more uniform energy deposition is produced compared to a normal beam with a flat phase. Such filaments can guide electric discharges over much longer distances. This technique could significantly extend the guiding range of laser filaments for lightning control and other long-range atmospheric experiments involving filamentation. (c) 2024 Optica Publishing Group

Electromagnetic radiation within the terahertz (THz) frequency range is of great interest for applications in remote sensing and time-domain spectroscopy. The laser-induced plasmas are promising mediums for generating THz radiation. It has been recently reported that focusing femtosecond Bessel pulses inside dielectrics induces a high aspect ratio over-critical plasmas. Here, we show that the intense resonantly driven electrostatic fields at the so-called critical surface lead to THz radiation emission. Through three-dimensional particle-in-cell simulation and analytical derivation, we have investigated the emission of THz radiation. We show that the THz radiation is associated with a hot population of electrons trapped in ambipolar electric fields of the double layers.

We demonstrated a method for in situ temporal characterization of an intense femtosecond laser pulse around its focus where the laser intensity exceeds 1014 W/cm2. Our method is based on the second harmonic generation (SHG) by a relatively weak femtosecond probe pulse and the intense femtosecond pulses under analysis in the gas plasma. With the increase in the gas pressure, it was found that the incident pulse evolves from a Gaussian profile to a more complicated structure featured by multiple peaks in the temporal domain. Numerical simulations of filamentation propagation support the experimental observations of temporal evolution. This simple method can be applied to many situations involving femtosecond laser–gas interaction, when the temporal profile of the femtosecond pump laser pulse with an intensity above 1014 W/cm2 cannot be measured in traditional ways.

Ultrahigh intensity contrast, short pulse laser-solid interactions offer an attractive platform for investigating high-energy density matter, particularly in the context of structured and ultra-thin targets that form hot, dense plasma conditions. Harmonic generation can improve the contrast of laser pulses by several orders of magnitude. In this study, we present the characterization of extreme contrast, relativistic intensity second-harmonic (SH) pulses at 400 nm, using the self-diffraction frequency-resolved optical gating (SDFROG) technique. The 400 nm pulses were generated at various input intensities using potassium dihydrogen phosphate (KDP) and lithium triborate (LBO) crystals. Our observations reveal the presence of spectral broadening, pulse compression, and complex structures at higher input intensities. We see that extreme contrast, a few tens of femtosecond pulses can have multiple ’prepulses’ at the 100s femtosecond scale as large as ten percent of the peak value. These can preionize a solid significantly and may influence the interaction. Simulations based on nonlinear pulse propagation equations reinforce our findings.

Cumulative effects are crucial for applications of laser filaments, such as for the remote transfer of energy and the control of electric discharges. Up to now, studies of cumulative effects in the air of high‐repetition‐rate pulse trains have been performed at lower rates than 10 kHz. Herein, the nonlinear effects associated with short plasma filaments produced by pulses of moderate energy (0.4 mJ per pulse) and repetition rates up to 100 kHz are experimentally characterized. With increasing repetition rate, a decrease in absorption, fluorescence emission, and breakdown voltage and concurrently an increase in peak intensity and third‐harmonic‐generation efficiency are observed. Hydrodynamic simulations of the heated gas show that the observed decreases are directly related to a quasi‐stationary state of reduced gas density in the filament. However, further investigations are required to fully understand the physics underpinning the observed sharp reduction of the breakdown voltage at 100 kHz repetition rates. The results may prove relevant for energy and information delivery applications by laser‐induced air waveguide or electric discharge and lightning control.

