Ultrashort Laser Pulses In Air Research Articles

Relativistic air-plasma far-infrared radiation effects by ultrashort laser pulses in air

Interactions of ultrashort laser pulses with gases have a wide variety of applications including lighting control, electromagnetic radiation and remote sensing. Considering the radiation effects, the ionization process, plasma formation and the current produced due to the formed plasmas, are the important fundamental processes. Here, a 2D relativistic fluid model is presented in which the interaction of an ultrashort laser pulse with air, leading to the electromagnetic pulse generation, is investigated. As the model is relativistic, it is appropriate for the investigation of the air-plasma radiation effects at high intensities. The results show that the intense laser-air interaction is suitable for the intense broadband far-infrared pulse generation due to the fact that the wave amplitude increases with the laser intensity. Moreover, for the laser intensities above 1020 W/m2, the laser pulse magnetic field gives rise to a strong axial electron current and a consequent axial wave component, which has to be accounted for in the proposed physical models. In addition, the Fourier transform analysis reveals that the radiated pulse is broadband and its frequency spectrum is not affected by the laser intensity.

Control of THz Radiation Divergence in Laser Filaments

Conditions for THz generation in the plasma of a filament produced by ultrashort laser pulses in air under aberrations in a one- and two-color optical scheme are observed. Possibilities are discussed of controlling the THz radiation pattern by application of an electric field and introduction of aberrations in the laser beam.

Self-referenced ultra-broadband transient terahertz spectroscopy using air-photonics.

Terahertz (THz) air-photonics employs nonlinear interactions of ultrashort laser pulses in air to generate and detect THz pulses. As air is virtually non-dispersive, the optical-THz phase matching condition is automatically met, thus permitting the generation and detection of ultra-broadband THz pulses covering the entire THz spectral range without any gaps. Air-photonics naturally offers unique opportunities for ultra-broadband transient THz spectroscopy, yet many critical challenges inherent to this technique must first be resolved. Here, we present explicit guidelines for ultra-broadband transient THz spectroscopy with air-photonics, including a novel method for self-referenced signal acquisition minimizing the phase error, and the numerically-accurate approach to the transient reflectance data analysis.

Acoustic emission of ultrashort laser pulses in air and aerosols

The effectiveness of photoacoustic diagnosis in studying ultrashort laser pulse propagation in different media is considered on the basis of results from laboratory experiments on recording acoustic emissions induced by terawatt femtosecond laser pulses affecting aerosols, individual droplets, and air.

Effects of higher-order Kerr nonlinearity and plasma diffraction on multiple filamentation of ultrashort laser pulses in air

The effect of higher-order Kerr nonlinearity on channel formation by, and filamentation of, ultrashort laser pulses propagating in air is considered. Filament patterns originating from multiphoton ionization of the air molecules with and without the higher-order Kerr and molecular-rotation effects are investigated. It is found that diverging multiple filaments are formed if only the plasma-induced defocusing effect is included. In the presence of the higher-order Kerr effects, the light channel can exist for a long distance. The effect of noise on the filament patterns is also discussed.

Filamentation in air with ultrashort mid-infrared pulses

We theoretically investigate filamentation of ultrashort laser pulses in air in the mid-infrared regime under conditions in which the group-velocity dispersion (GVD) is anomalous. When a high-power, ultra-short mid-infrared laser beam centered at 3.1-μm forms a filament, a spatial solitary wave is stabilized by the plasma formation and propagates several times its diffraction length. Compared with temporal self-compression in gases due to plasma formation and pulse splitting in the normal-GVD regime, the minimum achievable pulse duration (∼70 fs) is limited by the bandwidth of the anomalous-GVD region in air. For the relatively high powers, multiple pulse splitting due to the plasma effect and shock formation is observed, which is similar to that which occurs in solids. Our simulations show that the energy reservoir also plays a critical role for longer propagation of the air filament in the anomalous-GVD regime.

Peculiarities of filamentation of sharply focused ultrashort laser pulses in air

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of Rpl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.

Higher-Order Kerr Terms Allow Ionization-Free Filamentation in Gases

We show that higher-order nonlinear indices (n(4), n(6), n(8), n(10)) provide the main defocusing contribution to self-channeling of ultrashort laser pulses in air and argon at 800 nm, in contrast with the previously accepted mechanism of filamentation where plasma was considered as the dominant defocusing process. Their consideration allows us to reproduce experimentally observed intensities and plasma densities in self-guided filaments.

Curved plasma channels: Kerr lens and Airy prism

We analytically calculate the transverse energy fluxes induced in high-power Airy beams by the Kerr self-focusing and the Airy profile itself, respectively. In experimental condition representative of laser filamentation experiments of high-power, ultrashort laser pulses in air and condensed media, the Kerr lens induces transverse energy fluxes much larger than the Airy "prism" at the main peak. As a consequence, the curved plasma channels in Airy beams are not only a plasma spark on a curved focus, but indeed self-guided filaments, and their curved trajectory appears as a perturbation due to the linear Airy propagation regime.

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 clamped 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 harmonic. Maximal broadening is observed for large wavelengths and long filamentation ranges.

