Backward Terahertz Radiation from a Two-Color Femtosecond Laser Filament

A change in the quasistatic magnetic susceptibility in thin plates of iron borate (FeBO₃), which is a weak ferromagnet, has been revealed at adsorption of water molecules. The measurements have been performed at room temperature with the use of the magneto-optical Faraday effect. The change of the susceptibility in saturated water vapors is about 30%. The observed effect is reversible. The time of establishing the susceptibility after the introduction of water vapors is 1.5 min, which is twice as large as the time of establishing the susceptibility after the evacuation. The effect is explained by the appearance of uniaxial surface magnetic anisotropy in the basal plane because of the adsorption of water molecules.     

Diode pumped Nd:YAG laser with active Q-switching and mode locking for hole drilling

A diode-pumped Nd:YAG laser operating with active-passive Q-switch mode locking, has been developed. The acousto-optic repetition train was one kilohertz with generated pulse train widths 65 ns, single pulse widths 200 ps and an average power of 6.5 W. Improvement of efficiency of small diameter deep holes laser drilling in different materials was studied.     

Carbon-nanotube-based saturable absorbers for near infrared solid state lasers

We demonstrate the passive mode-locking of a diode-pumped Nd⁺³:YAG (central wavelength: 1.32 μm; pulse duration: 50 ps; output energy: up to 70 μJ) laser using a polymer film containing single-wall carbon nanotubes. The mode-locking regime is stable at a pump repetition rate of up to 1 kHz. We also investigate the temporal evolution of the light-induced absorption change of the polymer film containing carbon nanotubes in the spectral range of 1.3–1.5 μm by femtosecond time-resolved pump-probe measurements. The measurements reveal that light-induced transmission exhibits fast and slow components that last 280 fs and more than 10 ps, respectively. The third-order susceptibility of the polymer film containing single wall carbon nanotubes is as high as 10⁻¹¹ esu.     

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.     

To the theory of electronic states of epitaxial graphene formed on the surface of a metal substrate

The problem of electronic states of epitaxial graphene formed on the surface of a metal substrate has been considered. The electronic states of epitaxial graphene have been studied taking into account the indirect interaction of its atoms through the electron gas of the substrate.     

Interferometric diagnostics of femtosecond laser microplasma in gases

The laser plasma formed in gaseous media due to their optical breakdown under tightly focused femtosecond laser pulses has been experimentally investigated. Pump-probemicrointerferometry is chosen to perform spatial and temporal diagnostics of the plasma. Time dependences of the laser plasma electron density are obtained. It is shown that in breakdown of different gases (air, nitrogen, argon, and helium) at different pressures (in the range from 1 to 10 atm) the electron concentration continues to increase during ??1 ps when the laser irradiation is over. This effect is related to the impact ionization of the plasma by the hot electrons formed in interaction of intense femtosecond laser pulses with matter. The results of theoretical simulation of the post-ionization processes are presented.     

Kinetics and kinetic isotope effect in pentavalent plutonium disproportionation activated by power ultrasound

A kinetic isotope effect in Pu(V) disproportionation has been observed in nitric acid solution under the effect of power ultrasound with intensity 0.9W·cm^–2 and frequency 22 kHz. The isotope separation coefficient for²⁴²Pu/²³⁹Pu isotopes was found to be 1.0081 at 20°C. Without sonication the k.i.e. was not observed. The rate constant of Pu(V) disproportionation was found to be accelerated under sonication. The rate constant determined was (5.7±0.6)·10^–3 1²·mol^–2·s^–1 atl=0.9 W·cm^–2,v=22 kHz, [HNO₃]=0.5 mol·l^–1 andT=20°C. It is supposed that the acceleration of Pu(V) disproportionation and the kinetic isotope effect are due to the activation of plutonoyl groups in the interface between the cavitation bubble and the bulk solvent.     

Estimation of the absorptivity of a constructive material at unknown losses in ablation plume during laser ablation

Computation of the processes of laser heating of carbon silicon carbide composite material (CSCCM) samples in air (to temperatures above 2000°C for 1 s) by IR laser radiation with a wavelength of 1.3 μm and intensity of 3 kWcm⁻² in the presence of screening ablation plume have been carried out using the KARAT code. A comparison of the simulation results with the experimental dependences of spatial and temporal fields of sample temperatures made it possible to determine the absorptivity of thematerial, energy loss in the ablation plume, and, correspondingly, its influence on the heating and ablation of the material under study.     

Effect of phonon spectrum heating on the formation of laser-induced electron plasma in wide-gap insulators

The effect of lattice heating by laser pulses on the dynamics of electron plasma generation in transparent solids has been theoretically studied. Several ways of taking into account the contribution of the phonon spectrum heating to the electron avalanche dynamics, depending on the type of the effective (with respect to the field energy transfer to electrons) phonons and laser pulse duration, have been proposed. A comparative analysis of the results of Monte Carlo computation of electron gas heating in the laser pulse field, which were obtained for cold and heated lattices, has been performed. It is shown that the consideration of the effect of lattice heating on the probabilities of electron-phonon and electron-phonon-photon scattering leads to an increase in the avalanche rate, which is more pronounced at longer wavelengths of the incident radiation and under longer laser pulses. Some qualitative features of the redistribution of the energy, absorbed during a pulse, between the electron plasma and lattice are revealed, which suggest initiation of irreversible microscopic changes in the insulator. In particular, the ratio R of the energy accumulated in the electron subsystem to the excess (with respect to the initial equilibrium state) energy in the phonon subsystem has been calculated for different initial lattice temperatures. It is shown that this ratio increases with a decrease in the laser wavelength in the computation scheme with lattice heating disregarded and decreases at all pulse durations when the lattice heating is taken into account.