Table of contents

An investigation is reported of the stability of steady-state injection locking of a solid-state chip laser. It is shown that, in contrast to semiconductor lasers, the buildup of relaxation oscillations occurs only in a very small part of the injection-locking zone. The conditions are found for bistability in the steady-state injection-locking regime. It is demonstrated that detuning of the injected-signal frequency from the centre of the gain line results in a considerable asymmetry of the beat regimes on different sides of the injection-locking zone. Numerical simulation reveals bistable quasiperiodic regimes in which hf sinusoidal beats are modulated by pulses of self-excited relaxation oscillations.

The gains measured at the centres of vibrational — rotational lines of the main bands of the CO₂ molecule were used to determine, by the least-squares method, the populations of the laser-active levels and the vibrational temperatures of the active medium of a cw electric-discharge CO₂ laser.

An experimental investigation was made of the influence of the chemical composition of the oxidiser gas in an atomic-fluorine generator on the efficiency of generation of radiation representing the first overtone of the HF molecule in a self-contained supersonic cw chemical HF laser with the active medium 70 cm long. The optimal chemical composition was different for the fundamental and overtone transitions. A specific output energy of 84 J g^-1 at a specific mass flow rate of 0.13 g s^-1 cm^-2 through the nozzle array was achieved by optimisation of a linear three-mirror optical cavity at the 1.33 — 1.35 μm wavelengths. The overtone radiation power generated in the whole of the active medium was 7.5 kW, corresponding to a 41% efficiency of energy conversion to an overtone.

An experimental investigation was made of the energy and temporal characteristics of the pump process and of the output radiation of a pulsed gas-discharge excimer KrF laser (λ =248 nm) with He as the buffer gas. An RU-65 spark gap was used as the design base. The parameters of a high-voltage excitation circuit of the LC inverter type with automatic UV preionisation were optimised. A laser efficiency (based on the stored energy) of 2.4% and an output radiation energy of 0.57 J were reached for the first time for a gaseous medium of the He:Kr:F₂= 89.8:10:0.2 composition at a total pressure of 2.5 bar. The maximum output energy of the KrF laser was 0.82 J in the form of pulses of 24 ns duration at half-amplitude; the specific output energy was then 5.9 J litre^-1 and the efficiency was 2.0%.

An investigation was made of laser-induced face transitions of metals under confined surface conditions. The dynamics of the melting of indium was investigated by direct recording of a thermo-optically excited pressure pulse. The steep pressure rise observed when the absorbed energy exceeded the melting threshold was related to the volume expansion of the melt. The proposed method could be used to investigate phase diagrams of substances right up to their critical points.

A description is given of a new method for the formation of an ohmic contact with silicon carbide (SiC) single crystals, based on laser ablation of the crystal surface. It was found that regions of SiC ablated in air or in vacuum by copper vapour laser radiation (wavelength 510.6 nm, pulse duration 10 ns) become capable of reducing metals (Cu or Ni) from suitable solutions intended for autocatalytic deposition. The Raman spectra of the ablated regions demonstrate the presence of Si nanoclusters of 10–20 nm size, which are one of the reasons for chemical reduction of the metal. The deposited metal forms an ohmic contact with n-type SiC without additional annealing and the specific contact resistance depends weakly on the nature of the metal, amounting to 2.3 and 2.1 mΩ cm² for Cu and Ni, respectively. The mechanisms of ohmic contact formation by laser ablation are discussed.

The first precision measurements of the wavelengths and identifications were made of the satellites of the Heb line of the Si XIII ion in the emission spectrum of a laser plasma. These satellites were the result of radiation decay of the 1s3l₁2l₂ levels of the Si XII ion. The wavelengths were determined more accurately for the 1s3l₁3l₂ configurations. The experimental results obtained were compared with calculations carried out by various methods. The structure of the relative intensities of the 1s3l₁3l₂ satellites indicated that their emission occurred mainly in a thin overdense plasma region with N_e>N_e^cr.

A mathematical model and a numerical calculation scheme are proposed for diffusive redistribution of carbon, in the thermal influence zone of carbon steels and alloys, as a result of surface treatments with high-energy sources. Experimental and numerical investigations are reported of the influence of the interlamellar spacing and of the treatment rate on the depth of the hardened layer.

The intensities, widths, and shifts of the centres of the emission lines of a laser plasma generated on the surfaces of solid targets were determined experimentally as a function of the ambient gas pressure. The Stark broadening and the Stark shift of the emission lines were used to estimate the electron temperature and the electron density at various gas pressures.

Photopolymerisation of oxygen-bearing C₆₀ films by λ =459 nm laser radiation and by 100-fs λ=410 nm pulses was compared by visual observation of photodarkening spots and by recording the Raman spectra, in the range 600 — 1700 cm^-1, of the irradiated regions. When the average intensity was 10 — 20 W cm^-2, the irradiation dose needed for photopolymerisation was very different in the cw and pulsed cases. For the cw radiation, it was close to the dose for oxygen-free samples, amounting to 20 kJ cm^-2. For the femtosecond pulses, the threshold radiation dose was two orders of magnitude higher, which could be attributed to characteristic features of relaxation of the excited molecules.

An investigation was made of diffractive self-reproduction of a periodic wave field of two-dimensional arrays is studied. The conditions are found for the existence of this effect and expressions are derived for the self-reproduction distance. The critical gain increment for a phase-locked mode and the selectivity factor of two-dimensional arrays in Talbot cavities are found. It is shown that the efficiency of an array with a triangular occupancy configuration is higher than the efficiency of a square configuration.

