We propose an approach to stabilize the parameters of filament-generated infrared light bullet against pulse- to-pulse fluctuations in three domains: spectral, spatial and energy, by means of amplitude modulation of the initial laser beam. We studied the single-shot angle-wavelength spectra of filament-generated supercontinua for a beam modulated by a four-hole opaque mask and a beam without additional modulation. We demonstrated that amplitude modulation of the initial beam improves the bullet pulse-to-pulse stability in all the three domains with the bullet energy stability reaching as high as fivefold enhancement.

An O-shaped structure at wavelengths of 930–960 nm in the frequency–angular spectrum of the supercontinuum generated during the filamentation of a femtosecond laser pulse with a central wavelength of 740 nm on a 75-m path in air has been observed experimentally. This feature of the frequency–angular spectrum is due to the presence of the absorption band of water vapor in the range of 930–960 nm and the anomalous dispersion region associated with this absorption. This result opens prospects for the remote single-pulse detection of impurities in air.

We measured and simulated the spectrally resolved angular distributions of 0.3–1-THz emission from the two-color filament with its plasma length (∼40 mm) exceeding the dephasing length (∼25 mm) between the fundamental (740 nm) and the second harmonic (370 nm) pulses in air. We show that only the forwardly propagating on-axis terahertz (THz) radiation is sensitive to the variation of the phase offset φ between fundamental and second harmonics, while the ring-like THz beam carrying ≳80% of the overall THz yield is independent of φ. Utilization of the THz ring allows one to omit the tedious adjustment of the frequency-doubling crystal position in the experiment.

We propose an easy but effective approach to find a transition numerical aperture between the regimes of laser pulse filamentation with nonlinear focusing and with geometric focusing predominance. The suggested method based on the beam profile measurements allows correction of the data provided by spectra measurements. Using a simple semi-analytical model, we study the dependence of the transition numerical aperture on the pulse power and medium nonlinearity. The analysis shows that in condensed media the transition from the nonlinear to geometric focusing regime occurs at much tighter focusing than in air. Moreover, if the medium nonlinear refractive index is high enough, only the nonlinear focusing regime is observed even at numerical apertures close to one, which allows symmetric plasma channel formation.

Filamentation of high-power femtosecond laser pulses in air is accompanied by a fairly strong release of optical energy into the propagation medium due to laser-induced ionization of air molecules and production of an underdense plasma of charged species. We present the results of our laboratory experiments and numerical simulations aimed at estimating the energy deposition amount by laser filament upon propagation in air depending on the conditions of spatial focusing, pulse energy, and radiation wavelength. Importantly, our study reveals a more than 50% decrease in the filament energy deposited in air in the range of moderate numerical aperture values, approximately from 0.003 to 0.007, at carrier wavelengths of 740 and 470 nm. We attribute such a considerable reduction in the laser pulse energy release for femtosecond plasma to the competing effects of Kerr self-focusing and geometric divergence of focused laser pulse.

We demonstrate that modulation of the initial beam by a simple four-hole amplitude mask allows to enhance pulse-to-pulse stability of the red wing of filament-generated supercontinuum. In particular, we focus our attention on the spectral position and width, angular coordinate and energy of the brightest red-shifted maximum in the spectrum of a loosely-focused filament in air. This maximum, corresponding to a Raman bullet, bears about $40 \%$ of the initial pulse energy and is of interest for applications. The enhancement in pulse-to-pulse stability of the bullet parameters reaches 1.7 times for the bullet spectral position and width, 3.4 times for its angular position and 4.5 times for its energy.

Приводятся экспериментально полученные двумерные распределения терагерцового излучения, генерация которого осуществляется одним и четырьмя филаментами, формируемыми фазовыми оптическими элементами. Продемонстрировано, что применение фазовой маски примерно в полтора раза уменьшает углы распространения терагерцового пучка, что обусловленно интерференцией терагерцового излучения от четырех источников. Применение решетки Дамманна эти углы несколько увеличивает.

We have experimentally obtained two-dimensional distributions of terahertz radiation generated by one or four filaments formed by phase optical elements in air. It has been demonstrated that the use of the phase mask reduces the propagation angles of terahertz beam by approximately one and a half times, which is due to the interference of terahertz radiation from four sources. The use of the Dammann grating slightly enlarges these angles.

At the selected frequencies from 0.3 to 10 THz we measured the two-dimensional (2D) distributions of fluence and polarization of terahertz (THz) emission from a single-color femtosecond filament. At the majority of frequencies studied, the THz beam has a donut-like shape with azimuthal modulations and radial polarization. At the maximal modulation, THz beam takes the form of the two lobes and polarization of the THz field degenerates into orthogonal to the laser pulse polarization direction. Violation of the radially polarized donut beam shape is due to destructive interference of THz waves driven by light pressure directed along the laser beam propagation axis and ponderomotive force parallel to the laser polarization.

The terahertz (THz) radiation emitted by an air-based femtosecond filament biased by a static electric field is known to have on-axis shape and relatively low frequency spectrum in contrast to the unbiased single-color and two-color schemes. Here, we measure the THz emission of a 15-kV/cm-biased filament in air produced by a 740-nm, 1.8-mJ, 90-fs pulse and demonstrate that a flat-top on-axis THz angular distribution of the emission at 0.5-1 THz transforms into a contrast ring-shaped one at 10 THz.

Directivity patterns of terahertz emission by femtosecond laser filament plasma in air after radiation propagation in hollow tubes with metal and dielectric (polypropylene) walls are experimentally studied. It is shown that narrow directed radiation can be obtained using quasi-waveguide propagation in a tube with dielectric walls, while the terahertz pulse duration is not affected significantly.

The angular distributions of various spectral components of terahertz radiation generated by filament plasma are measured in a wide range of numerical apertures NA of the laser beam from 0.003 to 0.1. It is shown that propagation angles of terahertz radiation are inversely proportional to the square root of the terahertz frequency. It is shown that an increase in the numerical aperture of the laser beam leads to an increase in propagation angles of terahertz radiation in the entire range of observed frequencies.

Two-dimensional distribution patterns of terahertz radiation generated in a laser single-color filament plasma are measured at several frequencies. In the low-frequency region (0.1–0.5 THz), the radiation propagates in a cone with a minimum on the axis. At higher frequencies, the terahertz radiation pattern depends significantly on the laser pulse polarization. In the case of linear polarization, the axial symmetry is broken: terahertz radiation propagates into two maxima located along the axis perpendicular to the laser polarization. In the case of circular polarization, the axial symmetry of the terahertz radiation distribution is restored.

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.

We performed full characterization of postfilament formed by the radiation of the Ti: Sa laser system on an extended atmospheric path. Single-shot angle-wavelength spectra, beam diameter and self-correlation function measurements have been employed for this purpose. Using angle-wavelength spectra, the evolution of on-axis red-shifted humps has been traced, showing that their divergence does not exceed 0.5 mrad. Two zones in the postfilamentation process have been revealed: the Stockes zone with soliton-like propagation, where the number of the Stockes humps and their shift increases while the pulse duration remains almost constant, and the zone where the temporal and spectral postfilament characteristics changes like in linear propagation mode, while the beam divergence is negligible due to the Kerr nonlinearity.

We measured and simulated frequency-angular distribution of terahertz emission from the plasma channel of a single-color filament biased by electrostatic field. Pronounced on-axis maximum of terahertz emission was observed for the electric field above 3.2 kV/cm. Directional diagrams of frequencies <1 THz reveal a flat-top shape due to the constructive interference of terahertz waves emitted by dipole local sources.