Table of contents

A technique for obtaining a repetitively pulsed operating regime in high-power wide-aperture lasers is proposed and experimentally realised. In this regime, the laser emits a train of pulses with a duration of 0.1 — 1 μs and a pulse repetition rate of several tens of kilohertz. The main properties of the pulsed regime are theoretically analysed and the proposed technique is tested in detail employing a test-bench gas-dynamic laser. The results of the test confirmed the conclusions of the theoretical analysis. The possibility of realising a repetitively pulsed regime in high-power wide-aperture lasers without a reduction in the average output power is experimentally demonstrated.

The results of experimental studies of the dynamics of formation and development of a laser plasma produced in microvolumes of gases (air) and transparent solids (fused silica) by high-intensity [I⋍(1-2)×10¹⁴ W cm^-2], ~22-ps, 539-nm laser pulses tightly focused in a region of diameter 4 μm are presented. The spatiotemporal distributions of the refractive index and the electron density are studied by the interferometric method with a spatial resolution of ~1.6 μm and a temporal resolution of ~3 ps directly during the action of excitation picosecond laser pulses. An almost complete ionisation of the initial gas was shown to occur even in the initial stage of air plasma formation, within a few picoseconds after plasma production. The irradiation of solid transparent dielectrics (fused silica) by picosecond laser pulses resulted in a reversible production of a plasma with an electron density above 10²⁰ cm^-3, which did not cause the damage of dielectrics.

A method is developed for detecting protein antigens for fluorescent immunoassay using a model system based on the technique for preparation of Langmuir films. Fluorescein isothiocyanate and donor-acceptor energy-transfer pairs of markers (the Yb complex of tetraphenyl porphyrin — benzoyl trifluoroacetoneisothiocyanate and derivatives of tetra(carboxyphenyl) porphyrin — cyanine dye containing a five-membered polyene chain), which were nor studied earlier, were used as markers for detecting the binding of an antigen on the surface of Langmuir films of antibodies. Fluorescence was detected in the near-IR region (for the first pair) and in the visible spectral range (for the second pair). To reduce the nonspecific sorption of a protein (antigen), a method was proposed for the preparation of a nonpolar surface by applying an even number of layers of stearic acid as a substrate for the Langmuir — Blodgett film. A high sensitivity of model systems to a protein antigen in solution was achieved (~10^-11 M), the assay time being 6 — 8 min. The model system with the first donor — acceptor pair was tested in analysis of the blood plasma. The fluorescence of the Dy³⁺, Tm³⁺, and Yb³⁺ complexes of tetraphenyl porphyrin sensitised by diketonate complexes of lanthanides was studied for the first time and the enhancement of the IR fluorescence of these complexes in a Langmuir film was demonstrated.

The possible acoustic and optical manifestations of the induced transparency during laser evaporation of metals are analysed. It is shown that under certain conditions the photoacoustic effect related to the movement of the induced-transparency front can make an appreciable contribution to the total recoil pressure. The formation of the induced-transparent layer can be accompanied at the initial instant of time by both the decrease and the increase in the reflectance of the target. It is established that the laser intensity, required to realise the stationary regime of evaporation with induced transparency, is substantially lower than the threshold of plasma formation.

A spatially infinite plane wave, as one of the possible solutions of Maxwell's equations, is a mathematically correct but physically empty image. It is pointed out that physically realised solutions in the form of nonplane waves are not strictly transverse waves outside a material medium and are characterised by three polarisation components, the superluminal phase and subluminal group velocities, the dispersion, the necessary presence of the fragments of a standing wave ('stopped light'), and the existence of a mass-like quantity, which can be defined as a finite observable (but not immanent) inertial and gravitational photon rest mass. This mass cannot be distinguished in a number of thought 'gedanken' experiments from the rest mass in a standard treatment.

The parameters of a high-current krypton discharge in tubes of an increased diameter (5 — 7 mm) are studied and optimised. Increased lasing power on the lines of singly and doubly charged krypton ions is achieved: 14 W (647 — 676 nm) and 6 W (407 — 415 nm). One transverse and one longitudinal mode are selected with efficiencies of 85 and 70 % using a convex — concave resonator and a Fabry — Perot etalon, respectively. The shape of the Lamb dip at the 676-nm line is studied in the single-frequency oscillation mode. It is shown that, in accordance with the theory, the dip broadens by a factor of 2.6 due to Coulomb ion — ion collisions at an achieved electron concentration N_e ~ 5 × 10¹⁴ cm^-3.

