Table of contents

A method for studying the characteristics of laser-induced ablative acceleration of thin foils is described. The method is based on the electrocontact measurement of the time of foil flight through base distances in the air atmosphere. Results of experiments on the laser acceleration of foils in cylindrical and conical channels for laser radiation intensities up to 2×10¹¹ W cm^-2 are presented. The results obtained were used in experiments to determine the split-off strength of aluminium targets at high strain rates, the accelerated aluminium foils being used as strikers.

The motion of an electron in the fields of relativistic-intensity linearly and circularly polarised laser radiation is analysed. The analysis is based on the numerical solution of the Newton equation with the Lorentz force. The electromagnetic radiation of an electron interacting with a laser pulse is studied. It is shown that this radiation is emitted in the form of extremely short attosecond pulses. It is found that an initially immobile electron does not move along figure-eight trajectories in the field of a linearly polarised laser pulse.

Optimal conditions are determined for the development of efficient diode-pumped 1-μm solid-state lasers with output powers up to a few tens of kilowatts. The thermal operating conditions are analysed for various Yb³⁺- or Nd³⁺-doped active media used in high-power laser systems. The advantages and disadvantages of these active ions and various crystal matrices are discussed. The theoretical analysis and experimental simulations allow one to determine the application fields of various laser crystals. A new concept of a multibeam (multipoint) pumping of active media is proposed.

Methods for direct (without the use of the Kramers–Kronig relations) measuring the dielectric response (dynamic conductivity and permittivity) spectra of dielectrics, conductors, and superconductors in the terahertz frequency range (0.03–1.5 THz) are described. The methods are realised by using BWO (backward-wave oscillator) spectrometers developed at the A.M. Prokhorov General Physics Institute, RAS. Dielectric measurements can be performed at temperatures from 2 to 1000 K in magnetic fields up to 8 T. The dielectric response spectra are investigated for a number of materials with properties determined by electron correlation effects: superconductors, one-dimensional conductors, heavy fermion systems, spin-ladder cuprates, and spin glasses.

The generation of harmonics of a high-power-laser-pulse field in a gas during impact ionisation of atoms by oscillating electrons is studied theoretically. Fields are considered under conditions when the oscillation energy of electrons in the radiation field, remaining nonrelativistic, considerably exceeds the ionisation potential of an atom. In addition, the radiation field was assumed weak compared to the atomic field (E_a = 5.1×10⁹ V cm^-1), which allowed us to neglect the field ionisation of atoms, taking into account only the impact ionisation of atoms by oscillating electrons. Under such conditions, along with the elastic scattering of electrons, the inelastic scattering of oscillating electrons accompanied by ionisation of gas atoms can make a significant contribution to a nonlinear current induced in the plasma.

Work in the field of electron-optical diagnostics of laser-induced rapid processes performed for almost half a century at the Photoelectronics Department at the General Physics Institute, RAS, is summed up. New femtosecond image-converter tubes have been developed, which provide the time resolution no worse than 100 fs in the slit scan (streak) regime, and photoelectron guns have been built for time-resolved studies of electron diffraction. The results of recent studies on manifold (up to fifty-fold) compression of picosecond electron bunches in nonstationary focusing fields are demonstrated.

Single crystals of solid solutions of a high optical quality are grown in the concentration vicinity of the saddle point of the ternary CaF₂–SrF₂–YbF₃ system. Efficient lasing with a small Stokes shift (at 1025 nm) was obtained in 980-nm diode-pumped single crystals. The total lasing efficiency (with respect to the absorbed average pump power) was 59% and the slope efficiency was 83%.

The spectral and lasing parameters of Nd:GdVO₄, Nd:YVO₄, and Nd:Gd_0.7Y_0.3VO₄ vanadate crystals cut along the c axis are studied. Lasing is obtained for the first time in a nonselective resonator at the ⁴F_3/2–⁴I_11/2 transition at 1065.5 nm in a Nd:GdVO₄ crystal. Tuning is realised in the range from 1062.3 to 1066.1 nm and two-frequency lasing is obtained.

The four-photon laser radiation scattering spectra are obtained in the submillimetre range (75–95 cm^-1) for deionised water, aqueous solutions of DNA and α-chymotrypsin protein. Narrow resonances are recorded whose frequencies coincide (within the resolution power of a spectrometer) with rotational frequencies in the ground electronic state and vibrational state of ortho and para isomers of H₂O molecule in a gas phase and with the frequencies of the lines of H₂O₂ and OH^- molecules. It is shown that the resonance contribution of the rotational lines of ortho isomers of H₂O to the signal of four-photon scattering of native solutions of biopolymers increases by a factor of at least 8 compared to their contribution to the scattering signal in water, and becomes considerably larger than the contribution from the paraisomer lines. Denaturation of DNA after heating and cooling of the solution leads to the disappearance of such selectivity.

It is shown that the service life of erbium-doped fibres can be increased many times under conditions of an elevated radiation level by loading the fibre glass network with molecular hydrogen. Backdiffusion of hydrogen from the fibre in the process of its operation is virtually excluded for the fibre covered with a hermetic carbon coating. It is shown that this technique of fibre preparation allows one to slow down significantly degradation of the lasing properties of erbium fibres under the conditions characteristic of space applications.

