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 develop a vibronic coherence model for multiorbital ionization and excitation of nitrogen molecules, based on an up-to-date concept of coherent ionization into excited states [C. H. Yuen and C. D. Lin, Phys. Rev. A 108, 023123 (2023); 109, 033108 (2024)]. Compared to other models, where the coherence of excitation dynamics is described with fewer details, our model predicts larger populations at excited states, in particular, at levels corresponding to the transition wavelengths 391 or 428 nm relevant to air lasing in the atmospheric environment. The dependence of population inversion on the pump wavelength is investigated, showing that ionic coherence in ionization injection enhances the vibronic three-photon transition in the N2 postionization process.

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.

We demonstrate that a single-color ultrashort optical pulse generating a short plasma string in air can emit THz radiation along any direction with respect to its propagation axis. The emission angle can be adjusted by the flying focus technique [1], [2], which determines the speed and direction of the ionization front.

Laser-plasma accelerators emerge as ultra-compact and versatile sources for numerous applications. Although the acceleration length is short (a few millimeters), they typically require large-scale infrastructures including ultra-high-power lasers, vacuum chambers and strict stability for temperature and humidity. As a result, most experiments are conducted in laboratories in large areas with controlled environments. Here, we present a highly compact (footprint of ~ 9 m²) and transportable system capable of generating electrons and photons in the MeV range at high repetition rates (up to 10 Hz) with average charge levels of 0.5 nC and up to 1 nC. This achievement shows the feasibility of performing laser-plasma acceleration outside of laboratory environments with a transportable system, significantly expanding the potential for practical applications.

We demonstrate that pulsed THz radiation produced in air by a focused ultrashort laser pulse can be steered to large angles or even in the backward direction with respect to the laser propagation axis. The emission angle is adjusted by the flying focus technique, which determines the speed and direction of the ionization front created by the single-color laser pulse. This easily adjustable THz source, being well separated from the intense laser, opens exciting applications for remote THz spectroscopy.

The filamentation of ultrashort laser pulses in air with Laguerre–Gaussian beams opens up numerous possibilities for atmospheric applications. In this work, we demonstrate a novel method to measure the critical power for self-focusing of structured laser beam based on the acoustic detection of laser ionization. Using this method, we determine the critical power of different Laguerre–Gaussian beams at 800 nm with ultrashort (50 fs) and sub-picosecond (500 fs) pulse durations. We also discuss the effect of the numerical aperture of the beam on the critical power and present the first measurements of the energy deposited by Laguerre–Gaussian filaments in the air. Our results reveal an unexpected influence of the laser pulse duration on the filamentation of vortex beams.

The recent development of high average, high peak power lasers has revived the effort of using lasers as a potential tool to influence natural lightning. Although impressive, the current progress in laser lightning control technology may only be the beginning of a new area involving a positive feedback between powerful laser development and atmospheric research. In this review paper, we critically evaluate the past, present and future of Laser Lightning Control (LLC), considering both its technological and scientific significance in atmospheric research.

Laser filamentation is a spectacular phenomenon where the self-focusing of the laser pulse generates ionizing light channels. Many applications of filamentation, such as the laser lightning rod, require the generation of superfilaments, long plasma channels of higher electron density than normal filaments. Using a multifocal phase mask, we demonstrate an extension of the superfilamentation length of a focused terawatt laser beam. Optimized superfilaments show increased energy deposition compared to a normal gaussian beam and an extension of their length by at least a factor two. When put in contact with a high voltage electrode, the guiding of a single plasma column with a length of ∼1 m is observed. The length of an air waveguide generated by a vortex laser pulse is also increased by a factor 2 in the presence of the phase mask.

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

We report an increase of the superfilamentation length of a focused terawatt laser beam using a multifocal phase mask. Optimized superfilaments extend the length of laser guided leader and air waveguides by a factor two.(c) 2024 The Author(s)

High power femtosecond laser pulses launched in air undergo nonlinear filamentary propagation, featuring a bright and thin plasma channel in air with its length much longer than the Rayleigh length of the laser beam. During this nonlinear propagation process, the laser pulses experience rich and complex spatial and temporal transformations. With its applications ranging from supercontinuum generation, laser pulse compression, remote sensing to triggering of lightning, the underlying physical mechanism of filamentation has been intensively studied. In this review, we will focus on the fluorescence and cavity-free lasing effect of the plasma filaments in air. The different mechanisms underlying the fluorescence of the excited neutral nitrogen molecules will be throughly examined and it is concluded that the electron collision excitation is the dominant channel for the formation of the excited nitrogen molecules. The recently discovered "air lasing" effect, a cavity-free bidirectional lasing emission emitted by the filaments, will be introduced and its main properties will be emphasized. The applications of the fluorescence and lasing effect of the neutral nitrogen molecules will be introduced, with two examples on spectroscopy and detection of electric field. Finally, we discuss the quenching effect of the lasing effect in atmosphere and the mechanisms responsible will be analyzed. An outlook for the achievement of backward lasing in air will be briefly presented.