Effects of delayed Kerr nonlinearity and ionization on the filamentary ultrashort laser pulses in air

We present a systematic study of filamentary ultrashort laser pulses in air, through numerical solutions of the nonlinear Schrödinger equation for various contributions of the delayed Kerr nonlinearity. The results show that a relatively larger contribution of the delayed Kerr nonlinearity will lead to a longer stable filament. This is explained from the transfer of the nonlinear contributions from the frontier part to the back of the pulse and the counterbalanced action of the negative plasma induced nonlinearity by the delayed Kerr nonlinearity in the trailing part of the pulses. Furthermore, effect of ionization on the stability of the filament is investigated. Two formulas are used to generate the data of the ionization, i.e., the Perelemov, Popov, and Terent'ev (PPT) and the Ammosov, Delone, and Krainov formula. It is found that simulation with higher ionization rate (PPT) could generate a more stable and longer filament.

χ(5)susceptibility stabilizes the propagation of ultrashort laser pulses in air

The influence of higher-order optical nonlinearities on the self-guiding of femtosecond pulses in the atmosphere is investigated theoretically. ${\ensuremath{\chi}}^{(5)}$ susceptibility, even small, is shown to significantly affect the propagation dynamics and enhance the robustness of femtosecond filaments by lowering the threshold intensity at which the beam saturates and by enlarging the self-channeling range.

Longitudinal compression of short laser pulses in air

We have performed laboratory experiments to study long distance propagation of large bandwidth ultrashort laser pulses in air. Initial pulse length, frequency chirping, and laser pulse energy were varied where the maximum propagation distance was up to 105 m. We have demonstrated the compression of initially negatively chirped low intensity laser pulses due to the linear group velocity dispersion of air. The characteristics of the compressed pulse such as pulse duration and spectral chirping were found to be significantly affected by the laser pulse intensity, with higher intensities corresponding to longer minimum compressed pulse duration.

Propagation of ultra-short, intense laser pulses in air

Recent theoretical, computational, and experimental work carried out at the Naval Research Laboratory on the propagation of ultra-short laser pulses in air is presented. Fully time-dependent, three-dimensional, nonlinear equations describing the propagation of laser pulses in air under the influence of diffraction, group velocity dispersion, Kerr nonlinearity, stimulated Raman scattering, ionization, and plasma wakefield excitation are presented and analyzed. The propagation code, HELCAP [P. Sprangle, J. R. Peñano, and B. Hafizi, Phys. Rev. E 66, 046418 (2002)], is used to simulate the propagation of laser pulses in air under the influence of the physical processes mentioned above. Simulations of laser filamentation together with experimental measurements are used to confirm that the filamentation process is dependent on pulse duration. An equilibrium configuration for optical and plasma filaments in air is derived and the dynamic guiding and spectral broadening of a laser pulse is modeled. The effect of atmospheric turbulence on nonlinear self-focusing is demonstrated. Simulations of a recent electromagnetic pulse (EMP) generation experiment are also presented and the efficiency of EMP generation is determined and found to be extremely small.

White-light continuum generation and filamentation during the propagation of ultra-short laser pulses in air

We present numerical results on the propagation of femtosecond pulses in air including multiphoton ionization, group-velocity dispersion, space–time focusing, self-steepening, Raman response, and higher-order χ (5) defocusing. It is shown that the self-steepening terms cause strong blue shifting of the spectrum generating a white-light continuum. Our results are in good qualitative agreement with experimental observations.

Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air

Backward emission of the supercontinuum from a light filament induced by high-intensity femtosecond laser pulses propagating in air has been observed to be enhanced compared with linear Rayleigh-Mie scattering. This enhancement is interpreted as a nonlinear scattering process onto longitudinal refractive-index changes induced by the laser pulse itself. The spectral dependence of the supercontinuum angular distribution is also investigated.

Guiding large-scale spark discharges with ultrashort pulse laser filaments

Using the nonlinear propagation properties of ultrashort laser pulses in air, we were able to produce long ionized filaments that served to guide spark discharges. With a laser pulse energy of 20 mJ, one or two ionized filaments were created and could guide streamer discharges over 2 m air gaps, where the electric field was fairly uniform and had an average value of 0.6 MV/m. Such a guiding effect was observed for times of 1–3 μs after the laser pulse created the ionized filaments. Longer delays (10–15 μs) were recorded at higher laser pulse energy, with a larger number of filaments. Images of the early stages of the discharge of a uniform air gap show that the laser-produced ionized filaments do not initiate the discharge process but act rather as preferred channels where the leader growth is accelerated. In the end, these straight conductive channels carry the arc current as the voltage in the gap breaks down.

Conical emission from laser–plasma interactions in the filamentation of powerful ultrashort laser pulses in air

We performed detailed experimental and numerical investigations of the conical emission (CE) accompanying the filamentation of powerful ultrashort laser pulses in air. It was found that the CE originates from self-phase modulation in the plasma produced by the pulse during propagation. The experiment and the simulation agree on the essential features of the CE: The CE angle decreases with increasing wavelength and is independent of the position along the filament, and there is no CE at Stokes-shifted wavelengths.

Moving focus in the propagation of ultrashort laser pulses in air

The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated by use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplification laser system. A filament contained only a small fraction of the pulse energy and always ended at the diffraction length of the beam (~100 m), independently of the pulse energy. These features are explained by the moving-focus model, which is presented as an alternative to the self-channeling model. Computer simulations involving ionization of the air also support the moving-focus model.