Table of contents

It is shown that the use of a doubly resonant cavity provides a considerable increase in the intracavity SHG efficiency in microlasers in the case of small nonlinearity parameters.

The possibility of lasing on the third continuum in argon was studied theoretically. This possibility is considered from the point of view of two hypotheses on the nature of the third continua, which have survived through the mid-1990s. The gain was calculated under conditions when the oscillation is most probable, namely, in the pressure range between 1 and 30 bar for specific pump power in the range 10^-3—10 MW cm^-3 bar^-1. The gain is shown to be negative under the above conditions and, hence, lasing cannot be achieved.

The kinetics of exchange by vibrational quanta is studied theoretically and experimentally in highly excited CO molecules by analysing the recovery time of the inversion population on the selected vibrational-rotational transitions in a doubly Q-switched pulsed CO laser. A model of vibrational kinetics that takes into account the multiquantum vibrational exchange between CO molecules in the temperature range between 100 and 300 K is described in detail for the first time. This model is compared with the model of single-quantum vibrational exchange. Good agreement between experimental data and theoretical calculations performed using the model of multiquantum vibrational exchange is the first direct confirmation of the validity of this model.

Characteristics of a chemical H₂—F₂ laser initiated by resonant vibrational excitation of HF molecules upon the development of a thermal chain branching reaction are theoretically studied. After the addition of N₂F₂ or F₂SO₃ molecules with partial pressure of 20 Torr to the HF: H₂: F₂: O₂: He = 0.5: 76: 228: 23: 100-Torr mixture, a specific output energy of a hydrogen fluoride laser initiated by a 2-μs pulse from an H₂—F₂ laser with the energy density of 0.01–10 J cm^-2 amounted to 30–60 J litre^-1, exceeding the specific initiation energy by a factor of 100 —4000.

The excitation circuits of a copper vapour laser (CVL) with a partial or total discharge of the storage capacitor were analysed. Based on this analysis, the effect of prepulse electron density on the repetition rate-energy characteristics of a CVL was shown to be minimal when the following conditions are fulfilled: R>2(L/C)^1/2 and L/R<τ_c (for partial-discharge pump schemes; C is the storage capacitor capacity, L is the discharge circuit inductance, R is the prepulse resistance of the active medium, and τ_c is the switch turn-on time) and for a high frequency of free oscillations in the discharge circuit of a CVL. The conclusion was drawn that traditional pump schemes limit the energy capabilities of a CVL. The repetition rate-energy characteristics of a CVL can be improved by resorting to schemes with a shock or complex excitation circuit or their combination.

It is found that a laser with an anisotropic cavity containing an absorbing medium may exhibit no response of the output radiation intensity to the modulation of the optical density of the absorbing medium in the presence of a magnetic field. The conditions when this effect may be observed are determined, and its possible applications are briefly discussed.

An analysis is performed of the general relationships of the statistical theory of laser-induced damage in transparent solids caused by absorbing inclusions. It is shown that the structure of the statistical theory equations determines the interrelation of various dependences of the damage thresholds on physical parameters. This results in the equivalence of the spot-size dependence of the damage threshold and the reliability of the transparent solid, as well as in a similarity of the threshold dependences obtained upon single-shot and multishot irradiation. The predictions of the theory are in good agreement with the experimental data.

The dynamics of anisotropic local melting of monocrystalline semiconductors irradiated by intense pulses of coherent and incoherent light is studied. The time dependences of the size and the density (per unit area) of local melting region obtained in situ are interpreted using a model of a short-lived metastable state characterised by superheating in the solid phase. Experiments are discussed required to provide the final answer to the problem of the mechanism of the effect revealed.

Based on two-dimensional numerical simulations, a study was made of the radiation absorption, the energy transfer, and the plasma formation upon the interaction of a laser beam with a homogeneous medium consisting of light elements with a density not exceeding the critical plasma density, which corresponds to the plasma resonance for the wavelength of the driving laser radiation. The calculations were performed by the HEAT-3D code, which involved the solution of a two-dimensional heat conduction equation with an energy source describing the absorption of laser radiation due to inverse bremsstrahlung in the material. The simulations were performed of the interaction of laser beams of radii 10^-2—6×10^-2 cm, 10¹⁴—5×10¹⁵ W cm^-2 in intensity, and with wavelengths of 1.053 and 0.527 μm with materials composed of light elements with densities of 1–10 mg cm^-3. An analysis of the simulations showed that the spatial temperature distribution of the resultant plasma is determined by the anisotropy of energy transfer. In its turn, the degree of anisotropy depends on the relation between the beam radius and the laser radiation absorption length, which is a function of the density and the temperature of the resultant plasma. The results of simulations are compared with the findings of experiments on laser irradiation of targets composed of low-density materials.