The radiation parameters of a diffusion-cooled compact slab CO₂ laser pumped by microwave discharge at a frequency of 2.45 GHz are studied. A magnetron from a domestic microwave oven is used as the pump source. An average output power of 25 W and an efficiency of ∼13% are obtained at a wavelength 10.6 μm. A peak output power of 580 W is achieved for 20-μs pulses emitted at a pulse repetition rate of 400 Hz. The dependence of parameters of the CO₂ laser on the input pulse power in the range 0.8–8 kW, the composition and pressure of the working mixture and the pump pulse duration and repetition rate are studied experimentally. Optimal relations between these parameters are determined for the given design of the laser.

Parameters of high-speed ablation of ultradeep channels by controlled pulse trains from a single-mode phase-conjugate dynamic cavity Nd:YAG laser emitting 20–200-ns, 70–250-mJ pulses at a pulse repetition rate in a train of 40–250 kHz are studied. The optimal parameters of ablation are found, for which a long-lived region of a hot rarefied gas was maintained in the ultradeep channel, which suppressed the shielding action of the surface plasma. The control of the lasing process during ablation optimises not only the heating and plasma formation, but also the removal of the processed material in the pause between laser pulses. Adaptive regulation of lasing parameters during ablation made it possible to obtain ultradeep channels of length 8–27 mm and diameters 80–300 μm of the input and output holes in metals (aluminium, steel and Inconel 718 nickel superalloy) and ultrahard ceramics (Al₂O₃, AlN, SiC).

The formation and evolution dynamics of the laser plasma produced in the microvolumes of gases (air, nitrogen, argon, and helium) upon their multiple ionisation by high-intensity (4×10¹⁶ W cm^-2) tightly focused (to a region 1.7 μm in diameter) 400-nm, 100-fs second-harmonic pulses from a Ti:sapphire laser is studied. The spatiotemporal distribution profiles of the refractive index and electron microplasma density were recorded by ultrahigh-speed interferometry. The postionisation of a femtosecond laser plasma, i.e. the growth of the electron density known after the end of the exciting laser pulse, is detected for the first time. A theoretical model is proposed, which describes the mechanism of plasma postionisation by hot photoelectrons. The results of electron density calculations are in good agreement with experimental data.

Photoacoustic pressure signals produced in an absorbing substrate under a layer of a transparent liquid irradiated by two successive nanosecond laser pulses are studied experimentally. The first pulse causes the heating and explosive boiling up of the liquid layer in contact with the irradiated surface, while the second pulse reaches the target at the time when a vapour film has already formed on its surface. The second photoacoustic response contains additional information on heat-and-mass transfer between the liquid and substrate surfaces separated by the vapour film.

It is shown that, as the temperature of an Yb, Er:YAG crystal is increased from room temperature to 400–600°C, the efficiency of energy accumulation on the ⁴I_13/2 level of the Er³⁺ ion increases by several times upon optical pumping through Yb³⁺ ions. Under these conditions, up to 60% of erbium ions can be excited, which gives promise that lasing at a wavelength of 1.54 μm can be achieved in the Yb, Er:YAG crystal in the three-level scheme.

The operation of a passively Q-switched laser emitting the TEM_mn mode is described. The excited-state absorption losses and nonlinear absorption in the shaded ends of the active element are taken into account. We measured the laser pulse energy for the TEM₀₀ mode, the ratios of the output energy in transverse TEM_mn modes with m = 0–4, n = 0 and m = 0–3, n = 1 to the TEM₀₀ mode energy in an erbium-doped glass laser and determined its temporal parameters in the passive Q-switching regime achieved with the help of a Co²⁺ : ZnSe crystal. It is shown that on passing from one type of the transverse mode to another for invariable parameters of the resonator, the laser pulse energy changes discretely. A change in the radius of the transverse distribution of the TEM₀₀ mode at the output mirror of the passively Q-switched laser during the development of lasing is estimated. It is found that the relative change in the laser beam cross section on the output mirror under our conditions is less than 2%. The experimental data obtained in the study are in good agreement with our calculations.

Experimental investigations of surface waves at the interface between a homogeneous medium and a periodically stratified medium representing a restricted system of coupled waveguides are analysed. It is found that in all cases of the observation of surface waves a restricted system of waveguides has its own spectrum of guided modes and the spectrum of leaky modes, which become the surface waves of the system. It is shown that when two surface waves are used in a biosensor scheme, they should be realised as leaky modes of a Bragg waveguide in which a periodic system of waveguides was used as a distributed Bragg mirror of this waveguide. It is pointed out that the prism method of excitation of surface waves can be used in sensors, where these waves are employed as a detecting tool.

The temperature dependences of the critical reduced electric field strength (E/N)_cr in SF₆ and mixtures of SF₆ with C₂H₆ are obtained in the gas temperature range T_g = 293–1500 K in the absence of thermal dissociation. Gases were heated due to the VT relaxation of SF₆ molecules excited by a pulsed CO₂ laser. The values of (E/N)_cr were estimated by the quasi-stationary burning voltage of a self-sustained volume discharge ignited with a delay with respect to the laser pulse. It is shown that in the absence of dissociation, the reduced electric field strength in SF₆ and mixtures of SF₆ with C₂H₆ in the temperature range under study increases with T_g. The obtained temperature dependence of (E/N)_cr in SF₆ is compared with the results reported by other researchers.