A. A. Zemlyanov

Microaxicon-generated photonic nanojets

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.

Kerr-driven nonlinear refractive index of air at 800 and 400 nm measured through femtosecond laser pulse filamentation

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.

Evolution of effective characteristics of laser beam of femtosecond duration upon self-action in a gas medium

Based on numerical solution of the nonlinear Schrödinger 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.

Self-action of tightly focused femtosecond laser radiation in air in a filamentation regime: Laboratory and numerical experiments

The influence of self-focusing of strong femtosecond laser radiation on the spatial energy distribution of a laser beam was studied. Experimental data are provided on the transverse energy density distribution of the tightly focused radiation of the Ti-sapphire laser following its filamentation in air. The results are interpreted within the framework of the nonlinear Schrödinger equation model. The best agreement between the theory and the experiment is found in the model that takes into account the change of character of the nonlinear response of the medium induced by high-intensity radiation in the nonlinear focus, which includes the change of the ionization mechanism of molecules from multiphoton ionization to tunnel ionization, the completely instantaneous Kerr nonlinearity, and saturation of the latter due to higher-order nonlinearity. The mechanism of the ionization of molecules can change from purely multiphoton ionization to tunnel ionization.

Intensity of optical field inside a weakly absorbing spherical particle irradiated by a femtosecond laser pulse

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.

Peculiarities of filamentation of sharply focused ultrashort laser pulses in air

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of Rpl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.

Comparative analysis of spatial shapes of photonic jets from spherical dielectric microparticles

A photonic jet is a specific spatially-localized region in the near field of optical radiation scattering by a micron-size particle. The spatial shape and basic characteristics of photonic jets depend to a large extent on the internal structure and size of microparticles. A qualitative classification of morphological types of photonic jets from spherical dielectric microparticles formed in vicinity of the shadow surface is presented. The classification is based on the analysis of their spatial shapes by taking into consideration the dimensional and power characteristics of the jets. Particles with homogeneous and radially inhomogeneous changes in the refractive index are studied.

Energy light structures during femtosecond laser radiation filamentation in air. To the 50th anniversary of the first paper about light self-focusing

A class of energy light structures in nonlinear wave physics, so-called diffraction-beam tubes in a self-focusing dissipative medium, has been studied. It is shown that their interaction along a high-power femtosecond laser radiation propagation path produces areas of light energy localization, stable in physical characteristics, i.e., a filament and a post-filamentation channel, as well as an energy-conservative periphery. Stabilization of boundaries of the above areas is provided by additional contraction of a diffractive nature from the side of the periphery. The dependencies of filamentation length of a laser pulse on the power and beam radius and the fact of high intensity conservation in the post-filamentation light channel are interpreted using this concept.

Diffraction optics of a light filament generated during self-focusing of a femtosecond laser pulse in air

The equations for time-averaged diffraction beams describing self-focusing of femtosecond laser radiation in air are derived on the basis of the nonlinear Schrödinger equation. It is shown numerically that these beams pass through three stages along the propagation distance: 1) nonlinear focus formation; 2) light filament; and 3) postfilamentation evolution. The diffraction beams at the second and third stages are characterized by two structures; one of them localized in a paraxial area has an angular divergence less than the diffraction one after output from a filamentation zone. Another structure is a peripheral area with characteristic divergence corresponding to divergence of the laser beam generally after passing through the nonlinear focus zone. The principal role of the diffraction interaction between the central and peripheral parts of a laser beam in the formation of a stable light channel near the beam axis is established.

Controlling the parameters of photon nanojets of composite microspheres

We consider the spatial and amplitude characteristics of PNJ forming in the neighborhood of the shadow surface of micron-sized composite particles consisting of a nucleus and a shell with various refraction indices when laser radiation scatters on them. We study the longitudinal and transverse dimensions of a photon flux and its peak intensity depending on the microparticle shell thickness. We show that a certain choice of the refraction index of the shell relative to the nucleus in two-layer composite spherical microparticles can significantly lengthen the forming PNJ or increase their peak intensity. The width of the photon flux during this changes insignificantly.