Remote generation of electromagnetic waves in the terahertz (THz) by the volume of air medium gains increasing scientific interest due to the demand for this spectral range in solving various practical problems including those belonging to the nonlinear atmospheric optics. During self-focusing and filamentation of high-power femtosecond laser pulses in air, the source of THz radiation is the plasma of laser filaments. One of the main challenges of THz generation by a laser filament is increasing the value of optical energy conversion to the long-wavelength band. In this paper, we present the results of our studies on DC-biased THz generation during single-color filamentation in air of focused femtosecond Ti:Sapphire-laser pulses (800 nm, pulse energy up to 40 mJ). The distinctive feature of our study is that a femtosecond optical pulse propagates through a spatially-localized heated air layer containing randomly inhomogeneous refractive index perturbations, which mimics strong air turbulence. For the first time to our knowledge, we show, that the turbulent air layer formed at the beginning of the optical path allows increasing the yield of THz radiation up to 1.5 times due to the formation of multiple chaotically-arranged filaments resulting from random perturbations of the optical beam energy profile that reduces the subsequent THz (re)absorption in the filament plasma.

Remote control of high-intensity laser beams in the atmosphere is an important problem of atmospheric optics. It is of special interest for atmospheric sounding, where turbulence can affect beam propagation. We experimentally study the effect of a turbulent layer produced at the beginning of a laser radiation propagation path on the characteristics of the filamentation domain and generation of high-intensity plasma-free channels for laser beams 2.5 and 5 cm diameter, including under the phase control of the transverse beam structure with a deformable mirror. In the presence of turbulence, the beginning of multiple filamentation domain approaches, however, insignificantly (<10

Распространение мощного фемтосекундного лазерного излучения в среде сопровождается формированием плазмы в поперечном сечении пучка. Свечение плазмы области дает сведения о химическом составе вещества в области взаимодействия, что открывает возможности химического спектрального анализа объекта. В докладе представлены результаты эксперимента, в котором наблюдается зависимость свечения спектральных составляющих области филаментации мощного фемтосекундного импульса от его начальной длительности. Для наблюдения выбраны линии азота, составляющего атмосферу, примеси, в данном случае натрия, и водорода, характеризующего жидкий аэрозоль с примесью. Показано, что с увеличением длительности лазерного импульса интенсивность спектральных составляющих растет. The propagation of powerful femtosecond laser radiation in a medium is accompanied by the formation of plasma in the cross-section of the beam. The glow of the plasma region provides information about the chemical composition of the substance in the interaction area, opening up possibilities for chemical spectral analysis of the object. This report presents the results of an experiment observing the dependence of the glow of spectral components in the filamentation region of a powerful femtosecond pulse on its initial duration. The observations focused on nitrogen lines, making up the atmosphere, as well as sodium and hydrogen impurities characterizing a liquid aerosol with impurities. It is shown that with increasing duration of the laser pulse, the intensity of the spectral components increases.

Генерация электромагнитных волн в терагерцовом диапазоне представляет интерес в силу востребованности данного диапазона для решения многих прикладных задач. Плазма филамента, возникающего при самофокусировке лазерных импульсов фемтосекундной длительности, является источником ТГц излучения. В настоящей работе представлены результаты исследования ТГц излучения филаментом в условиях его распространения через слой с неоднородным показателем преломления. Показано, что сформированный в оптическом канале турбулентный слой повышает эффективность генерации ТГц излучения в 1,4 раза за счет случайного возмущения энергетического профиля пучка и формирования множественной филаментации. The generation of electromagnetic waves in the terahertz range is of interest due to the demand for this range in solving various applied problems. The filament plasma generated during the self-focusing of femtosecond laser pulses serves as a source of THz radiation. This work presents the results of the study of THz radiation from the filament while propagating through a layer with an inhomogeneous refractive index. It is shown that the turbulent layer formed within the optical channel increases the efficiency of THz radiation generation by 1.4 times due to random perturbation of the beam’s energy profile and the formation of multiple filamentation.

Для дистанционного зондирования атмосферы уже долгое время активно применяется метод флуоресцентной спектроскопии (LIFS), который является одним из наиболее чувствительных и селективных методов определения малых концентраций веществ. Применение мощных фемтосекундных лазерных источников для дистанционного зондирования атмосферы методом флуоресцентной спектроскопии способствует развитию лидаров нового поколения. Целью исследований было изучение возможности применения интенсивных световых каналов, сформированных турбулентным экраном для реализации метода флуоресцентной спектроскопии жидкофазного аэрозоля. Показана возможность удаленного детектирования спектров флуоресценции и увеличения регистрируемого свечение при сегментировании лазерного пучка. For atmospheric remote sensing, the method of fluorescence spectroscopy (LIFS) has been actively used for a long time, which is one of the most sensitive and selective methods for determining trace concentrations of substances. The use of powerful femtosecond laser sources for remote atmospheric sensing by fluorescence spectroscopy contributes to the development of next-generation lidars. The purpose of the research was to investigate the feasibility of using intense light channels formed by a turbulent screen for implementing the fluorescence spectroscopy method for liquid-phase aerosol. The possibility of remote detection of fluorescence spectra and increased registered luminescence due to segmentation of the laser beam was demonstrated.

