[en] Experimental study of the nonlinear propagation of near-IR gigawatt femtosecond laser pulses in air in the self-focusing and filamentation regimes have been performed in open air and in laboratory. The influence of the initial geometric divergence (both positive and negative) of the laser beam with an irregular intensity profile on the transverse light energy distribution at the end of the path is studied. It is shown experimentally that the displacement of the filamentation region due to geometric focusing or defocusing, makes it possible to control the number and spatial location of light energy density peaks in the receiving plane. The conditions under which a light filament can be reconstructed after the beam transmission through a linear focal waist are determined. A semi-empirical threshold relation is obtained for the beam focusing force and the beam power, when light beam undergoes filamentation behind the geometric focus of the optical system. (extreme light fields and their applications)

[en] We report the investigation results for spatially-localised light structures (photonic nanojets) under near-field optical radiation scattering on phase diffraction gratings. Main parameters of photonic nanojets from gratings with sawtooth, rectangular and hemispherical groove profiles are obtained by numerical electrodynamic simulation. It is found that by varying a period, degree of filling, groove shape and parameters of optically contrast coating of the diffraction grating one can control the characteristics of the produced photonic jets in a wide range. (laser beams)

[en] The propagation of focused high-power femtosecond laser pulses in air is numerically simulated. The dependences of the effective average size of a focal spot and the maximum achievable radiation intensity in the focal beam waist on the peak power of incident radiation are studied. It is shown that in the regime of nonstationary self-action of radiation, due to photoionisation of the medium and formation of plasma, it becomes impossible to focus radiation into a spot of diffraction-limited size predicted by a linear theory. (nonlinear optical phenomena)

[en] We consider a specific spatially localised light structure, namely, a 'photonic jet' formed in the near field upon scattering of an optical wave in a dielectric micron particle. Dimensional parameters and intensity of a photonic jet from microaxicons of different spatial orientation are studied theoretically. It is found for the first time that an axicon-generated photonic jet has in this case a substantially larger length compared with the case of a jet formed on a spherical particle. (scattering of light)

[en] This paper examines the propagation of focused femtosecond gigawatt laser pulses in air under normal and reduced pressure in the case of Kerr self-focusing and photoionisation of the medium. The influence of gas density on the beam dimensions and power and the electron density in the plasma column in the nonlinear focus zone of the laser beam has been studied experimentally and by numerical simulation. It has been shown that, in rarefied air, the radiation-induced reduction in the rate of plasma formation diminishes the blocking effect of the plasma on the growth of the beam intensity in the case of tight focusing. This allows higher power densities of ultrashort laser pulses to be reached in the focal waist region in comparison with beam self-focusing under atmospheric pressure.

[en] A model of optical nonlinearity of air (atmospheric nitrogen and oxygen) is developed. This model can be used to calculate numerically the propagation of radiation with a wavelength close to 10 μm. It takes into account the electronic Kerr effect, higher order nonlinearities, ionisation of a gas medium by electron impact, and pulse group-velocity dispersion. The applicability limits of the Drude approximation for calculating the impact-ionisation rate are also considered. (nonlinear optical phenomena)

[en] The propagation of high-power femtosecond laser pulses is considered in the self-focusing and filamentation formation regime. Similarities and differences in the character of the laser beam filamentation are analysed qualitatively and quantitatively in the atmospheric air and water. Based on the numerical results, the most important parameters (radius, intensity, free electron density) in light and plasma filaments produced by ultrashort laser pulses are shown to depend strongly on the optical parameters of the propagation medium, and, first of all, on the Kerr nonlinearity coefficients and the photoionisation rate of the medium molecules. (nonlinear-optics phenomena)

[en] 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). (paper)

[en] The spatial and amplitude characteristics of photonic nanojets from micrometre-sized composite particles consisting of a nucleus and several shells with different refractive indices were considered. We investigated the longitudinal and transverse dimensions of the photon jet as well as the dependence of its peak intensity on the optical contrast of the shells. It was shown that, by varying the refractive index of the neighbouring shells in composite spherical microparticles, it is possible to manipulate the photonic nanojet parameters, in particular, increase its length or raise the peak intensity of the photon flux. (interaction of laser radiation with matter. laser plasma)

[en] Spatially localised light structures (photonic nanojets) formed in the near field of light scattering from an ordered ensemble of single-layer clusters of transparent glass microspheres have been theoretically studied. The main parameters of photonic nanojets (length, width, focal distance and intensity) are calculated based on numerical solution of Maxwell equations, and their behaviour under conditions of interference between neighbouring microparticles is analysed. The degree of manifestation of collective effects during the formation of a nanojet array is found to depend on the spatial structure of the ensemble of microspheres, their size and repetition period. It is found that in some cases, having chosen an appropriate configuration of a microsphere ensemble, one can improve significantly the characteristics of the photonic nanojets formed by this ensemble. (interaction of laser radiation with matter)