V. Yu. Fedorov

Interaction/Laser Radiation with Matter Filamentation of Laser Pulses with Different Wavelengths in Air

Propagation in the air of laser radiation with wavelengths in the range from 248 to 1240 nm are investigated numerically. It was shown that the intensity in a filament weakly depends on the wavelength in a long-wave region and decreases significantly in the UV region. Electron concentration in a plasma channel increases with a wavelength decrease with a dependence close to quadratic. With an increase in the wavelength characteristic scale of the intensity variation in the pulse cross section, the radii of the filament and plasma channel increase.

A nonlinear optical model of an air medium in the problem of filamentation of femtosecond laser pulses of different wavelengths

A frequency-dependent model of nonlinear optical response of atmospheric air upon propagation of a femtosecond laser pulse is constructed. The model is derived on the basis of generalized experimental and theoretical data on the cubic susceptibility and photoionization of gaseous components of air. The model was proved by solving the problems of filamentation of a femtosecond laser pulse with a wavelength lying in the range 0.24 to 1.2 μm.

Filamentation of a femtosecond laser pulse with the initial beam ellipticity

The effect of transverse elliptical intensity distribution on multiple filament formation in a femtosecond laser pulse is numerically studied. Two types of filament control are demonstrated. First, the beginning of filamentation can be varied by changing the degree of ellipticity. Second, random multiple filaments can be arranged in a spatial order predetermined by the specific shape of the ellipse. The latter is valid for pulses with a peak power more than ten times higher than the critical power for self-focusing in the medium.

Third harmonic generation by ultrashort laser pulses tightly focused in air

The influence of focusing conditions (numerical apertures from 0.004 to 0.06) on absolute energetic characteristics of third harmonic generation (THG) in air was experimentally studied for pumping 1R (744 nm wavelength) femtosecond laser pulses. THG was observed both for sub-critical and super-critical laser pulses in the linear and non-linear propagation modes, respectively. The maximum THG efficiency of 1.6 × 10−3 was obtained in our experiments at the tight focusing conditions and the sub-critical pulse powers.

Light bullets from Mid-IR femtosecond filament in air

We numerically investigated the formation of light bullets under filamentation of 3.8- μm femtosecond pulse in presence of anomalous GVD in humid air. The dispersion of humid air in infrared spectral region was characterized by model fits of the real part of refractive index from HITRAN database. The fit for the 2.8-4.2 μm region describes the areas of anomalous GVD near 3.1 μm and 4 μm in humid air. During the nonlinear propagation of femtosecond pulse in humid air, the compressed in space and time wave packet – light bullet – appears in the central time layers of the pulse and moves to the pulse tail. The duration of light bullet reaches the few-cycle value and the energy fluence in the in the light bullet cross-section is above 1 J/cm2. Together with the light bullet formation the spectrum of the pulse broadens strongly and covers the spectral region from 3 μm to 5 μm. The interference model was used for investigation of the frequency-angular distribution of the supercontinuum spectrum components.

Calculation of the growth kinetics and dispersity of K2SO4 crystals in drops of an evaporating solution

Experimental data on the evaporation kinetics of saturated K2SO4 solution drops and the nucleation kinetics of the first crystals are used to develop a simple procedure for the calculation of the solution concentration and the number, size, and dispersity of growing crystals. The calculation results agree well with the experimental data on the growth kinetics of K2SO4 crystals and their dispersity after complete evaporation of water. The dispersity of crystals is shown to linearly depend on the reciprocal time of evaporation of drops having different initial heights.

THz generation by two-color femtosecond filaments with complex polarization states: four-wave mixing versus photocurrent contributions

Two-color filamenation in gases is known to produce intense and broadband THz radiation. There are two physical mechanisms responsible for the THz generation in this scheme: four-wave mixing and emission from the induced plasma currents. The case when the main and second harmonic are linearly polarized is well studied including the impact from each of the above mechanisms. However, for the cases when the two-color fields have complex polarization states the role of the four-wave mixing and plasma mechanisms in the formation of the THz polarization is still under-explored. Here we use both the four-wave mixing and photocurrent models in order to consider the THz generation by two-color fields with arbitrary polarizations. We show that under specific polarizations of the two-color field components it is possible to determine which of the mechanisms is responsible for the THz polarization formation.

Effect of dynamic diffusion of air, nitrogen, and helium gaseous media on the microhardness of ionic crystals with juvenile surfaces

The load dependences of the microhardness of surface layers of NaCl and LiF ionic single crystals with juvenile surfaces and surfaces exposed to air for a long time measured in the air, nitrogen, and helium gaseous media have been investigated. It has been found that there is a change in the sign of the derivative of the microhardness as a function of the load for LiF crystals indented in helium and after their aging in air, as well as a weaker effect of the nitrogen and air gaseous media on the studied dependences as compared to NaCl crystals. It has also been found that, after the aging of the surface of NaCl crystals in air, there is a change in the sign of the derivative of the microhardness in the nitrogen and air gaseous media, as well as a pronounced change in the microhardness as a function of the time of aging the samples in air as compared to the weaker effect of the gaseous medium for LiF crystals. The obtained data have been analyzed in terms of the phenomenon of dislocation-dynamic diffusion of particles from the external medium into crystalline materials during their plastic deformation along the nucleating and moving dislocations. It has been shown that this phenomenon affects the microhardness through changes in the intensity of dislocation multiplication upon the formation of indentation rosettes in different gaseous media. The performed investigation of the microhardness of the juvenile surface of NaCl and LiF crystals in different gaseous media has revealed for the first time a different character of dislocation-dynamic diffusion of these media in a “pure” form.

Elasticity, anelasticity, and microplasticity of directionally crystallized aluminum-germanium alloys

The structure, Young’s modulus defect, and internal friction in aluminum-germanium alloys have been studied under conditions of longitudinal elastic vibrations with a strain amplitude in the range of 10−6−3 × 10−4 at frequencies about 100 kHz. The ribbon-shaped samples of the alloys with the germanium content from 35 to 64 wt % have been produced by drawing from the melt by the Stepanov method at a rate of 0.1 mm/s. It has been shown that the dependences of the Young’s modulus defect, logarithmic decrement, and vibration stress amplitude on the germanium content in the alloy at a constant strain amplitude have an extremum at 53 wt % Ge. This composition corresponds to the eutectic composition. The dependences of the Young’s modulus defect, the decrement, and vibration stress amplitude at a constant microstrain amplitude have been explained by the vibrational displacements of dislocations, which depend on the alloy structure.

Dispersity and growth kinetics of K2SO4 crystals in drops of an evaporating solution

Growth of K2SO4 crystals is studied in solution drops that have different initial heights and evaporate in different times. The dependences of the crystal size on the crystal growth time are obtained. The following three crystal growth modes are detected: rapid crystal growth in a supersaturated solution, a stop in the growth as a result of complete removal of supersaturation, and slow growth at a quasi-equilibrium solution concentration. The dispersities of the crystals that are retained at the bottom of the drop after complete evaporation of the solvent are calculated. A linear relation between the crystal dispersity and the reciprocal crystal growth time is revealed. The dispersity of K2SO4 crystals and the dispersity of the solid-solution dendrites in aluminum alloys are found to exhibit the same character of their dependences on the reciprocal crystal growth time.