Controlled deposition of high laser-power density at remote distances still remains a challenge. Previous experimental works using terawatt peak-power laser systems to initiate filamentation at distances of hundreds or even thousands of meters in the atmosphere have documented limited control on laser energy deposition due to the effects of diffraction and turbulence. In this work, we demonstrate a promising scenario for the projection of high power densities at kilometric distances in air, which requires multiple-gigawatt laser pulses reshaped into ring-Airy beams. We show that a power distribution over a relatively large primary ring accompanied by the inward power flux characterizing a ring-Airy beam limits the occurrence of nonlinear effects to the vicinity of the focal point. Close to focus, self-focusing sets in, which accelerates the convergence process of the beam. We quantify the influence of self-focusing on the focal shift and propose an empirical law that fits our numerical simulation results for the position of the nonlinear focus as a function of the input power and the apodization factor of the ring-Airy beam. We show that once the intensity of the beam exceeds the ionization threshold of air, a short filament is generated whose power content rather accurately corresponds to the critical power for self-focusing, independent of the input parameters.

Ultrashort high-energy visible pulses have enabled unprecedented opportunities in temporally resolving ultrafast dynamics in physics, chemistry, and biology. Until now, high-energy sub-10 fs visible pulses have been mostly obtained through complex non-collinear optical parametric amplification setups, followed by some pulse post-compression technique. Here, we present an alternative approach, which relies on considering the typically-undesired multimode nature of large-core hollow-core capillary fibers (HCFs) as an essential asset. In our experiments, 1 mJ 175-fs-long pulses centered at 1035 nm, emitted by an Yb:KGW (Pharos – Light Conversion) laser, were coupled into a 3-m-long HCF (few-cycle Inc.) filled with Argon gas. At a selected pressure of 2.9 bar, a fast energy transfer from the laser broadened via self-phase modulation towards the arising visible light was observed starting at around 0.8 mJ laser pulse energy. At the maximum pulse energy of 0.94 mJ, a continuous spectrum of visible light between 800 nm and 400 nm was measured, with an overall energy of approximately 30 µJ. To understand this process, we implemented 3D carrier-resolved pulse propagation simulations based on the guided mode theory. The simulations predict the direct formation of a pulse of about 5 fs right at the exit of the fiber, considering the visible spectrum in the range 525 - 750 nm. We found that the presence of higher-order modes is crucial to generate such visible pulses and that the Kerr effect is the dominant nonlinearity enabling the modal energy transfer. Experimentally, we characterized the visible pulses by means of a transient-grating frequency-resolved optical gating setup (TG-FROG). At 0.94 mJ and 2.9 bar, a visible pulse duration of 4.6 fs was measured. We also implemented a cross-correlation TG-XFROG, using the separately-compressed laser light and the visible pulses, which demonstrates the possibility of directly implementing high-energy NIR-pump VIS-probe measurements on a sub-10-fs scale.

Starting from 175-fs-long pulses at 1035 nm, we directly generate 20 µJ, 4.6 fs visible pulses through the nonlinear mixing between the spatial modes of a 3-m-long Ar-filled hollow-core fiber, without any pulse post-compression.

The creation of high-energy-density (≳106 joules per cm3) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with sub-wavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two limiting cases are considered: one is a homogeneous step-like profile that can sustain plasmon formation, and the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case, an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region.

Precise control of the filamentation of an ultrashort and intense near-infrared laser pulse in air is crucial for many applications but remains challenging. By combining inverse design for shaping of a pulse at the output of a laser with simulation of the nonlinear propagation of the pulse in air, we numerically investigate the way to simultaneously control the length and position of a plasma filament at an arbitrary remote distance for the first time. An intermediate state, with a Bessel-Gauss beam profile, is introduced between the laser output and the filament. Through forward and backward propagations of the intermediate state, an end-to-end design of the initial laser pulse is achieved to match the desired filament characteristics. Our calculations show that a properly engineered torus pulse can project such a Bessel-Gauss beam at remote distance and generate a long plasma filament without requiring any feedback loop. We also proposed an experimental arrangement to project such an intermediate state and generate the corresponding filament remotely. The method can be further extended to other types of filaments or targets that may be reached from different intermediate states.

We study the propagation of ultrashort laser pulse in filamentation and postfilamentation regimes at the distances up to 95 m. In order to control the start of the filament and spectrum broadening we insert meshes inside the beam. For all beam configurations we found distances range where laser pulse triggers high-voltage discharge.

Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser–plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two-surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons.