Table of contents

It is shown that basic properties of the nonlinear response of high-temperature superconductors (HTSCs) observed in femtosecond and picosecond pump—probe experiments at high and low pump levels in various variants of the pump—probe spectroscopy, including one- and two-photon excited-state probing, can be interpreted by using two assumptions. The spectral and temperature properties of the HTSC response at low pump levels can be explained taking into account the contributions from interband electronic transitions to the dielectric constant. At the same time, drastic variations in the HTSC response kinetics (temporal features) observed at high pump levels (for a typical pump pulse energy of ∼10^-7 J in a focal spot of diameter 150 μm) can be explained by assuming the existence of a 'frozen' (metastable) energy gap in the electronic spectrum of a HTSC. In this case, all the conditions required for the interpretation of a drastic decrease in the relaxation rate of a nonlinear response (degeneracy) are realised due to the specific distribution of the electronic state density immediately after the formation of the energy gap in the electronic spectrum of the HTSC.

The light—current and spectral characteristics of semiconductor lasers are simulated by using models with and without inverse mass for radiative transitions with and without fulfilment of the wave-vector selection rules. The best agreement between the theory and experiment is obtained for a model without inverse mass with radiative transitions without fulfilment of the selection rules. The obtained results are discussed.

The power and spatial characteristics of a longitudinally diode-pumped laser based on a composite YVO₄/Nd:YVO₄ crystal are studied. It is shown that the use of a composite crystal allows one to increase the external slope quantum efficiency from 36% to 41% and decrease the quality factor M² of the output beam from 2 to 1.5 compared to these parameters for a Nd:YVO₄ crystal.

A method is proposed for describing absorption of an electron-heating femtosecond laser pulse that interacts with a metal under conditions of high-frequency skin effect. It is shown that the effective frequencies of electron—electron collisions accompanied by umklapp processes can be determined by measuring the absorption or reflection coefficients of a femtosecond pulse.

A three-level neural network is considered which contains twenty five inputs, two hidden elements, and four outputs and is trained to recognise four situations at the input: the appearance of the fundamental mode of a Fabry—Perot resonator, the superposition of the fundamental and the first mode of the resonator with zero phases, the superposition of the fundamental and first mode of the resonator with phases 0 and π, and the appearance of the second mode and the superposition of the fundamental and second modes. It is shown that the network can recognise correctly variations in the mode composition of the Fabry—Perot resonator over the modulus of the transverse distribution of the field during the development of two-mode lasing. The operation of the network is studied for the two response times: the time shorter than the resonator time and the time determined by the response time of a pyroelectric detector of laser radiation.

A method is proposed for compensating thermally induced phase distortions of laser radiation in absorbing optical elements. The method is based on supplementary heating of the peripheral region of the distorting element by the radiation from an auxiliary laser. A programme code has been developed for calculating the optimal parameters of supplementary radiation for minimising phase distortions. This code is based on the numerical solution of the thermal conductivity and static elasticity equations for a nonuniformly heated solid of cylindrical symmetry. Experiments reveal a high efficiency of the method for compensating distortions resulting from absorption of radiation with a Gaussian intensity profile.

Optical nonlinear effects of self-phase and cross-phase modulation and stimulated Brillouin scattering (SBS) determining the data transmission quality in fibreoptic communication lines are studied. The Stokes radiation power upon SBS is calculated taking into account the nonlinear broadening of the optical signal spectrum caused by self-phase modulation. The mechanism of signal distortion caused by the joint action of self-phase modulation and chromatic dispersion is considered. The main theoretical results are verified experimentally.

We consider normalised intensity correlation functions (CFs) of different orders for light emitted via parametric down-conversion (PDC) and their dependence on the number of photons per mode. The main problem in measuring such correlation functions is their extremely small width, which considerably reduces their contrast. It is shown that if the radiation under study is modulated by a periodic sequence of pulses that are short compared to the CF width, no decrease in the contrast occurs. A procedure is proposed for measuring normalised CFs of various orders in the pulsed regime. For nanosecond-pulsed PDC radiation, normalised second-order CF is measured experimentally as a function of the mean photon number.

An ytterbium-doped fibre amplifier with the hexagonal structure containing seven waveguides is simulated by using a numerical program based on the scalar paraxial optics approximation. The influence of the scatter in parameters of active fibre cores on spontaneous phasing discovered earlier is studied numerically for the first time. The critical scatter in the core parameters at which the phase locking of radiation is preserved is found. It is shown that a decrease in the numerical aperture of fibre cores facilitates the stabilisation of phase locking.

A model of a sensor of an external action on a fibreoptic cable used in extended-perimeter security systems with localisation of the action site is developed. The detection principle is based on phase-sensitive reflectometry with the use of a narrowband pulsed radiation source. The model has a high sensitivity inherent in interference sensors. The maximum detection range and spatial resolution are 35 km and 5 m, respectively.

The possibilities of apodization of fibre Bragg gratings written by the holographic method with a Gaussian beam are considered. A simple method is demonstrated for smoothing the spectral profile of highly reflecting gratings by illuminating the written interference pattern by a homogeneous Gaussian beam.

The spatial intensity distribution of radiation propagating in semi-infinite directional coupler arrays with different dependences of coupling constants on the fibre number is studied.

The formation of a dense array of nanospikes is studied upon ablation of a silver target immersed into water or ethanol by 350-ps pulses from a neodymium laser. The average height of nanospikes is 50 nm and their density amounts to 10¹⁰cm^-2. The irradiated surface is studied by using reflection spectroscopy in the 200–600-nm range and an atomic force microscope. Small lateral dimensions of nanospikes confine plasmon oscillations of electrons, which is manifested in the colouring of the irradiated surface. The plasmon resonance of nanospikes is observed at 380 nm and shifts to the visible region upon oxidation in air. The initial spectrum of nanospikes is recovered after processing of their oxidised surface in an aqueous solution of ammonium. The use of nanospikes for observing surface enhanced Raman scattering is discussed.

The energy and spectral parameters of a barrier discharge in a mixture of argon with hydroxyl ^•OH are studied experimentally. A sealed lamp with the radiation intensity maximum at λ = 309.2 nm, an emitting surface area of ∼700 cm², and a radiant excitance of 1.5 mW cm^-2 has been fabricated. The radiant power of the lamp is 1.1 W.

The parameters of the femtosecond vibration—rotation molecular dynamics of liquid acetonitrile CH₃CN, trimethylacetonitrile (CH₃)₃CCN, propionitrile CH₃CH₂CN, fluoroform CHF₃, and chloroform CHCl₃ are found by analysing the ultrafast optical Kerr effect. The influence of the molecular structure on the features of rotational (diffusion and libration) motions is studied. It is shown that the distribution of libration frequencies is described by the Maxwell distribution.