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The problem of controlling the parameters of the filamentation region of high-power femtosecond laser pulses for amplitude modulation of radiation by a metal mesh mask is theoretically considered. In this case, the initial laser beam is split into individual lower energy subbeams. This leads to a pronounced regularization of the spatial structure of the filaments, which is formed at the stage of radiation self-focusing due to diffraction interaction between subbeams in a nonlinear medium. Using numerical simulation, it is found that the total length of the filamentation region of femtosecond laser radiation in air is reduced when using modulation meshes. At the same time, the longitudinal continuity of laser plasma in such filaments can significantly increase. It is shown that the spatial parameters of filaments (coordinate of the beginning, length, and continuity) can be controlled over a wide range by changing mesh parameters (crosshair thickness and cell size), as well as the position of the mesh mask relative to the laser beam center. The results are important for predicting the propagation of high-power femtosecond laser radiation in a nonlinear medium, in particular, along atmospheric paths. [ABSTRACT FROM AUTHOR]

Results of numerical simulations of optical radiation focusing by a cluster of identical nanospheres close packed into a spherical volume (a "metaparticle") are presented. Parameters of the focal region (intensity and longitudinal and transverse sizes) formed by metaparticles with different internal structure are calculated. It is shown that the problem of optical radiation focusing by a spherical cluster of nanoparticles can be reduced in certain cases to the problem of light focusing by a homogeneous spherical particle with an effective refractive index derived from the effective medium theory. Moreover, certain spherical cluster topologies make it possible to improve optical focusing in the near-field region, in particular, to increase the peak intensity or enhance its spatial localization. [ABSTRACT FROM AUTHOR]

We simulate and examine the absorption dynamics of the near-infrared optical radiation in a spherical microcapsule surrounded by solid nanoparticles of different optical properties (metal, biocompatible dielectric) with the use of numerical finite-difference time-domain (FDTD) calculations. A model microcapsule resembles a microcontainer used in modern bio- and medical technologies for targeted delivery of therapeutic nanodoses of drugs to the desired region of biological tissues. We show that the optical field superlocalization in the "hot regions" on the microcapsule surface takes place due to light scattering on nanoparticles. The three-fold light absorption enhancement can be achieved due to the addition of buffer nanoparticles. [ABSTRACT FROM AUTHOR]

Results of numerical simulation of the optical radiation focusing by Fresnel zone plates (ZP) with the wavelength-scale (mesowavelength) dimensions and the two-layer profile of phase ridges are presented. We show that the dimensions and intensity of the focusing area are sensitive to the ZP design. By changing the phase step shape (height and width of plate faces) it is possible to significantly improve the light field characteristics in the focusing region due to smoothing the wave phase profile after passing through individual ZP layers. The comparison is made with the case of conventional binary ZP. [ABSTRACT FROM AUTHOR]

Optical resonance of the internal field of a dielectric microparticle occurs when the frequency of the incident light wave is tuned to the frequency of one of the particle spatial eigenmodes and leads to a sharp increase in the intensity and localization of the field near the surface with the formation of annular-periodic structures in the form of standing waves, the so-called "whispering gallery" modes (WGMs). We theoretically consider the case where a dielectric microsphere is placed near a flexible light reflecting membrane which acts as an external pressure sensor. In this case, due to reflection from the reflecting membrane, the WGMs of the sphere are excited twice by direct and backward reflected radiation which then couples within the microparticle volume. The optical intensity of the resulting WGM field carries information about the position of the flexible loaded membrane. This allows one to propose a physical concept of a new miniature all-optical pressure sensor. It is shown that the pressure sensitivity of such a sensor depends on the Q factor of the excited resonance mode, as well as on the geometrical and mechanical parameters of the flexible membrane. Important advantages of the sensor proposed are the miniature design (the size of the sensor is determined by the diameter of the flexible membrane) and the noncontact type of the pressure sensor placement. [ABSTRACT FROM AUTHOR]

