The numerical simulations of propagation of high-power femtosecond laser pulses in air under conditions of superposed spatial phase modulation are carried out on the basis of the reduced form of nonlinear Schrödinger equation for time-averaged electric field envelope. Initial spatial modulations are applied to pulse wavefront profiling by a staggered phase mask with variable phase jumps between adjacent elements. It is shown that with specific phase modulations, the pulse filamentation region in air can be markedly shifted further and elongated compared to a non-modulated pulse.

The evolution of the small-scale transverse structure of high-power femtosecond laser radiation propagating in air in the filamentation mode has been experimentally and theoretically studied. Experimental results were obtained using wideaperture (centimeter) collimated beams of femtosecond pulses of a titanium-sapphire laser. As a result, the features of propagation of spatially isolated high-intensity light channels, the diameter of which is several millimeters, were determined. They are formed because of Kerr self-focusing of intensity inhomogeneities in the initial transverse profile of the laser beam. It is shown that the formation of a filament (a localized light structure, the existence of which is associated with the plasma formation and the generation of conical emission) does not occur in each of these channels. The theoretical evaluation of this light channels characteristics was carried out. It is based on the diffraction-ray model of single filamentation of femtosecond laser pulses. Studying of the evolution transverse profile of a laser beam with a centimeter radius and subterawatt power along air path showed that the initial radius of intensity inhomogeneities, for which laser filamentation occur, is equal several (2.5-3.5) millimeters. The power in these inhomogeneities varies from 19 to 26 GW. Differences in the values of the radius and power of these inhomogeneities are the cause of different distances from the laser pulse source at which their self-focusing occurs.

Results of numerical simulation of self-action in air of a sequence of ultrashort laser pulses with a carrier in the near and mid-IR regions are presented. We show that the use of a 10.6-mu m pulse train allows significant elongation of the plasma channel generated during pulse filamentation and enhancement of its spatial connectivity. The filamentation of a submicron pulse train does not visibly change filamentation region parameters.

The near field of scattering of an optical wave (zone of photonic jet (PhJ)) at radially symmetric nonabsorbing micron-sized dielectric particles is numerically simulated with an emphasis at the study of size and amplitude parameters of photonic jets from quartz microparticles of various spatial shape and orientation. Photonic jets from hemispheres are shown to have the long length, but relatively low intensity. The use of conical particles of a certain shape gives a record increase of the PhJ length up to two tens of wavelengths of the incident radiation (at the fixed intensity level) at the preserved subwavelength transverse dimension of the photonic jet.

The results of numerical modeling of the near-field scattering of the light wave (photonic (nano) jet - PNJ) on the composite particles, representing a truncated circular cone with attached hemispheres are presented. For the first time it is shown that the combination of spherical and conical light focusing by composite particles leads to the formation of highly localized photonic jets with peak intensity being considerable higher than that for isolated microaxicons of the same cross-section.

Features of formation "photonic nanojets" (PNJs) near the surface of spherical dielectric microparticles irradiated by a spatially limited laser beam are theoretically investigated. The influence of the waist size of a light beam with the Gaussian transverse intensity profile on PNJ spatial parameters and peak intensity is found for the first time.

The possibility of ultrahigh localization of the optical field near micrometer-sized spherical dielectric particles (zone of "photonic jet") excited by the pulsed laser radiation is studied theoretically. It is shown for the first time that the photonic jet formed at the nonstationary de-excitation of high-Q resonance modes of a particle can have the sub-diffraction cross-dimensional size.

We report on the experiments on the interaction of gigawatt femtosecond laser pulses with suspended millimeter-sized water droplets. The transparent droplets experienced laser-induced breakdown and explosive boiling up and emitted a broadband radiation. This radiation covers the spectral range from 450 to 1100 nm and consists of the spectrum of laser pulse scattered and transformed by the droplet due to self-phase modulation and plasma emission produced in water during photoionization. The droplet emission spectrum showed remarkable broadening at all viewing angles and is maximal in the direction of the laser exit from the droplet. The enlargement of the droplet results in additional spectral spreading of the emitted radiation. The depth and amount of laser pulse spectral self-transformations upon propagation through the water droplet are simulated by means of numerical calculations. (C) 2011 Optical Society of America

The peculiarities of resonant optical field excitation inside a water microdroplet under illumination by a spatially bounded Gaussian beam with a temporal regime of a single chirped ultrashort laser pulse and a chirped pulse train are considered. It is established that the coupling efficiency of incident radiation to a selected high-Q whispering-gallery mode significantly depends on the interpulse interval in the train and chirping parameter of pumped laser radiation. The influence of the geometry of particle illumination by a laser beam and of the number of pulses in the train on the whispering-gallery mode buildup and its peak intensity is investigated.

We report numerical calculations of scattering and backscattering efficiency factors of water droplets illuminated by a quasi-white light radiation (a supercontinuum. radiation) that results from self-focusing of a high-power ferntosecond laser pulse in the atmosphere. The behavior of these integral optical characteristics for a wide range of droplet sizes that covers the size spectra of fog and cloud particles is analyzed. The results are compared to the values of the scattering factors obtained by the Lorentz-Mie theory.