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Peculiarities of internal optical field resonant excitation inside micron-sized spherical transparent dielectric cavity illuminated by a train of ultrashort laser pulses are investigated. On an example of water microdroplet is shown that optimal tuning of incident radiation to a selected high-Q resonance electromagnetic cavity mode can be realized by varying temporal interpulse interval in a laser train together with linear frequency modulation of every pulse (chirp). Efficiency of resonant particle excitation by a chirped picosecond pulses train can be considerably increased as compared to unchirped pulse train and CW excitation also.

Based on numerical solution of the nonlinear Schrodinger equation for complex envelope of the light-wave electric field strength, we study regularities in variation of effective parameters of a femtosecond laser beam (energy transfer ratio, root-mean-square radius, duration, and width of temporal spectrum) propagating in the air with formation of a single filament. We derive approximate formulas for the effective radius of the femtosecond laser beam in the regime of solitary filamentation for three characteristic zones corresponding to different stages of transient self-focusing of the beam. We also analyze the problem of effective radius of the laser beam with a nonideal Gaussian profile in the regime of multiple filamentation. At the later stages of evolution of the effective radius, for identical initial energy and dimensional characteristics of the ideal and nonideal light beams, we establish similarity of their limiting angular divergences.

Analytical expressions are derived for integral optical characteristics (the scattering, absorption, and backscattering efficiency factors) of aerosol particles irradiated by a femtosecond laser pulse train. The behavior of these integral characteristics attendant to variations of the particle size and pulse period-to-pulse duration ratio is investigated based on the results of numerical calculations.

The particular qualities of formation of the optical field in the composite spherical particles–microcapsules consisting of a polymer absorbing shell and a non-absorbing liquid core are presented. The thickness of the shell and the coefficient of its own radiation absorption determine the nature of spatial distribution and amplitude characteristics of the absorbed power which are revealed by the numerical simulation. The variation of these parameters allows changing position and peak values of the areas the effective absorption volume of the particle. Therefore the conditions conducive for the opening shells in the appropriate spatial areas in order to liberate the contents of the microcapsules are ensured.

Angular distribution of spontaneous fluorescence excited by femtosecond laser radiation inside of a spherical microparticle is considered. The optical field distribution inside the particle is investigated for multiphoton excitation of the process. The location and effective volume of fluorescence sources radiating inside the particle are calculated numerically as functions of the particle radius in the case of one-, two-, and three-photon fluorescence. Within the framework of the geometrical optics approximation, an expression for the ratio of the average fluorescence brightness values for the forward and backward hemispheres of the particle is derived. It is demonstrated that asymmetry arises in the angular spontaneous fluorescence distribution in the forward/backward directions when the fluorescence source is localized near the shadowed hemisphere of the particle. The asymmetry becomes more pronounced with increasing order of the multiphoton process.

The generic scenario of intense femtosecond laser pulse propagation in the air from the viewpoint of evolution of its integral effective parameters (energy transfer coefficient, effective radius, effective duration, limiting angular divergence) is considered. The analysis of variation of the effective parameters along the propagation path in the single and multiply filamentation scenarios based on numerical calculations is presented. It is shown that the process of self-action of the ultrashort radiation is characterized by the formation in a medium of the nonlinearity layer, after which optical pulse propagates quasi-linearly with the limiting angular divergence that depends mainly on initial pulse power. The effective pulse temporal duration and the effective beam radius increase after the passage through the nonlinearity layer, and their values are mostly determined by the initial beam power also. The coefficient of energy transmission of femtosecond laser radiation is lower than in the linear medium and has a tendency to decrease with the increase of the pulse power.

Filamentation behavior of femtosecond laser pulses in the air in presence of aerosol on the optical path is simulated numerically on basis of (3D + 1)-dimensional nonlinear Schrodinger equation. The effect of a localized attenuating layer modeling an aerosol medium, on spatial extension and evolution of the light filament is investigated. For the first time, it is shown that filamentation length depends not only on the optical depth of the aerosol layer and pulse peak power, but also on aerosol location on the propagation path and its spatial extension along the beam. The length of the filament is maximal if the nonlinear focus of the beam is located behind the aerosol layer rather than before it.

