Table of contents

A solution of D'Alembert's equation for the vector potential of an electromagnetic field is found in the form of a wave packet, which does not spread in time and space. The expression obtained for the vector potential of a photon is used for the solution of some problems.

A problem of scattering of an ensemble of photons by material particles is solved. The vector potential of each of the incident photons scattered by particles is described by a nonspreading wave packet. The expressions for cross sections for elastic and inelastic scattering of electromagnetic radiation are derived taking the space — time localisation of photons into account. The possible experiments for verifying these theoretical results are discussed.

Reflection and transmission of a plane electromagnetic wave propagating in a layered dielectric structure with an arbitrary number of layers of various thicknesses are investigated. For the general case of oblique incidence of the wave on this structure, the reflection and transmission coefficients are calculated for both TE and TM waves using a multiple reflection method. An algorithm to apply the obtained formulas for numerical and analytical calculations is suggested.

It is shown that the delay time of a weak signal propagating in an amplifying medium on the wings of the spectral amplification line may be shorter than the time of propagation of the signal with the velocity of light in vacuum. It is found that in this case, the time dependence of the signal is exactly 'reconstructed' at the point of detection, and the detection of the signal continues even if it is abruptly terminated at the point of transmission. It is also shown that using the complex time of group delay of the signal, it is possible to improve the accuracy of the results in the first order of dispersion theory within this approximation.

The propagation of Hermitian beams in a medium with a distributed quadratic inhomogeneity is studied and is shown that any solution can be represented as a function of some particular solution. This is accomplished by establishing a one-to-one correspondence between optical fields in a homogeneous medium and in a medium with an arbitrary quadratic inhomogeneity. The stability of optical resonators is studied and the condition for their stability is found. Several solutions are found using the method developed.

Based on the equation for the second-order coherence function, the propagation of partially coherent radiation is studied under the combined action of diffraction and refraction caused by the inhomogeneous distribution of the real and imaginary parts of the perturbation of the permittivity of a medium in the case of inhomogeneous absorption (amplification). The limits of application of the method of geometrical optics for inhomogeneously absorbing (amplifying) media are studied, as well as of the methods that neglect refraction caused by the inhomogeneous distribution of the imaginary part of the permittivity of the medium.

Simple formulas are obtained for determining the population of the vibrational levels of singlet oxygen generated chemically in a singlet-oxygen generator. The rate of decrease in the vibrational energy of oxygen is limited by the exchange between its first vibrational level and the bending mode of the water molecule. It is shown that the populations of singlet oxygen molecules at the second and third vibrational levels are comparable with the population of oxygen in the excited electronic state b¹Σ_g⁺. The possibility of formation of electronically excited iodine in the reaction O₂(α¹Δ_g, ν=2) +I₂(X) → O₂(X³Σ_g^-) +O₂(Λ³Π_1u), which may be the intermediate state in the process of dissociation of iodine in singlet-oxygen medium, is substantiated.

Epitaxial films with the atomic concentration of neodymium varied in the range from 1 to 15 % were grown on Gd₃Ga₅O₁₂ substrates with the orientation (111) by the method of liquid-phase epitaxy from a PbO — B₂O₃ overcooled melt solution. The absorption spectra of the films are recorded and their luminescence spectra and luminescence lifetimes of active ions are measured upon diode laser pumping. The concentration dependence of the luminescence lifetime of Nd³⁺ ions is determined.

A high power erbium-doped fibre amplifier pumped by single-mode radiation at 1478 nm is fabricated and tested. A Raman converter based on a fibre with a phosphosilicate core was used as the pumping source. The maximum output power achieved in spectral ranges C (1.53 — 1.56 μm) and L (1.56 — 1.60 μm) is 28 dBm.

It is shown that a weak electromagnetic pulse interacting with a copropagating ionisation front is converted in the general case into three electromagnetic pulses with higher and lower frequencies, which propagate in different directions. The coefficients of conversion to these pulses (for intensities) were found as functions of the frequency. The electromagnetic energy is shown to decrease during this conversion because of the losses for the residual electron energy.

A system is proposed for excitation of luminescence with the help of diffraction axicons with a ring aperture to detect luminescent images. The excitation beams are visualised using LiF crystal radiophotoluminophores. It is shown experimentally that spatially truncated Bessel beams formed in this case provide the local excitation of luminescence in the region of diameter 1.6μm and length 3 mm.

Phase conjugation of laser radiation is studied by SBS in a capillary filled with TiCl₄. A combination of a relatively low SBS threshold for spatially inhomogeneous radiation, a high quality of phase conjugation, and a wide dynamic range of the phase conjugated radiation energy was obtained. PBr₃ is proposed and investigated as a nonlinear optical medium for further decreasing the SBS threshold.