Propagation of high-power phase-modulated femtosecond laser pulses in air is simulated numerically. Spatial modulation of the initial wavefront of the pulse was implemented using a programmable phase mask consisting of checkered zones with a varied phase jump of the wave. Within the framework of the numerical solution of the nonlinear Schrödinger equation for the time-averaged amplitude of the electric field, modes of self-focusing, filamentation, and postfilamentation channeling of radiation for phase-modulating masks with different phase shifts at boundaries of adjacent segments are studied. It is shown that for certain types of phase modulation of the pulse the filamentation domain in air can be significantly shifted (in the coordinate) and elongated as compared with a nonmodulated pulse. In addition, it is established that using phase modulation makes it possible to decrease angular divergence of high-intensity light channels forming at the stage of postfilamentation propagation. This provides the possibility of radiation self-channeling at distances multiple exceeding the Rayleigh length. [ABSTRACT FROM AUTHOR]

Results of laboratory experiments on control of the filamentation domain of a focused femtosecond laser beam with a spatially structured (with a deformable mirror) wavefront are described. Aberration beams consisting of coherent interleaved ring subapertures obtained with a deformable mirror of specific form are studied. This allows producing inhomogeneities in the distribution of the optical field amplitude during laser beam propagation; these inhomogeneities can subsequently become seeds for light filaments. This approach to control high-power radiation filamentation does not require compensation for distortions of the initial beam profile; on the contrary, it is based on a controllable introduction of pre-calculated wavefront aberrations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Results of numerical simulation of binary phase Fresnel zone plates (FZPs) with a mesowave length size and a large aperture angle (NA ∼ 1) are presented. The study is aimed at revealing the main features in the formation of the spatial near-field structure, as well as optimization of FZP parameters for obtaining maximally intense foci. It is shown that dimensional parameters of the FZP focal region turn out to be sensitive both to the zone etch depth and to the type of constructive design of the plate. An increase in the number of plate zones leads to the formation of local outer foci of the plate with a higher intensity. [ABSTRACT FROM AUTHOR]

Theoretical aspects of nonlinear propagation of high-power ultrashort laser radiation in air in the filamentation mode with a transverse coronal-type intensity distribution formed by incoherent addition of several radiating subapertures arranged in a ring are considered. Using a numerical solution of the nonlinear Schrödinger equation averaged over time, transformations of the intensity profile of synthesized beams along the optical path are studied under changes in the number and power of partial radiators constituting them. It is shown that synthesized beams with a ring-shaped profile have a number of advantages from the viewpoint of control over the radiation filamentation domain. In particular, by changing the number and size of the subapertures, one can significantly delay the beginning of filamentation of the whole beam and increase the distance of its existence as compared to beams with a traditional unimodal profile (Gaussian or plateau-like). [ABSTRACT FROM AUTHOR]

The results of experimental and theoretical studies of the evolution of the small-scale transverse structure of high-power femtosecond laser radiation propagating in air in the multiple filamentation mode are presented. It has been found that the presence of intensity inhomogeneities in the initial transverse profile of the laser beam leads to the formation of spatially isolated light channels due to the Kerr self-focusing effect. When the power in these channels exceeds a certain threshold value (the critical power), radiation filamentation occurs in them. The parameters of these light channels are theoretically estimated on the basis of the diffraction-ray model of single filamentation. It is shown that for a laser beam with a centimeter radius and subterawatt power, the initial radius of intensity inhomogeneities in the transverse profile has a characteristic value of several millimeters. [ABSTRACT FROM AUTHOR]

Spatial resolution of conventional optics, which is necessary for nondestructive trapping of microobjects, is limited by diffraction to a value equal to half of the radiation wavelength. Despite this limitation, use of optical methods is one of the main directions in biological and biomedical researches because only these methods have a minimal impact on living organisms. The rapid advance in this area is largely owing to the development of new optical technologies and the considerable advance in mesoscale photonics, which has allowed researchers to develop techniques for controlling structured beams for optical traps. In this work, we consider some recent trends in the field of optical manipulation based on mesoscale dielectric particles. [ABSTRACT FROM AUTHOR]