Using the Fourier technique in combination with the Mie theory, we study numerically the spatiotemporal evolution of the intensity of the internal optical field inside micron-sized weakly absorbing spherical particles upon diffraction by these particles of a femtosecond laser field. A number of specific features of the dynamics of the spatial intensity distribution of the femtosecond pulses inside the particles are found to depend on the pulse width, the shape of the laser beam, the size of the particles, and the geometry of their irradiation. It is shown that, under conditions of nonstationary diffraction, the internal optical field is usually excited in a resonance way, with the eigenfrequencies of one or several high-Q resonance modes of the particle falling into the central part of the original pulse spectrum. This causes a time delay of the light in the particle and a reduction of the absolute maximum in the time dependence of the internal field intensity as compared with a stationary regime. The greatest reduction of the peak occurs at exact resonance. In this case, the decrease in the peak intensity may reach several orders of magnitude. Irradiation of a particle by a narrow Gaussian beam of femtosecond duration directed toward the particle center enhances the internal field intensity as compared with the case of near-edge incidence.

The results of numerical simulation of multiple filamentation of terawatt femtosecond pulse Ti:Sapphire laser performed on the experimental data obtained in the airway of a length of 106 m when changing the initial spatial focusing and laser power.

We report on the new optical tool for precise manipulation of various microobjects. This tool is referred to as a “photonic nanojet” (PJ) and corresponds to specific spatially localized and high-intensity area formed near micron-sized transparent spherical dielectric particles illuminated by a visible laser radiation The descriptive analysis of the morphological shapes of photonic nanojets is presented. The PJ shape characterization is based on the numerical calculations of the near-field distribution according to the Mie theory and accounts for jet dimensions and shape complexity.

Self-action in air of a high-power femtosecond laser pulse with the spatial form of a ring-Gaussian beam ('dressed' beam) is studied theoretically. Pulse self-focusing and filamentation is analyzed in detail through the numerical solution of the spectral propagation equation, taking into account medium optical nonlinearity and plasma generation. Pulse propagation dynamics and energy fluxes inside the beam are visualized by means of averaged diffraction ray tracing. We clearly show that, in terms of diffraction optics, the outer ring forms a specific nonmaterial diffractive waveguide, favoring long-range self-channeling of the central part of a beam by delivering optical energy to a filament. The spatial robustness and stability of such diffractive waveguides strongly depends on the energy stored in the ring, as well as on its position relative to the beam axis. The striking advantage of such 'dressed' beams is their reduced angular divergence during plasma-free (post-filamentation) evolution.

By virtue of the finite element method, the numerical simulations of the temperature field near a poly-layer spherical nanoparticle exposed to a pulsed optical radiation are carried out. Such particles model the double-shell nanocapsules with an aqueous core and composite light absorbing SiO2/TiO2 shells used in various biomedical and chemical applications. For the first time to our knowledge, we study in detail the spatial distribution of absorbed light power inside the double-shell nanocapsules as well as the heating dynamics by varying particle absorbance and laser irradiation time interval. We show that the maximal achievable temperature inside a nanocapsule could be increased by shortening the laser pulse and simultaneously tuning particle absorption, which determines the amplitude and spatial localization of the internal heat sources.

Self-focusing and filamentation of externally focused ultrashort laser radiation in air are discussed. The influence of initial beam focusing on the filamentation length is numerically studied, and the conditions under which a light filament can resurrect after the linear focus are determined. We have found that the probability of this post-focal filamentation increases with an increase of laser power regardless of the type of filamentation (single/multiple).

An experimental and theoretical study of the post-filamentation stage of focused high-power Ti:Sa-laser pulses in air is presented. For the first time to our knowledge, the angular and spatial characteristics of specific spatially localized light structures, the ionization-free post-filament channels (PFCs), formed inside the laser beam in the post-filamentation region are systematically quantified under different external focusing and energy of initial pulse. We show that PFC angular divergence tends to decrease with the increase of the laser pulse energy and beam focal distance. These findings are discussed in the framework of the Bessel–Gauss-like beam formation in a course of pulse filamentation stage.

We investigated numerically the specific spatially localized intense optical structure, a photonic nanojet (PNJ), formed in the near-field scattering of optical radiation at phase diffraction gratings. The finite-difference time-domain technique was employed to study the PNJ key parameters (length, width, focal distance, and intensity) produced by diffraction gratings with the saw-tooth, rectangle, and hemispheric line profiles. Our analysis showed that each type of diffraction gratings produces a photonic jet with unique characteristics. Based on the numerical calculations, we demonstrate that the PNJ could be manipulated in a wide range through the variation of period, duty cycle, and shape of diffraction grating rulings.

The special features of an optical field formed inside a micron-sized weakly absorbing spherical dielectric particle illuminated by a spatially bounded Gaussian beam in the form of a single pulse or an ultrashort laser pulse train are considered. It is established that the efficiency of incident radiation energy transfer to a given high-Q natural resonance of the particle illuminated by a focused light beam significantly depends on the pulse period-to-pulse duration ratio in the train and linear frequency modulation depth of each pulse. The influence of the geometry of particle illumination and of the number of pulses in the train on the time dependence of the internal particle field and its peak intensity is investigated.