We report on high average-power, high-energy picosecond fourth-harmonic generation in LBO. The first stages of a Yb:YAG laser chain operating at 1 kHz repetition rate generate few-picosecond 220 mJ chirped pulses at 1030 nm fundamental wavelength. They are frequency-converted in a cascade of three LBO crystals to generate the second-, third-, and fourth-harmonics at 515 nm, 343 nm and 257 nm respectively. Crystals thicknesses and angular phase-matching detuning were calculated as a function of pulse duration through broadband nonlinear optical numerical simulations. Last crystal is both conduction-cooled on edge and surface-cooled at center through forced-air flow to mitigate heating due to nonlinear absorption in the deep-UV and reduce temperature gradients. Chirped-pulse duration was experimentally adjusted to achieve stable 20% overall conversion efficiency. Near-field beam profiles were continuously recorded at 10 Hz, for all four wavelengths involved, together with corresponding energies, showing no significant beam degradation over 50 hours. Temperatures of the two last crystals were monitored, and will help optimize surface cooling for future power ramping-up.

We study the influence of the gravitational force on the generation of low-density channels of air left in the path of femtosecond laser filaments at high repetition rate. We observe a more important density variation along the filament longitudinal axis in the case of a vertically created filament as compared to a horizontal one. This leads to a more important reduction of the electrical breakdown field using vertical filament. This geometry induced difference is only observed at high repetition rate because it is directly related to the cumulative effect appearing above 100 Hz.

Quantum interference occurs frequently in the interaction of laser radiation with materials, leading to a series of fascinating effects such as lasing without inversion, electromagnetically induced transparency, Fano resonance, etc. Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter, regardless of how many optical frequencies are involved. Here, we report on quantum interference driven by multiple photons in the emission spectroscopy of nitrogen ions that are resonantly pumped by ultrafast infrared laser pulses. In the spectral domain, Fano resonance is observed in the emission spectrum, where a laser-assisted dynamic Stark effect creates the continuum. In the time domain, the fast-evolving emission is measured, revealing the nature of free-induction decay arising from quantum radiation and molecular cooperativity. These findings clarify the mechanism of coherent emission of nitrogen ions pumped with mid-infrared pump laser and are found to be universal. The present work opens a route to explore the important role of quantum interference during the interaction of intense laser pulses with materials near multiple photon resonance.

We investigate the pump wavelength dependence of population inversion in a nitrogen ion when nitrogen gas is irradiated by a femtosecond laser pulse in the wavelength range from ultraviolet to near-infrared. It is found that the population inversions corresponding to the transitions at 391 and 428 nm are very sensitive to the laser wavelength, with extreme values near 400 and 1000 nm. For the ultraviolet pump, the population inversions for 391 and 428 nm transitions occur at different wavelengths: a maximum inversion for the first corresponds to a minimum for the second. This is attributed to the near-resonance single-photon transition combined with the Raman process. A significantly stronger inversion is observed for the near-infrared laser, with the maximum depending on the laser intensity, ascribed to the ac Stark effect. The Floquet theory confirms this conclusion and reveals that the three-photon transition assisted by the strong laser field leads to this enhanced population inversion. This study provides the optimal conditions for air lasing based on ionic transitions of nitrogen molecules.

The nonadiabatic molecular alignment effect is commonly found helpful in enhancing nonlinear processes such as high-order harmonics and strong-field ionization. In this work, an opposite phenomenon is observed, in which the nonadiabatic molecular alignment effect prepared with a linear-polarized prepulse strongly suppresses the generation of N2 lasing induced by circularly polarized 800-nm femtosecond laser pulse in filament plasma. The presence of a weak prepulse periodically suppresses the lasing at each revival timing of the rotational wave packet of the N2 molecule. The fine structures of the lasing suppression at different revivals exhibits distinct features. The underlying mechanism of the lasing signal suppression is attributed to sudden change of the polarization ellipticity of the pump laser pulse as it experiences ultrafast birefringence induced by the linear prepulse. Theoretical simulations by numerically solving the time-dependent Schr & ouml;dinger equation with the molecular alignment effect included confirm sensitive changes of the polarization ellipticity of the pump laser at every alignment revival, which reproduces most features of the fine structures of lasing suppression and hence largely supports our interpretation.

We study the creation and evolution of meter-scale long-lived laser-guided electric discharges and the interaction between consecutive guided discharges. The lifetime of guided discharges from a Tesla high voltage generator is first increased up to several milliseconds by the injection of additional current. The subsequent discharge evolution is measured by recording the electric current and by Schlieren and fluorescence imaging. A thermodynamic model of the gas evolution is developed to explain the discharge evolution. Finally, we analyze the succession of laser-guided discharges generated at 10 Hz.