An experimental setup and experiments on measurements and calculation of light pressure on constructional materials at different light angles are presented. This calculation is necessary to determine the lighting properties of a material and the total solar pressure moment. Industrial metal-knitted textile was used as a constructional material. Metal-knitted textile is a material for large-diameter spacecraft antennas. More than 4000 experimental measurements of the transmittance and reflection coefficients of the metal-knitted textile were carried out for the calculation. [ABSTRACT FROM AUTHOR]

Characteristics of the domain of multiple filamentation in air are estimated based on the numerical solution of a nonlinear Schrödinger equation in the single filamentation statement. The method of diffraction-ray tubes was used to describe the single filamentation of laser pulses. The efficiency of this method for interpreting experimental results and predicting effects is shown, which is important when planning experiments. The characteristic size of small-scale intensity inhomogeneities in the laser centimeter-radius beam profile, which induce a multiple filamentation of femtosecond pulses, is shown to be several millimeters. An increase in the initial laser beam radius during telescoping increases the sizes of the initial small-scale intensity inhomogeneities, and reduces their power. This leads to an increase in the distance to the start of filamentation. An increase in the initial beam power promotes the elongation of the filaments and increases their number. [ABSTRACT FROM AUTHOR]

The results are presented of experiments on controlling the characteristics of the multiple filamentation domain of femtosecond laser pulses along atmospheric paths via variations in the initial spatial focusing, beam radius, and pulse energy, as well as the optical field structure at the initial beam aperture. [ABSTRACT FROM AUTHOR]

Based on numerical simulation and qualitative analysis, the effect of the group velocity dispersion on the formation of light structures during self-focusing and filamentation of femtosecond Ti:sapphire laser pulses in air is studied. The main features of the filamentation have been determined at various pulse duration, initial beam radii, and peak radiation powers based on the results of numerical solutions of the nonlinear Schrödinger equation in a Kerr-plasma dissipative dispersion medium and using the diffraction-beam tube approach. Dispersion is detected in the cases where the dispersion length is not a minimum at the scale of the process. It is shown that the relative (normalized to Rayleigh length) coordinate of the beginning of filamentation increases as the dispersion distortions of the pulse increase, and the filamentation channel length is reduced. For shorter laser pulses (tens of femtoseconds), the filamentation ceases as the laser beam radius increases. The size of the energy-replenishing diffraction-beam tube and the angular divergence of postfilamentation light channels increase for this class of pulses. [ABSTRACT FROM AUTHOR]

Results of the theoretical study of the propagation of femtosecond pulses of a Ti:Sapphire laser in air under self-focusing and filamentation are presented. The self-focusing of laser pulses is analyzed on the basis of the method of diffraction-beam tubes. The analysis established that specific light structures were formed in a laser beam during self-focusing. One of such structures is an energy-replenishing diffraction-beam tube, which provides the necessary energy for filamentation and has the form of a high-intensity light channel during postfilamentation pulse propagation. The dependences of the radius and power of the energy-replenishing tube on the initial beam radius and peak power at a fixed pulse length are derived. It is revealed that the energy consumption of radiation for the filamentation decreases with an increase in the beam radius. The peak power contained in the energy-replenishing light tube during the postfilamentation laser pulse propagation does not exceed the critical self-focusing power of a Gaussian beam and weakly depends on the initial pulse parameters. [ABSTRACT FROM AUTHOR]

