Table of contents

A review is given of the original theoretical results obtained in studies of self-organisation of micrometre and nanometre defect — deformation structures in solids under the action of laser radiation causing development of generation — diffusion — deformation instabilities. A good agreement with experimental results is demonstrated.

A review is given of the published theoretical and experimental investigations of the interference stabilisation of Rydberg atoms by a strong optical field. The physical nature of the effect, the models used to describe it, the alternative stabilisation mechanisms, and the extent to which the theoretical results match the published experimental data are discussed.

An experiment providing evidence in support of the hypothesis that the bioluminescent reaction is dissociative in character was proposed and carried out. The processes occurring in the native bioluminescent system were modelled by the luciferase — luciferin complex. The experiment was performed in accordance with the pump-and-probe scheme involving probing by picosecond laser pulses and was based on the steady-state quenching of the intrinsic tryptophan fluorescence of luciferase when the latter is bound in a complex with luciferin. An increase in the intensity of the tryptophan fluorescence on photoexcitation of luciferin was regarded as confirmation of the hypothesis of the dissociation of the complex and it made it possible to estimate the rate of propagation of the corresponding conformational changes through the protein molecule.

A scheme for an injection optical parametric oscillator (OPO) pumped by a forsterite laser is examined (λ = 1.25 μm). The possibility of efficient generation of femtosecond pulses in the mid-IR range is demonstrated for RbTiOAsO₄ and AgGaSe₂ crystals, ensuring group-velocity matching of the interacting waves. A two-stage system designed to increase the contrast of the femtosecond laser radiation by parametric amplification and generation of the sum frequency in the group-velocity-matching regime is proposed.

A numerical simulation was conducted of the generation of short-wavelength radiation in a gas target driven by a short laser pulse, taking into account the diffraction of the laser beam and the generated fields. It is shown that a sequence of attosecond x-ray pulses of about 100 as duration is formed in the far field of diffraction. The feasibility of extracting a single attosecond pulse out of the total generated field employing pinholes is demonstrated.

It is shown that collinear three- and five-wave mixing schemes may operate in an isotropic gyrotropic medium (the Koroteev problem in chiral nonlinear optics) when waveguide propagation of optical waves is employed. The normal modes (eigenmodes) of the waveguide field are characterised by the presence of a longitudinal electric field component, which leads to the appearance of a transverse nonlinear polarisation component and lifts the prohibition of collinear generation of the sum and difference frequencies on the basis of a quadratic nonlinearity, and of a bioCARS signal based on a fourth-order nonlinearity. Expressions are obtained for the amplitudes of signals arising as a result of such nonlinear optical mixing.

The problem of steady-state propagation of several mutually incoherent optical waves — components of 'dark' multicomponent solitons and cnoidal waves — through a photorefractive crystal with a drift nonlinearity of the defocusing type is considered and solved. Analytical expressions are obtained for the distributions of the optical field between the components of the resulting solutions, containing up to three self-consistent components inclusive. It is shown that these solutions are stable and that their spatial structure is retained in mutual collisions and after stochastic perturbations of the intensity distributions.

A simple method for the production of ultrashort light pulses (USPs) with suppressed photon fluctuations is considered, which is based on self-phase modulation (SPM) of a USP in a nonlinear medium, say, an optical fibre and subsequent transmission of a pulse through a dispersive optical element.

Exact self-bending soliton solutions for the equation of a beam in a photorefractive medium with diffusion and drift components of the nonlinear response are obtained and their stability is analysed. It is shown that the interaction of solitons in a medium with nonlocal nonlinearity is accompanied by energy exchange, which causes, as a rule, the annihilation of one of the solitons.

Ultrathin (5 — 25 nm thick) nickel films were investigated experimentally by the picosecond biharmonic pumping method. Characteristic side resonances in the wing of the dependence of the self-diffraction efficiency on the frequency detuning of the biharmonic pumping components were observed. A model of the electronic part of the cubic nonlinear susceptibility was constructed. It takes into account the real structure of the spectrum of the electronic states of nickel, its quantum-size renormalisation and spin splitting, interband and intraband relaxation, as well as selection rules for the electronic transitions. For interband polarisation relaxation times T₂ > 300 fs, a qualitative agreement was obtained between the calculated relationships and the experimental data. In the frequency-degenerate case, points at which the nonlinear response should be absent were found on the frequency scale.

A theory of second-harmonic generation in spatially periodic media is developed. This theory takes account of distortions of the atomic wave functions in the field of an external electromagnetic wave and also of the spatial dispersion. The results of experiments on second-harmonic generation in one-dimensional periodic media are discussed. The angular and polarisation dependences of the reflected second harmonic are calculated. A comparison of the experimental results with the theoretical calculations shows that they are in good agreement.

It is proposed to use bichromatic laser fields to increase the efficiency of generation of coherent short-wavelength radiation by atoms in the process of above-threshold tunnel ionisation. It is shown in the framework of a semiclassical model that the use of a relatively weak static field, along with the basic pump radiation, results in the increase of the fraction of recombining photoelectrons and in the rise of their kinetic energy at the moment of recombination. It is shown that the experimental realisation of this effect is possible with the use of high-power CO₂ laser radiation (instead of the static field) and ultrashort pulses of the basic pump component with wavelength λ ~ 1 μm. Numerical calculations in the framework of a quantum mechanical model confirm the conclusions of the semiclassical model and show that the addition of long-wavelength radiation enables tripling the maximum generation frequency. Under specific intensity of CO₂ laser radiation, the so-called multi-plateau structure was discovered near the high-frequency edge of the generated radiation spectrum. An interpretation of this effect is given in the framework of a semiclassical model.

The photoinduced processes in azo-compound-containing side-chain polymer films with liquid-crystal properties are examined theoretically. A model is proposed whereby it is possible to consider the dynamics of the optical response of a medium taking into account the anisotropic saturation in the angular distribution of the azo-dye isomers as well as the intermolecular interaction. The influence of the liquid-crystal ordering in the polymer is taken into account by introducing a phenomenological mean-field factor. Analytical solutions describing changes in the optical properties of a polymer film during the initial illumination stages are in good agreement with experimental data.

Transient self-interaction of low-power He — Ne laser radiation (1 < 50 mW cm^-2 ) in a liquid-crystal polymer film containing chemically bound azodye molecules was observed experimentally. The self-interaction occurred in the region of a temperature-induced phase transition in the polymer film and was accompanied by the formation of quasi-periodic ring-shaped structures in the distribution of the transmitted light intensity.