The nonlinear absorption of light in silicon nanocrystals suspended in glycerol is studied by the Z-scan method. The experimental data are used for calculating the nonlinear absorption coefficient β_Si-gl for silicon nanocrystals in glycerol (with a volume filling factor f=2×10^-4), and the coefficient β_Si for pure silicon with a hypothetical volume filling factor f≈1. For the laser radiation wavelength λ=497 nm and the pulse duration τ=0.5 ns, these coefficients are β_Si-gl=1.2×10^-8 cm W^-1 and β_Si =7.36×10^-5 cm W^-1, while the corresponding values for λ=532 nm and τ=10 ns are β_Si-gl=5.36×10^-5 cm W^-1 and β_Si =0.25 cm W^-1. Experiments with 540-nm, 20-ps laser pulses performed for two different filling factors equal to 2×10^-4 and 3×10^-3 gave nonlinear absorption coefficients β_Si-gl=2×10^-7 and 3.6×10^-6 cm W^-1, respectively. Optical absorption and Raman scattering spectra of silicon nanocrystals are also studied. A theoretical analysis of the experimental results shows that optical absorption can be related to the localisation of photoexcited carriers in the conduction band. The localisation is caused by the action of strong static electric fields on an electron in a nanoparticle.

The space — time structure of radiation from an industrial electric-discharge, flowing CO₂ laser is studied experimentally. For the optimal design of the optical and gas-dynamic sections, a total suppression of high-frequency radiation fluctuations and a lowering of the amplitude of low-frequency fluctuations down to 6% — 10% were observed with increasing output power. At the same time, the intensity distribution in the near-field diffraction zone was complicated and unstable. 'Hot' spots and screw wave-front dislocations were observed in the laser-beam cross section. Numerical simulations showed that the properties of the near-field radiation structure are determined by random caustics and phase singularities formed due to the diffraction transformation of the initial wave-front perturbations.

A new method for scanning a single-frequency laser with a Littman — Metcalf cavity using an electrooptic deflector is proposed and realised experimentally. The main advantages of such a scanning technique are its high precision and high frequency tuning rate, as well as the absence of mobile elements. It is shown that for a certain configuration of the cavity, the variation of the laser radiation wavelength can be synchronised with the optical length of the cavity, thus preventing mode hopping during scanning.

A new scheme of an interferometric scanning autocorrelator for measuring ultrashort light pulse durations in the femtosecond and subpicosecond ranges is considered. The distinguishing features of this scheme are its compact structure and a very simple adjustment. A variable time delay of ultrashort light pulse components is produced by a swinging birefringent plate made of a uniaxial crystal. Analytic expressions are obtained for the width of the scanning range and an optimal orientation of the optical axis of the crystal. The effect of disbalance in the intensities of waves that have propagated different optical paths on the form of the autocorrelation function (ACF) is analysed numerically. An experimental ACF for Ti:sapphire laser pulses of duration ∼25 fs is presented.

New possibilities of laser fluorimetry offered by the use of algorithms for solving inverse problems based on artificial neural networks are demonstrated. A two-component mixture of polyatomic organic compounds is analysed by three methods of laser fluorimetry: a direct analysis of the fluorescence band, the kinetic fluorimetry (when durations of the laser pulse and the detector gate pulse are comparable with the fluorescence lifetimes or exceed them), and the saturation fluorimetry. The numerical experiments showed that the use of artificial neural networks in these methods provides a high practical stability of the solution of inverse problems and ensures a high sensitivity and a high accuracy of determining the contribution of components to fluorescence and of measuring molecular photophysical parameters, which can be used for the identification of components.

To decrease the contrast of the speckle of the optical field formed at the output of a multimode optical fibre, it is proposed to use the acoustooptical Bragg diffraction providing a collinear propagation of beams having different polarisation and diffracted to the +1 and -1 orders, and to focus them at the fibre input. This regime provides the 90 % diffraction efficiency. It is shown experimentally that the properties of diffraction in a TeO₂ anisotropic crystal provide an efficient control of the speckle contrast by varying the acoustic frequency, which allows a strong suppression of the speckle.

Reflection holograms in thick layers of bichromated gelatin are recorded by using 50-fs pulses of the second harmonic of a Ti:sapphire laser.

Attention is drawn to the insufficient validity of a number of conclusions concerning the fundamentals of statistical physics made in a paper of A M Tkachev and S I Yakovlenko [Quantum Electron.30 1077 (2000)]