We consider the regularities of the femtosecond laser pulse propagation from the viewpoint of evolution of its effective parameters (energy transfer coefficient, effective beam radius, effective pulse duration, limiting angular divergence, and effective intensity). Based on the concepts of the spatial self-action zones we present a thorough analysis of beam effective parameters variation along the propagation path, discussing the regime of formation of a single axial filament. Filamentation behavior of femtosecond laser pulses in the air in the presence of aerosol on the optical path is considered also. The effect of a localized attenuating layer modeling an aerosol medium, on spatial extension and evolution of the light filament, is investigated.

The numerical simulations results of resonance excitation of internal optical field of transparent spherical microparticles under irradiation by ultra-short laser pulse train are presented. It was determined that the most optimal tuning of incident radiation to assigned high-Q eigenresonance of a particle might be realized using varying a porousness of pulse in a train with linear frequency modulation of each pulse in a train (chirping). Analytical expressions were found for calculation of these parameters depending on laser pulse duration and frequency location of excited resonance.

We report on the calculations of optical radiation scattering at micrometer-sized conical-shape dielectric particles (microaxicons) with special attention given to the specific spatially localized near-field area constituting a photonic nanojet (PNJ). By virtue of finite-difference time-domain simulations we show for the first time, to the best our knowledge, that the PNJ produced by a microaxicon of specific spatial orientation can exhibit extreme axial elongation up to ∼25λ (at defined intensity level) while retaining high peak intensity and subwavelength transverse width.

The filamentation behavior of femtosecond laser pulses in air in the presence of aerosol on the optical path is simulated numerically on the base of (3D+1)-dimensional nonlinear Schrodinger equation. The effect of a localized attenuating layer modeling an aerosol medium extinction on the spatial extension and evolution of a light filament is investigated. It is shown for the first time that the filamentation length depends not only on the optical depth of the aerosol layer and pulse peak power but also on aerosol location on the propagation path. The length of the filament is maximal if the nonlinear focus of the beam is located behind the aerosol layer rather than before it.

The filamentation dynamics of 15 GW femtosecond laser pulses at 800 and 400 nm wavelengths upon their tight focusing in air is studied experimentally. The spatial position and extent of plasma channel formed within the filamentation zone as a function of laser pulse power are investigated. The processing of the experimental data according to the Marburger formula by J. H. Marburger [Prog. Quantum. Electron. 4, 35 (1975)] and the dispersion relations for air cubic nonlinearity has allowed to estimate the effective value of the Kerr-driven air refractive index on 400 nm as 5.36 × 10−19 cm2/W with a 5% error.

The appearance of droplet deformations is discussed at the expense of jet shock resulting from local surface ejections of water-vapor mix under the high-rate laser heating of the droplet.

The theoretical aspects of propagation of laser radiation along slanted paths in the atmosphere at high altitudes under conditions of stimulated Raman scattering (SRS) by molecular nitrogen are considered. The SRS power conversion coefficient, the transverse intensity distribution, and the effective size of laser beams with different initial spatial profiles are numerically simulated along the propagation path. The behavior of these parameters is studied for different ratios between the optical path length and the length of free diffraction of the initial laser beam. It is shown that, at a given level of the increment of the SRS amplification, the diffraction can both increase and decrease the efficiency of the Raman interaction of waves owing to the intensity redistribution in the transverse beam profile. This effect is the most pronounced for beams with non-Gaussian initial spatial intensity profiles.

Experimental investigations of acoustic signals generated by individual laser-irradiated water droplets are reported. The dependence of droplet destruction thresholds on droplet radius and radiative heating rate is determined. A theoretical explanation of our experimental results is provided in terms of a model that includes the processes of droplet evaporation and fragmentation in response to intense laser heating.

The results of numerical calculations of water-droplet explosions initiated by intense CO(2)-laser radiation are presented. The theoretical model for this process is based on the solution of the value of the thermal-boundary problem in an inhomogeneously heated droplet, including the kinetic equation describingvapor generation in a superheated liquid. The main characteristics of droplet explosions (e.g., degree of explosive evaporation and time of explosion) are calculated. It is established that these characteristics depend on the heating rate of the droplet and on its radius. The results point to the fact that two droplet-heating regimes can be distinguished-slow heating and rapid heating-based on the behavior of the explosive boiling process. This division represents the competition of real physical processes in an irradiated droplet and makes it possible to separate the basic, specific features of the explosion process.