Table of contents

Experiments allowing the production of carriers at high concentrations (5 × 10¹⁹-10²⁰ cm^-3) in a semiconductor irradiated by femtosecond laser pulses are discussed. At such high concentrations, and at room temperature the formation of the Bose condensate of an electron — hole plasma is expected, which is accompanied by superradiance. A minimal (threshold) energy is determined which should be absorbed in an elementary event for producing a certain concentration of free carriers. When the carrier concentration is high, carriers with opposite mass signs can appear in the conduction band of gallium arsenide and, hence, the efficient Coulomb attraction is possible between carriers in the same band, which should be taken into account in the formation of a correlated state of carriers.

When a 10 μW — 3 W laser beam (for example, at 632.8 nm) is focused into a usual soap film of thickness between 10 nm and 10 μm from the side or through a defect on the film surface, the self-channeling of the beam propagating in the film is observed. The beam also exhibits self-branching into submicron filaments, the so-called whiskers of length several tens of centimetres. The results of experiments on the dynamics of behaviour of these whiskers in a soap film, their polariton nature, and possible applications are discussed.

A review is presented of the state of the art in a new direction in quantum electronics based on the use of femtosecond lasers for precision frequency measurements and the development of optical frequency and time standards.

The amplitude — time and spectral characteristics of laser radiation and discharge characteristics in mixtures of SF₆ with hydrogen and hydrocarbons at a high lasing efficiency are investigated. Lasing efficiencies of ~10 % with respect to the deposited energy are obtained under the pumping from capacitive and inductive energy storage units. It is shown that, during a pump pulse, the maximum efficiencies are achieved at high values of the parameter E/p in the laser gap. When profiled electrodes and UV preionisation are used, a specific output energy of a nonchain HF laser of ~140 J L^-1 atm^-1 and a lasing efficiency of ~4.5 % with respect to the stored energy were obtained. It is shown that the emission spectrum of a nonchain HF laser operating at high lasing efficiencies becomes significantly wider and cascade lasing is realised on the ν(3–2) → ν(2–1) → ν(1–0) vibrational transitions at several rotational lines.

The lasing characteristics of a Cr²⁺ : ZnSe crystal grown by the method of free growth on a single-crystal seed using physical transport in helium are studied. The crystal was doped from a vapour phase during its growth. The achieved concentration of dopant Cr²⁺ ions (5×10¹⁸ cm^-3) provides the absorption coefficient of 4.5 cm^-1 at the maximum of the pump band (1.78 μm). Upon pumping by a pulsed Co : MgF₂ laser at 1.67 μm, the slope efficiency equal to 59 % with respect to the absorbed energy was achieved, which corresponds to the quantum efficiency 87 %.

A method for increasing the peak radiation pulse power of self-terminated amplifiers without changing the pump power of an active medium and energy extraction is discussed. The first experimental results obtained by using the active media of copper vapour lasers are presented. A nearly twofold increase in the output pulse peak power is achieved.

A UV-radiation source based on a single-stage multipass copper-vapour laser amplifier and a nonlinear DKDP crystal is discussed. The peak pulse power of the laser radiation at the nonlinear crystal input was enhanced considerably by using a special design of the multipass amplifier, resulting in efficient lasing at the sum frequency (at 0.271 μm) of the 0.51-μm and 0.578-μm copper lines.

A transverse discharge in a narrow dielectric gap with extended electrodes is investigated in the case of an additional pulsed ionisation at a repetition rate of 10 — 30 kHz. The electrical characteristics of the discharge and the limiting energy deposition are studied experimentally and theoretically. It is shown that the limits of the discharge homogeneity are determined by the ionisation — overheating instability and can be varied by changing the preionisation power. The output power of a CO₂ laser obtained from a unit discharge surface is 1.5 W cm^-2.

The effect of parasitic elements of a laser on its modulation characteristic is investigated theoretically and experimentally. It is shown that this characteristic depends on the thickness d of the insulating dielectric film, differential resistance R_d, and length L of the laser resonator. For example, at d=0.15 μm, R_d = 9 Ω, and L = 300 μm, the upper modulation frequency at a level of —3 dB is 500 — 1500 MHz, and at d = 1.2 μm, R_d = 5 Ω, and L = 200 μm, it reaches 12 GHz.

