Table of contents

Basic methods of laser photothermal spectroscopy, which are used to study photoinduced absorption in various media, are briefly considered. Comparative analysis of these methods is performed and the latest results obtained in this field are discussed. Different schemes and examples of their practical implementation are considered.

Based on the calculation of the electrostatic field potential of space charges, we have analysed the characteristic features of light-induced scattering of extraordinary polarised light in photorefractive (PR) crystals (for example, an SBN-75 crystal). Using the method of images, the electrostatic field is analysed for surface (aperiodic) waves along the crystal – dielectric (air) interface. It is shown that the field distributions satisfying the boundary conditions can emerge only upon accumulation of a screening electric charge in a narrow transition layer of thickness ∼1 mm, the sign of the charge being opposite that of the space charge in the illuminated region of the crystal. A model is proposed to explain the observed features of the surface PR waves in a metal-film coated PR crystal. In considering the contact potential difference at the PR crystal – film interface it is shown that in the crystal layer (adjacent to the film) enriched with charge carriers, i.e., electrons, the refractive index can be significantly reduced. In the case of small excitation angles (0 – 1.5°), this layer can act as an optical barrier, the reflection from which can result in near-surface waves; a characteristic difference from the previously observed oscillatory surface waves is the presence of a broadened intensity distribution shifted inside the crystal.

The boundary problem on the development of a smallscale instability of arbitrarily polarised plane waves is solved. The influence of polarisation on the spatial-perturbation gain is analysed. The maximally allowable (i.e., causing no destruction of a nonlinear medium) average intensity at a specified input noise is determined for arbitrary polarisation.

The filamentation of IR and UV laser pulses has been studied numerically and experimentally for different initial beam focusing geometries, and linear electron density profiles along the plasma channel of filaments have been obtained. The results demonstrate that changes in laser beam focusing have a stronger effect on filament and plasma channel parameters for UV radiation than for IR radiation. Focusing causes individual high fluence regions produced by refocusing to merge to form a continuous extended filament with a continuous plasma channel.

A generalised theory has been used to carry out a comparative analysis of the use of various four-level and quasi-threelevel media for the self-starting of degenerate four-wave mixing PCW generation directly in a laser medium placed in a loop resonator. It has been shown that quasi-three-level media can compete with four-level media at long upper laser level lifetimes and increased pump intensities. The most attractive solid-state laser media for four-wave PCW generation have been identified that have the highest deposited energy at a given pump intensity. In addition to neodymium-doped crystals, which are already widely used for four-wave PCW generation, promising materials are fourlevel chromium-doped media, e.g. alexandrite and Cr : LiCAF, and quasi-three-level media with the longest upper laser level lifetime, such as Yb : YAG and Tm, Ho : YAG, at high pump intensities.

We have studied temporal and amplitude characteristics of the electrical signal arising under irradiation of the free water surface by a 2.94-μm high-power laser. The investigations have been conducted using a specially designed electrode system, which makes it possible to record the motion of the electric charge after the end of the laser pulse. We have measured the dependences of the signal parameters on the radiation energy density at the water surface.

A new method was developed for the mass spectrometric analysis of organic and bioorganic compounds, which involves laser ablation with the ionisation of its products by laser-plasma radiation and enables analysing gaseous, liquid, and solid substances at atmospheric pressure without sample preparation. The capabilities of this method were demonstrated by the examples of fast pharmaceutical composition screening, real-time atmosphere composition analysis, and construction of the mass spectrometric images of organic compound distributions in biological materials.

Ceramic and crystalline weakly guiding optical fibres with the core – cladding refractive index difference of 10^-2 – 10^-4 are fabricated by a hot pressing method. The waveguides with one or several cores for operation in the spectral range 0.2 – 5 μm are produced. The waveguides are based on CaF₂, SrF₂, and BaF₂ ceramics and crystals and their solid solutions doped with trivalent Pr, Nd, Tb, Dy, Yb, Ho, Er, and Tm ions, as well as on LiF ceramics and crystals with colour centres. The first results of investigation of the lasing properties of ceramic SrF₂ : NdF waveguides under diode pumping are presented, and the prospects of further investigation are discussed.

