Table of contents

The dependences of the dynamic parameters of the growth of a vapour-gas channel and the depth of the melting zone on the shape of laser pulses and the spatial beam parameters are studied. Based on this study, the requirements are formulated to the characteristics of radiation providing a many-fold increase in the melting depth with respect to the depth that was previously considered as limiting. A new laser setup for deep high-quality welding was built. The role of the buffer volume of a melt in the suppression of ejection of molten metal from the laser-heated region upon deflection of the melt surface is substantiated. An increase in the rate of growth of a vapour-gas channel during an increase in its length (the self-concentration of a heat source) was experimentally observed. At the depths corresponding to the preferential vertical orientation of fluid flows, a large-scale vortex motion of the melt was observed. The dependence of the penetration depth of the melting front on the aperture angle β was experimentally found to have an extremum. The maximum penetration depth was obtained for β = 0.075 — 0.80 rad. Recommendations for designing pulsed laser technological systems for high-quality welding of metals are formulated.

The spectral, luminescent, and lasing properties of aqueous solutions of a cationic dye rhodamine 6G with additions of anion polyelectrolytes — polyacrylic and polymethacrylic acids — are studied. It is found that the energy and spectral properties of lasing of these solutions depend on the ratio of concentrations of polyelectrolyte and molecules. It is also found that the lasing parameters of aqueous-polyelectrolyte dye solutions can be controlled by changing the structure of the molecular system. The variation in the structure of aqueous-polyelectrolyte dye solutions of rhodamine 6G resulted in an almost five-fold increase in the lasing efficiency compared to that in aqueous dye solutions.

The mechanical, optical, and thermooptical properties of a microporous glass-polymer (MPG—P) composite used as a matrix for solid-state dye lasers are studied. It is shown that the composite has a high mechanical hardness, good transparency, excellent thermooptical parameters, and high laser damage resistance, and can be also readily doped with various dyes. The analysis of physical properties of the MPG—P composite showed its advantages over other solid matrices (bulk polymers and sol-gel glasses) for applications in efficient solid-state dye lasers.

The lasing dynamics of a wide-aperture laser with an intracavity saturable absorber is theoretically studied. The saturable absorber inserting gives rise to an autowave profile of the optical field. The characteristic equation for the perturbations of the laser field is derived and solved. The spatial spectrum of autowaves is determined. The relevant set of equations was numerically solved for two types of resonant boundary conditions: total reflection of light from the side boundaries of the cavity and a coaxial laser geometry.

A thermodynamically consistent model for calculating the equation of plasma state in the average-ion approximation is proposed. The model takes into account the chemical bonds in a solid, the pressure of electron shells of an ion, which arises upon the ion compression, and also the degeneracy of the electron gas at high densities and low temperatures. The calculations of shock compression of different materials, such as liquid deuterium, Al, Be, Fe and Au, carried out using this model showed that it provides a satisfactory description both of the existing experimental results and of the results of calculations involving the Thomas — Fermi model over a broad range of pressure behind the shock front. The isotherms of the above materials calculated in the average-ion approximation and using the Thomas — Fermi model are compared for different temperatures.

The generation of harmonics from a solid surface irradiated by 27-ps pulses (I≤1.5×10¹⁵ W cm^-2) from a neodymium laser is studied. The conversion efficiency of p-polarised laser radiation was more than ten times greater than that of the s-polarised radiation for the second harmonic generation and more than one hundred times greater for the third harmonic generation. The optical rotation of the second harmonic generated by s- and p-polarised laser radiation is studied. The efficiencies of the second, third, and fourth harmonic generation were 2×10^-8, 2×10^-10, and 5×10^-12, respectively. The intensities of the second, third, and fourth harmonics exhibit a power dependence on the laser radiation intensity with exponents equal to 1.5, 1.8, and 3.8, respectively.

A decrease in the refractive index of water in its transparency region caused by irradiation by high-power 100-ps pulses from a 2.94-μm erbium laser is found and measured. The possible reasons for this phenomenon are discussed.

The theory of the interaction of a centrosymmetric atom with a superstrong spatially inhomogeneous laser field is developed. This theory employs the two-level approximation to describe the dynamics of spatially nonlocal interactions related to variations in the populations of the levels. We propose a model that includes spatially nonlocal interactions, such as magnetic-dipole and quadrupole interactions and interactions due to the gradient of the ponderomotive potential of the field. We consider the interaction of a homogeneous medium of centrosymmetric atoms with a superstrong laser field, which is represented as a superposition of two plane-wave ultrashort pulses propagating at some angle with respect to each other. Perturbation theory, which is valid for the fields of moderate intensity, is developed for the atomic response. The results of numerical simulations are compared with the predictions of this perturbation theory. The specific features of the nonlinear-optical response of an atom in a superstrong field are investigated. The angular distribution of the second- and third-harmonic emission is calculated in the constant-field approximation for different polarisations of the incident field.