We propose new schemes of high-resolution coherent four-photon spectroscopy where chirped pulses are employed for the Raman excitation of a nonlinear medium. The use of linearly chirped pulses in coherent anti-Stokes Raman scattering spectroscopy allows the spectra of nonlinear susceptibility to be studied with a terahertz resolution by measuring the intensity of the signal emerging from four-wave mixing as a function of the delay time between the pump pulses. This approach can be also employed for single-pulse spectroscopy of nonlinear susceptibility.

The uniqueness and stability of the solutions of two- and three-parameter inverse problems in nonlinear fluorimetry (saturation fluorimetry) of complex organic compounds is studied by the method of computer simulations. The main attention is paid to a practical stability of such problems with respect to the input data noise and variations in the model. The use of the technique of artificial neural networks (including their training on noisy input data) provides the solution stability under real experimental conditions.

A nonlinear optical system with spatially distributed feedback was studied both theoretically and experimentally. The phase-to-intensity transformation in this system was performed by a spatial filter capable of suppressing low spatial frequencies. Hard excitation of stationary spatial structures observed in this system was explained by an analysis of the phase space structure of the amplitude equations. The developed theoretical approach, which uses a step-function approximation of the steady-state solution, allows one to determine the main quantitative characteristics of the generated structures. The basic properties of the response of the system to external perturbations with various symmetries were investigated experimentally. The obtained experimental data qualitatively agree with the results of the theoretical analysis.

The propagation and interaction of ultrashort pulses in a nonresonance quadratically nonlinear medium is considered. A stationary solution was found analytically. It was demonstrated numerically that pulses with an energy much lower than the energy of a stationary pulse decay under the influence of dispersion during propagation, whereas pulses with an energy higher than the energy of a stationary pulse disintegrate into a series of pulses moving like stationary ones. The effect of additive and multiplicative amplitude modulation on the pulse propagation was investigated. Stationary pulses were shown to be stable upon weak modulation and in collisions with each other.

Optical superradiance was experimentally obtained for the first time on the ³P₀—³H₄(0) transition in a Van Vleck paramagnet — a Pr³⁺:LaF₃ crystal. Superradiance at a wavelength of 477.7 nm was emitted by a sample 8 mm long, with resonant absorption coefficient α=4.7 cm^-1, excited by 10-ns laser pulses at a temperature of 2.2 K.

A high-efficiency extracavity parametric oscillator based on KTP crystals for converting radiation of Nd³⁺: YAG lasers at 1.064 μm to radiation at 1.573 μm is made and its oscillation characteristics are studied. A differential efficiency of 56.5% and a threshold energy density of 0.06 J cm^-2 were obtained. The angular divergence of parametric emission did not exceed four diffraction limits in a wide range of pump energies. The feasibility of stable operation for a more than 30-fold excess of pump energy over the threshold value was obtained.

The dynamics of holographic recording in a film of azo-containing liquid-crystal polymer is studied both experimentally and theoretically in the presence of the orienting electric field. The possibility is demonstrated to efficiently control the optical recording regime by varying the applied field. The specificity of hologram formation in the case of different polarisation of the interacting light beams is considered. A theoretical model describing the holographic recording dynamics in a film of azo-containing nematic polymer is presented; numerical simulation of the processes of recording and read-out in anisotropic holograms is carried out as well.

A search is performed for a frequency reference in the 732-nm region, which is required to calibrate a laser spectrometer in experiments on precision measurements of the 1S—2S transition frequency in muonium. The adequate reference is found in the absorption spectrum of molecular iodine vapours heated to 600°C. The reference frequency is less than 1 GHz away from the required spectrometer frequency tuned to the resonance in muonium.

t is proved analytically and numerically that, under certain conditions, the reversible spontaneous emission of two-level atoms moving in a high-Q resonator and described quantum-classically can be chaotic in the sense of the exponential sensitivity with respect to the initial conditions. The wavelet analysis of the vacuum Rabi oscillations showed that this chaos is structural. The numerical estimates showed that a Rydberg atom maser with a superconducting microwave resonator operating in a strong coupling mode is a promising device for detecting mani-festations of the dynamic chaos in the reversible spontaneous emission.

A laser-induced thermomechanical hydrodynamic flow is theoretically predicted and experimentally detected in a horizontal layer of a nematic liquid crystal with a homogeneous distribution of the director in the presence of vertical temperature gradient. This effect is caused by the gradient in the director distribution (which is necessary for the usual thermomechanical effect) produced by the light field due to photoinduced Freedericksz transition.

A review of reports made at the IX seminar, which was held at the 'Applied Chemistry' Russian Research Centre (Smolyachkovo, Leningrad region), is presented.