The characteristics of a random laser based on a nanocomposite medium consisting of a transparent dielectric and scattering doped nanocrystals are calculated. It is proposed to use ytterbium laser media with a high concentration of active ions as nanocrystals and to use gases, liquids, or solid dielectrics with a refractive index lower than that of nanocrystals as dielectric matrices for nanocrystals. Based on the concept of nonresonant distributed feedback due to the Rayleigh scattering, an expression is obtained for the minimum length of a nanocomposite laser medium at which the random lasing threshold is overcome. Expressions are found for the critical (maximum) and the optimal size of nanocrystals, as well as for the optimal relative refractive index of nanocomposites that corresponds not only to the maximum gain but also to the minimum of the medium threshold length at the optimal size of nanocrystals. It is shown that the optimal relative refractive index of a nanocomposite increases with increasing pump level, but is independent of the other nanocomposite parameters.

Optical absorptions of the low-lying states with higher angular momenta of the D⁻ system in a spherical quantum dot (QD) with the Gaussian potential are studied by using the matrix diagonalisation method and the compact density-matrix approach. The linear, nonlinear third-order and total optical absorption coefficients are calculated for the ¹P⁻ → ¹D⁺ and ¹D⁺ → ¹F⁻ transitions. Numerical results for GaAs/Ga_1-xAl_xAs QDs are presented. The calculated results show that with increasing quantum numbers describing the angular momenta of transitions the optical absorption peaks shift towards lower energies and their intensities increase.

The model is proposed, which allows one to solve the problem of finding the energy spectrum and the wave function of an electron in a type-I core – shell quantum dot. It is shown that the size of the core and shell can serve as control parameters for the optimisation of the energy structure of the quantum dot in order to obtain the real structures with desired electrophysical and optical properties.

Employing the methods of linear absorption spectroscopy and nonlinear four-wave mixing spectroscopy using laserinduced gratings we have simultaneously measured the local concentrations of H₂O molecules and the gas temperature in the process of the H₂ – O₂ mixture heating. During the measurements of the deactivation rates of pulsed-laser excited singlet oxygen O₂ (b ¹Σ⁺_g) in collisions with H₂ in the range 294 – 850 K, the joint use of the two methods made it possible to determine the degree of hydrogen oxidation at a given temperature. As the mixture is heated, H₂O molecules are formed by 'dark' reactions of H₂ with O₂ in the ground state. The experiments have shown that the measurements of tunable diode laser radiation absorption along an optical path through the inhomogeneously heated gas mixture in a cell allows high-accuracy determination of the local H₂O concentration in the O₂ laser excitation volume, if the gas temperature in this volume is known. When studying the collisional deactivation of O₂ (b ¹Σ⁺_g) molecules, the necessary measurements of the local temperature can be implemented using laser-induced gratings, arising due to spatially periodic excitation of O₂ (X³Σ^-_g) molecules to the b ¹Σ⁺_g state by radiation of the pump laser of the four-wave mixing spectrometer.

A method of computer generation and projection recording of microholograms for holographic memory systems is presented; the results of mathematical modelling and experimental implementation of the method are demonstrated.

We have theoretically studied the effect of difference in particle shapes on the appearance of the diffraction pattern, which arises in the scattering of a laser beam on a dilute suspension of erythrocytes in an ectacytometer. We have proposed data processing algorithms allowing one to estimate the red blood cell shape parameter variance under conditions of laser ectacytometry. The conclusions of the theoretical analysis are verified experimentally.

The publishing house 'Avers' has published O.D. Volpyan's and A.I. Kuzmichev's book 'Negative Refraction of Waves'.