An analytic solution is obtained for the equations of resonance coherent SRS by neglecting the population of the final level of the Raman transition for the systems with the active-medium length that is smaller than the wavelength of the incident light. For the extended systems, a numerical solution is obtained. The energy distribution of the Stokes pulses is found. The large-scale (about 100%) fluctuations of the Stokes radiation energy were observed in the case of unsaturated amplified spontaneous emission.

The specific features of the propagation of soliton-like light beams through a fully ionised two-dimensional cold plasma are considered employing analytical and numerical methods commonly used in nonlinear optics. Exact soliton profiles for the lower and upper soliton branches are found numerically in the presence of optical bistability. It is shown that the interaction of incoherent soliton-like laser beams in such a plasma may result both in the destruction of one of the beams and in production of new ones. The regime of the modulation instability of a plane wave propagating through a cold laser-produced plasma is studied.

The results are presented of calculating the stimulated Raman scattering (SRS) of femtosecond pulses in a periodic structure consisting of alternating quarter-wave plates of fused silica and a Nd:KGW crystal. The analysis of the calculations shows that efficient SRS conversion of the pump can be obtained only if the density of the electromagnetic field states is increased by varying the spectral properties of the periodic structure.

The effect of relaxation of nonlinearity on the formation of a shock wave of the intensity envelope of ultrashort laser pulses is studied on the basis of numerical analysis of the corresponding truncated wave equation. The relaxation of nonlinearity is shown to lead to the stabilisation of the envelope sharpening process, with the minimum width of the shock-wave front being limited by the relaxation time of nonlinearity. It is established that depending on the nonlinearity sign, the relaxation causes compression or spreading of the propagating pulse.

A version of the perturbation theory is developed for determining the field distribution of spatial solitons with a 2D transverse profile in a medium with saturable absorption and gain in the case of small deviations from paraxial conditions. Starting from the unperturbed paraxial soliton with the linear polarisation of radiation, an approximate master equation is derived for transverse components of the electric field in the case of wide solitons. It is shown that its solution represents a stable weakly nonparaxial dissipative optical vector soliton with an axially asymmetric field distribution.

Analytic expressions are obtained for spin and orbital moments of multidimensional optical solitons — two-dimensional beams and three-dimensional light bullets. It is shown that for given directions of the incidence of light bullets, the time delay in the pulse sequence determines the direction and value of the orbital moment, and can be used as a parameter to control the structure appearing during the mutual capture.

The study of the propagation of an electromagnetic ultrashort pulse in a Kerr medium with impurity atoms under quasi-resonance conditions is reported, which demonstrates the efficiency of the unitary transformation method in nonlinear optics. An equation is derived, which differs from the known equations describing the evolution of a pulse envelope and takes into account the dispersion of the nonlinear response and the dispersion of group velocities.

Excitation of the presumed laser levels of Fe I and Fe II in electron — atom collisions is studied by the techniques of extended intersecting beams and optical spectroscopy. The total excitation cross sections for the z⁵P₃^o, x⁵D₄^o, and z⁵F₅^o levels of the iron atom were determined. The excitation cross sections were measured for transitions arising from the odd levels of Fe II with an excitation energy ranging from 38 000 to 48 000 cm^-1 relative to the ground level of Fe II.

An experiment is suggested for high-precision verification of the assumption concerning isotropy of the ambient space. The key element of the experimental setup is a ring laser with the cavity partially filled with a condensed matter.

The ways of raising the limiting density of cooled atoms and the extension of the volume occupied by them are considered. These include raising the laser beam intensity above the saturation intensity of the working transition, the use of a noncollinear geometry of the laser beam and the atomic medium, and coherent cooling by sequences of counterpropagating π-pulses.

Acoustic monitoring of a plastic operation for reshaping the porcine ear using radiation from a Ho:YAG laser was performed to control a change in the elasticity of the ear cartilage. Variations in the cartilage elasticity were controlled by changes in the amplitude and shape of an acoustic wave during the laser action. It is shown that the optoacoustic signal amplitude exponentially decreases at least by a factor of 2–2.5 at the moment of the cartilage reshaping caused by the action of radiation pulses from a Ho:YAG laser.