Two-photon fluorescence was observed for 18 organic dyes in a polymethyl methacrylate (PMMA) matrix excited by a femtosecond Ti:sapphire laser. The product of the cross section for two-photon absorption by the quantum yield of fluorescence (two-photon fluorescence cross section) is estimated by comparing it with fluorescence of Rhodamine 6G in ethanol. Using this parameter, dyes are selected that exhibit the most intense fluorescence in PMMA and their concentrations in PMMA are optimised. Coumarin and rhodamine dyes in polymer matrices are proposed for using as visualisers of femtosecond radiation of a Ti:sapphire laser and as detectors in self-triggering systems.

The spectral, luminescent and lasing properties of eight organic molecules, substituted pyrans (DCM), are studied upon pumping by an exciplex XeCl laser at 308 nm and by the second harmonic from a Nd:YAG laser at 532 nm. These molecules exhibit lasing in the red spectral region between 600 and 780 nm with the efficiency of 45%. Lasing was also obtained in bis-substituted pyrans having the quantum yield of fluorescence equal to 0.01. The possibility of preparation of solid active media for tunable lasers based on polymer matrices doped with substituted pyrans is discussed.

The presence of vibrationally excited oxygen in the active medium of a chemical oxygen — iodine laser is discovered using the emission technique. Using the analysis of the luminescence spectra of oxygen molecules on O₂(b¹Σ_g⁺, v=i)→O₂(X³Σ_g^-, v'=i) (i=0, 1, 2) electronic vibrational — rotational transitions, it is shown that ~22 % of O₂(b¹Σ_g⁺) molecules are at the first vibrational level and ~10 % are at the second one. Moreover, due to the fast EE energy exchange, a mean number of vibrational quanta per one molecule in each of the O₂(X³Σ_g^-), O₂(a¹Δ_g) and O₂(b¹Σ_g⁺) components is approximately the same and amounts to 30 % — 40 %.

The phenomenon of plasmon resonance in ellipsoidal nanoparticles with shells is considered. Based on the geometrical sizes of a nanoparticle and its components, the theory is developed which allows the calculation of absorption spectra. Using the Maxwell — Garnett theory, a collective plasmon resonance is considered, which represents a nonlinear dependence of the plasmon frequency on the concentration of nanoparticles.

A pulsating optical discharge moving in air at a subsonic speed (~250 — 300 m s^-1) and creating in front of it a high-intensity low-frequency wave propagating in the same direction is considered. The range of dimensionless parameters in which this effect is manifested is determined.

Parametric generation in a HgGa₂S₄ crystal pumped by a nanosecond Nd:YAG laser is obtained and investigated for the first time. The experimental results demonstrate the outlook for using HgGa₂S₄ crystals for the development of efficient optical parametric oscillators in the mid-IR range.

The possibility of controlling the pulse shape and duration and the beam divergence of a copper vapour laser operating in the mode of double pump pulses, when the first pulse excites lasing in the active medium and the second amplifies it. It is shown that a change in the delay of the second pump pulse relative to the laser pulse initiated by the first pump pulse allows an efficient control of the laser-radiation characteristics. In this case, the coefficient of laser-radiation conversion into a beam with a diffraction-limited divergence may reach ~80 %.

A Nd:YLF laser emitting 2-ns pulses synchronised with a femtosecond Cr:forsterite laser is built. The pulse duration and synchronisation are ensured by two Pockels cells, in which voltage pulses are synchronised with the femtosecond laser by fast emitter-coupled logic elements. One of the Pockels cells ensures Q-switching, while the other cuts a short pulse from a 15-ns Q-switched pulse. The experimental results show that the two-step scheme proposed for synchronisation of a Q-switched laser and a passively mode-locked laser provides quite simple and reliable synchronisation of these lasers with a jitter of a few tens of picoseconds.

A scheme for amplification of ultrashort laser pulses is studied, which is used in experiments on symmetrisation of ablation pressure with the help of a prepulse upon acceleration of foils by laser radiation of high brightness. The possibility of direct amplification of short pulses before their expansion in order to increase the energy contrast is considered. In experiments performed on the PICO facility, the amplification of a 10-ps pulse with a power density exceeding 100 GW cm^-2 is demonstrated with the gain equal to 1.2 and the inversion drop above 30 %.