The results of theoretical studies of spatially localized near-field light structures (photonic nanojets) formed when laser radiation is scattered on a meta-surface having the form of an ordered single-layer assembly of dielectric microparticles (spheres, cones) embedded in a transparent matrix (silicone film) are presented. The behavior of the main parameters of the localized light structures (length, width, and peak intensity) under the effects of light fields of neighboring microparticles is thoroughly analyzed by using computational electrodynamics to solve Maxwell's equations with the finite-difference time-domain method (FDTD). It is ascertained that the main factors which affect the parameters of photonic nanojets under study are the spatial orientation of the microcones and the depth of their immersion in the silicone matrix. It is shown that several spatial configurations of the microassembly of the cones allow the creation of an ensemble of photonic nanojets with specific parameters, which are unattainable for isolated microcones. Ordered clusters of spherical particles have an advantage in terms of a comprehensive assessment of the parameters of photonic nanojets. [ABSTRACT FROM AUTHOR]

The results of experimental and theoretical studies of femtosecond laser pulse filamentation with use of a bimorph deformable mirror are presented. The mirror allows controlling the position of the filamentation domain throughout a model path due to phase distortions of different parts of a laser beam, determining localization of filaments and high-intensity channels in a beam cross section, and forming long (>100 m) high-intensity (1011–1012 W/cm2) weakly diverging plasma-free channels. [ABSTRACT FROM AUTHOR]

The effect of photonic nanojets (PNJ) in the scattering near field nearby the surface of dielectric axisymmetric microparticles (hemisphere, axicon, and combined particles) under optical illumination is theoretically considered. Key PNJ parameters (length, width, and peak intensity) are calculated using the discrete-dipole approximation; the dependence of these parameters on the geometric shape of microparticles is analyzed. We show that the use of a special type of combined transparent particle, which consists of an axicon and two attached hemispheres, can produce ultralocalized light fluxes with a peak intensity which considerably exceeds the corresponding values for simple particles (hemisphere and axicon). [ABSTRACT FROM AUTHOR]

Based on the diffraction-ray representation of nonlinear propagation of laser pulses, some examples of visualization of the filamentation of high-power femtosecond laser pulses in air are considered in the form of evolutionary paths of diffraction beams and light tubes. It is ascertained that a filament is supported by the light energy of the part of the beam which is inside a beam tube with the initial peak power higher than the critical power of self-focusing. This light tube is an energy reservoir for the filament. The light beam periphery holds the diffraction-beam tube which includes the filament by means of formation of a "diffractive" waveguide. [ABSTRACT FROM AUTHOR]

The concept of nonstationary diffraction-beam optics of high-power femtosecond laser pulses is presented. According to the concept, the power of a beam propagates along specific light structures—diffraction- beam tubes. These tubes do not intersect and do not exchange energy, but changes in their shape and cross sections during propagation show the effect of physical processes that occur with radiation in the medium. The nonstationary theory is supplemented with evolutionary equations for time-averaged diffraction rays and effective squared radii of diffraction tubes. [ABSTRACT FROM AUTHOR]

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-μm 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. [ABSTRACT FROM AUTHOR]

The results of laboratory experiments on measurements of the length and position of the filamentation zone of femtosecond GW laser radiation at wavelengths of 800 and 400 nm propagating in air under conditions of sharp external focusing are presented. Dependencies of the length of the radiation filamentation zone on its initial radius and power are studied. Qualitatively new regularity is obtained, which consists in invariance of the observed length of the beam filamentation under variations in its diameter with the assumption of equality of the initial intensities. The feature is not typical for collimated radiation and is connected with the dominance of geometrical focusing over Kerr self-focusing of a beam during filament formation. [ABSTRACT FROM AUTHOR]

The single (axial) filamentation regime of high-power collimated femtosecond laser radiation in the atmosphere is theoretically investigated. The effect of initial diameter of a laser beam on characteristics of a filament is analyzed. We consider three filament parameters: the coordinate of the filament start, its length and longitudinal continuity. We found that for the same initial beam power all the above characteristics of the filaments show a dependence on the beam radius. As the main physical cause for this circumstance the diffraction interaction of various beam areas during the filamentation process is considered; the strength of this interaction increases as the beam widens. [ABSTRACT FROM AUTHOR]