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.

The generation of terahertz radiation in a BNA crystal pumped by 1.24-µm femtosecond laser radiation from a Cr:forsterite laser system with a pulse duration of 100 and 35 fs and a pump density of 10 mJ/cm 2 has been realized. The achieved generation efficiency is 0.1%. It is found that a decrease in the pump pulse duration from 100 to 35 fs leads to the generation of high-frequency components in the ranges of 2.5–6.5 THz and 9‒10.5 THz in the generated radiation spectrum. Simulation of the terahertz radiation generation based on the solution of Maxwell’s equations by the finite-difference time-domain method has made it possible to adequately describe the measured spectra. The generation of broadband high-frequency terahertz radiation in the BNA crystal pumped by the Cr:forsterite laser system allows one to consider this schematic as an alternative to sources based on the BNA crystal pumped by a Ti:sapphire laser system.

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.

It has been shown that the spectrum of intense few-cycle terahertz radiation generated in a DAST organic crystal can be controlled by chirping 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system. It has been found that an increase in the linear chirp of generating radiation results in the narrowing of the spectrum of terahertz radiation and its redshift. The simulation of the generation of terahertz radiation within the model of three-wave mixing has shown that this effect is due to a change in the phase matching width of the degenerate generation of the difference frequency of terahertz range. In addition, the comparative analysis of terahertz radiation spectra generated in DAST, DSTMS, OH1, and BNA organic crystals indicates that the spectral–temporal properties of terahertz radiation can be more roughly controlled by choosing an appropriate crystal. The proposed approach to control the terahertz radiation spectrum by chirping the pump pulse provides the foundation for spectroscopic studies using intense terahertz radiation with controlled spectral–temporal properties.

В работе показана возможность управления спектром мощного малопериодного терагерцового излучения, генерируемого в органическом кристалле DAST, путем чирпирования излучения накачки, в качестве которого использовалось фемтосекундное лазерное излучение системы на кристалле хромфорстерита с длиной волны 1.24 мкм. Установлено, что увеличение линейного чирпа генерирующего излучения приводит к обужению спектра терагерцового излучения и его смещению в низкочастотную область. Проведенное моделирование процесса генерации терагерцового излучения в рамках модели трехволнового смешения показывает, что в основе данного эффекта лежит изменение ширины фазового синхронизма вырожденного процесса генерации разностной частоты терагерцового диапазона. Также проведен сравнительный анализ спектров терагерцового излучения, генерируемого в органических кристаллах DAST, DSTMS, OH1 и BNA, свидетельствующий о возможности более грубого управления спектрально-временными свойствами терагерцового излучения путем выбора требуемого кристалла. Продемонстрированный подход к управлению спектром терагерцового излучения путем чирпирования импульса накачки закладывает основу для проведения спектроскопических исследований с использованием мощного терагерцового излучения с управляемыми спектрально-временными свойствами.

It has been shown that the spectrum of intense few-cycle terahertz radiation generated in a DAST organic crystal can be controlled by chirping 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system. It has been found that an increase in the linear chirp of generating radiation results in the narrowing of the spectrum of terahertz radiation and its redshift. The simulation of the generation of terahertz radiation within the model of three-wave mixing has shown that this effect is due to a change in the phase matching width of the degenerate generation of the difference frequency of terahertz range. In addition, the comparative analysis of terahertz radiation spectra generated in DAST, DSTMS, OH1, and BNA organic crystals indicates that the spectral–temporal properties of terahertz radiation can be more roughly controlled by choosing an appropriate crystal. The proposed approach to control the terahertz radiation spectrum by chirping the pump pulse provides the foundation for spectroscopic studies using intense terahertz radiation with controlled spectral–temporal properties.

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.

A two-stage optical parametric amplifier is fabricated on the basis of type-II BBO crystals pumped by the intense radiation of a Ti:sapphire laser. Femtosecond radiation tunable from the near to mid-infrared range at wavelengths of 1.1–1.6 μm (signal wave) and 1.6–2.6 μm (idler wave) is generated with a total energy conversion efficiency of 8%. The output energy of generated infrared pulses at wavelengths of 1.3 and 2 μm is 840 and 280 μJ, respectively. It is experimentally demonstrated that terahertz radiation can be subsequently generated in a DAST organic crystal using the optical rectification process under the pumping by generated mid-infrared radiation. The developed model of the generation of terahertz radiation shows that the optical–terahertz conversion efficiency to 3.6% can be increased by chirping femtosecond mid-infrared radiation (~2 μm) to 200 fs.

We compare transverse structure evolution and energy deposition into the medium within focused multifilament arrays created using two different types of diffraction optical elements (DOEs): TEM11 phase plate and a Dammann grating. We show that the employment of the Dammann grating provides a robust way to create regular multifilament arrays, which is far less dependent on laser beam quality than one using the phase plate.

Nonlinear increase of energy deposition results in superfilamentation under femtosecond multifilamentation in air with NA ≪ 10−2 focusing, while for NA & 10−2 it grows linearly with filament number.