A liquid-crystal lens with the focal distance depending on the transmitted light intensity is proposed and studied. A theoretical model is developed which adequately describes the wave-front formation by the lens. The results of visualisation of the wave-front control in a setup with crossed Polaroids and the intensity distributions, which characterise the focusing properties of the lens, are presented. To illustrate the application of the lens, an adaptive-optics system is built for stabilisation of radiation power on a 1-mm diaphragm, which reduces the power fluctuations by a factor of 30.

The solutions of the dispersion equation for the perturbations of the front of laser evaporation of metals are studied for different Mach numbers M in the vaporised material flow. It is shown that the simultaneous manifestation of the hydrodynamic perturbations in the condensed medium and the gas-dynamic perturbations in the vapour flow for M < 1 significantly changes the dispersion curves compared to the cases of evaporation instability (M=1) and Darrieus — Landau (M ≪ 1) instability.

Nonlinear refraction caused by the orientation of molecules of a nonlinear-optical chromophore induced by pulsed 532-nm laser radiation in a polymer based on bisphenol-A diglycydyl ether with 4-aminoazobenzene molecules covalently bonded with the polymer chain is studied. The induced anisotropy of the refractive index caused by the orientation of azobenzene molecules is studied using the polarisation and pump-probe techniques in real time. The induced anisotropy appears in the polymer during 50 — 70 μs and relaxes, as a rule, for 5 — 10 s. It is found that nonlinear refraction in the range from microseconds to tens of seconds in this polymer and, hence, the writing of dynamic holographic gratings by nanosecond laser pulses are mainly caused by the orientation of azobenzene molecules by polarised radiation.

The self-focusing of Gaussian light beams and the propagation of screening spatial solitons in a cubic optically active photorefractive crystal with the (0) cut plane are studied for an arbitrary orientation of an external electric field in this plane. A change in the rotation of the polarisation plane caused by the nonlinear interaction of light with the crystal is studied theoretically. It is shown that optical activity enhances self-focusing in some cases. Screening self-focusing was observed in an incoherently illuminated Bi₁₂SiO₂₀ crystal without additional laser irradiation. The experimental data well agree with the proposed theoretical model.

A new technique for investigating the surface plasmons by means of a scanning acoustic microscope is proposed. Within this technique, the surface electromagnetic wave (plasmon polariton) is excited by laser radiation on one side of a metal film, while a scanning acoustic microscope excites surface acoustic waves on the other side of the film. Obtained for the first time, the acoustic images of plasmons, propagating on the grating surface, demonstrate the possibility of studying the plasmon wave field distribution by means of a scanning acoustic microscope.

It is shown that stimulated Raman self-scattering (SRSS) can be efficiently used for the compression of femtosecond optical solitons in optical fibres with the spectrally inhomogeneous frequency dependence of the group-velocity dispersion. The SRS dynamics is studied in detail near the point of the second-order zero dispersion. The saturation of compression of femtosecond solitons in spectrally inhomogeneous fibres in the zero-dispersion region is predicted.

The decay of an N-soliton optical pulse in optical fibres induced by the nonlinear interaction with a perturbing pulse is analysed numerically. The main attention is paid to the analysis of conditions under which the separation of soliton components occurs over a minimal distance. The analysis was performed by varying the carrier frequency of the perturbing pulse, its shift in time, and the phase difference. The numerical calculations are confirmed for the zero time shift by analytic calculations performed by the method of inverse scattering problem.

A scheme for recording and reconstruction of 'relief' waveguide Fourier holograms of two-dimensional objects is described. Distortions appearing in the reconstruction of an object by a waveguide mode are analysed. A computer model is proposed which provides the qualitative and quantitative estimates of the influence of different parameters of the recording — reconstruction scheme and the waveguide parameters on the quality of the reconstructed image of a two-dimensional object. The effect of the structure of the recorded two-dimensional image on the noise level arising under the waveguide reconstruction is demonstrated.