A new more accurate calculation algorithm (with a variable grid-mesh size) is proposed for the propagation of radiation considered in the paraxial approximation based on the Fresnel — Kirchhoff formula. The algorithm was used to develop the FRESNEL program intended for simulation of the propagation of radiation through laser systems. Examples of calculations carried out with the new algorithm are given and they are compared with the traditional method.

An expression is obtained for the aberration operator of a plane-parallel plate and it is shown that its use in the far-field zone makes it possible to take full account of electromagnetic field perturbations. Examples are given of calculations in accordance with the proposed algorithm when the radiation field is split into 1024 × 1024 discrete elements.

An analysis is made of an increase in the radiation divergence in a self-starting laser system with four-wave mirrors based on saturated-gain gratings in the active medium of an Nd : YAlO₃ laser. The experimental results obtained lead to the conclusion that in such systems with a loop cavity the full phase conjugation does not occur because at least two gain gratings shifted in phase by π are formed in the active medium of the laser.

Picosecond absorption spectroscopy was used in an investigation of nonlinear optical processes in nanostructured composites based on polytungstic acid, which were photocoloured by preliminary UV irradiation. The absorption bands responsible for the photocoloration and associated with intervalent charge transfer were bleached by the action of powerful laser pulses. The characteristic monoexponential relaxation time of the process was ~300 ps. The results obtained were analysed taking account of the special features of the splitting of the energy levels of tungsten ions with different degrees of oxidation and of delocalisation of the valence states within separate elements of the polytungstic acid structure.

An investigation was made of the photoluminescence and differential absorption spectra of phosphate glasses containing embedded CdSe nanoparticles. These spectra were determined by the pump-and-probe method. The photoluminescence spectrum included a short-wavelength band resulting from direct recombination of electron — hole (eh) pairs and a long-wavelength band associated with carrier recombination via deep levels of defects. Two peaks, attributed to the 1S_e → 1S_3/2 and 1S_e → 2S_3/2 transitions, were observed in the short-wavelength photoluminescence band at high excitation rates. The kinetics of the differential absorption spectra was explained by saturation of the absorption of the exciton (eh) transitions and by the appearance, in the excited nanoparticles, of the absorption corresponding to two-pair (2eh) states.

An experimental investigation was made of phase conjugation in the course of intracavity degenerate four-wave mixing of radiation in the active medium of a pulsed electron-beam-controlled discharge CO₂ laser generating long pulses. An analysis was carried out of the dependences of the energy and temporal characteristics of a phase-conjugated signal on the specific input energy deposited in the electron-beam-controlled discharge, on the Q-factor of the laser cavity, on the composition and pressure of the laser-active mixture, on the angle of interaction and optical delay between the probe and reference waves, on the ratio of the intensities of these two waves, and on the polarisation and spectral composition of laser radiation. The energy reflection coefficient of the phase-conjugated signal increased with increase in the input energy and with reduction in the probe wave intensity. An increase in the proportion of nitrogen and a reduction in the proportion of helium in the laser-active mixture increased the reflection coefficient and gave rise to a specific profile of the phase-conjugated signal with two maxima, attributed to amplitude and phase nonlinearities of the active medium. An increase in the specific input energy and of the cavity Q-factor reduced the response time of the phase-conjugate signal.

The spectra of the two-photon-excited luminescence were determined and measurements were made of the two-photon absorption cross section of deoxyribonucleotides (5' dAMP, 5' dGMP, 5' dTMP, 5' dCMP) and of deoxyribonucleic acid (DNA) at the wavelength of the second harmonic of an Nd³⁺ : YAG laser (λ = 532 nm).

A novel approach is proposed for reconstruction of the temporal characteristics of radiation from the results of nonlinear—optical measurements. A description is given of an algorithm for the processing of a two-dimensional set of data, based on inversion of the Gabor transform. This algorithm makes it possible to reconstruct arbitrary temporal signals, including multipulse signals, in the absence of any a priori information on their profile.

A numerical simulation is reported of dynamic multiple (isotropic and anisotropic) scattering of light in a randomly inhomogeneous medium representing a suspension of light-scattering Brownian particles. The results of a simulation of the temporal autocorrelation function of backscattered light are compared with calculations carried out within the framework of the diffusion approximation for the radiation transport equation. It is shown that, although the diffusion approximation describes incorrectly low-order scattering, it is quite acceptable for dealing with high-order scattering.

An automated IR spectrometer was constructed on the basis of a pulsed optical parametric oscillator tunable in the range 1.5 — 3.7 μm. An interferometer optothermal method with a phase-contrast system for recording the signal was adopted. When the energy of the parametric oscillator radiation was 10 mJ, the minimum detectable absorption coefficient was α ~10^-6 cm^-1

Photodynamic effects were observed for the first time in laser Doppler anemometry: the Doppler frequency shift of light scattered by a particle was found to depend on the action of probe light beams on this particle.

An analysis is made of physical models of selective three-stage photoionisation of a vapour in a three-dimensional cavity. These models have been used in theoretical analyses of preparation of significant amounts of highly enriched ¹⁶⁸Yb by the AVLIS (atomic-vapour laser isotope separation) method. A comparison of the model predictions with experiments shows that a three-dimensional model ignoring the absorption of laser radiation in a medium describes well the experimental results obtained for the second and third ionisation stages. The first excitation stage is described satisfactorily by a model in which absorption is included. A comparison of the calculated and experimental results is used to find the coefficient (65%) representing ion extraction from a laser plasma.