The work presents the results on the influence of the temperature of a turbulent layer on the transverse energy structure of femtosecond (multi-ring and Gaussian) laser beams. Gaussian and multi-ring beam profiles exhibit a monotonous growth in the number of intense light maxima with increasing the strength turbulence of air jet and increasing laser pulse energy. The formed intense light channels (~ 1011 W/cm2) make it possible to excite two-photon fluorescence of dyes with a signal level that makes it possible to take at a distance of 100 m.

Представляются экспериментальные результаты исследования особенностей формирования и распространения интенсивных световых каналов в условиях структурирования начального профиля пучка мощного фемтосекундного лазерного излучения в виде отдельных субапертур. Такое особым образом структурированное излучение получено путем амплитудной модуляции лазерного пучка с использованием масок. Применение амплитудных масок позволяет получить заданное число высокоинтенсивных световых каналов, и осуществить их управляемое распространение в конкретном нелинейном режиме, преимущественно на небольших (десятки метров) расстояниях в атмосфере.

The results of analysis of aerosol in aqueous NaCl solution by the femtosecond-pulse laser-induced breakdown spectroscopy (FS-LIBS) method are considered. Estimates are presented for the impurity (Na) concentration by the analytical pair method using the additive method. We chose, as an internal standard line, the radiation of the atomic nitrogen ion N+ at a wavelength of 500.515 nm, formed from atmospheric nitrogen molecules in the filamentation domain. A possibility of estimating the concentration of a chemical compound (NaCl) from emission of its constituent part (Na) in aqueous aerosol by the FS-LIBS method is interesting for developing the femtosecond lidar sounding techniques.

В работе представлены результаты по влиянию турбулентного слоя вначале трассы на характеристики области филаментации для пучков диаметром 2,5 см и 5 см. Внесение турбулентности приводит к приближению начала области множественной филаментации (ОМФ) к источнику. Сокращение дистанции до начала и конца ОМФ, при наличии турбулентности, является несущественным (< 10%). Сформированный в начале трассы турбулентный слой приводит к кратному увеличению количества интенсивных каналов (~ 1011-1012 Вт/см2), протяженностью > 100 м.

The results of experimental studies of the features of formation and path propagation of high-intensity light channels under conditions of distortions of the initial profile of a high-power femtosecond laser radiation beam are presented. The distortions are introduced by amplitude modulation of the laser beam using masks. The use of amplitude masks allows generation of a preset number of high-intensity light channels and their controllable filamentation mainly at short (tens of meters) distances of radiation propagation in air.

The paper presents the femtosecond laser filamentation, which splits the liquid spray into two jets. The dependence is suggested for the acoustic signal and the femtosecond pulse energy, which identifies the liquid spray threshold.

According to the results of studying the interaction of high-power femtosecond pulsed radiation from a titanium–sapphire laser with the aerosol particles of an aqueous solution of NaCl, we estimate the directional pattern of Na emission from the filamentation region. It has been found that this directional pattern is characterized by minima in angular directions of 0° and 180° and a maximum in an angular direction between 20° and 160°. Based on the results of recording of supercontinuum radiation formed in the air and scattered by aerosol particles, the features of the aerosol particle size distribution have been determined. Gradient optimization methods using graphic processors were employed to speed up calculations. The neural network was used as an optimization method.

Представлены результаты экспериментов по возбуждению и регистрации двухфотонной флуоресценции красителей интенсивными световыми каналами, сформированными при управлении волновым фронтом фемтосекундных лазерных импульсов. Свечение красителей и прием обратного сигнала осуществлялся по лидарной схеме на удалении 100 м от источника.

In laboratory conditions, the fluorescence spectra of rhodamine 6G were obtained in the domain of post-filamentation channels upon interaction with femtosecond laser radiation. A fluorescence signal was received from a distance of 7.5 m from the aerosol and the droplet according to the lidar sensing scheme.

Results of the complex experimental study of the spectral and temporal characteristics of the emission glow of control samples of several solid-state substances (imitation of topographic targets in the atmosphere) and of solid aerosols (imitation of air pollutants) under the action of femtosecond pulses of a Ti:Sa laser (at a carrier wavelength of 800 nm) and nonlinear optical effects are presented.

The report presents the results of estimating the angular distribution of emission of impurity from the breakdown region of aerosol by femtosecond pulses in the range from 0о to 180о and the intensity of the emission line depending on the concentration of the impurity for the problem of remote sensing by the femtosecond laser-induced breakdown method.

Experimental results of measurements of the threshold power density of stimulated radiation in solutions of the coumarin-30 dye with ZnO nanoparticles irradiated with femtosecond laser pulses are presented. It is shown that the thresholds of stimulated emission in solutions of the coumarin-30 dye with ZnO nanoparticles are approximately three orders of magnitude higher than the thresholds of stimulated emission in solutions of the coumarin-30 dye with ZnO nanoparticles irradiated with nanosecond laser pulses.

The results of measure of the duration of a femtosecond laser pulse propagating in a mixture of dry air and water vapor (atmosphere) on a hundred-meter atmospheric track have been presented. Measurements have been made for a spectrally limited pulse and a pulse with initial frequency-phase modulation. A model for calculating the humidity and refractive index of a multicomponent medium for estimating the dispersion spread of an ultrashort pulse in the atmosphere has been discussed.