P. M. Mikheev

Excitation and decay of low-lying nuclear states in a dense plasma produced by a subpicosecond laser pulse

The excitation of low-lying nuclear levels in a hot, dense plasma, produced by a subpicosecond pulse with intensity exceeding 1016 W/cm2, is investigated theoretically and experimentally. The basic channels of electronic (inelastic scattering and inverse internal electron convergence) and photon (photoexcitation) excitations of such states as well as the influence of the broadening of a nuclear level on the excitation efficiency and the presence of hot electronic component are examined. The experimental data from measurements of the decay kinetics of the low-lying nuclear level 6.238 keV of the stable isotope 181Ta, which were obtained on two experimental laser systems, are presented.

Neutron generation in dense femtosecond laser plasma of a structured solid target

We report neutron production by the 2H(d, n)3He reaction induced upon the illumination of a solid nanostructured target by femtosecond laser pulses of intensity 20 PW/cm2 (1 PW = 1015 W). The target was structured through the preliminary illumination by a laser pulse of the same intensity.

Generation of High-Energy Negative Hydrogen Ions upon the Interaction of Superintense Femtosecond Laser Radiation with a Solid Target

The formation of a high-energy (∼35 keV) beam of negative hydrogen ions was observed in the expanding femtosecond laser plasma produced at the surface of a solid target by radiation with an intensity of up to 2× 1016 W/cm2. The energy spectra of the H+ and H−-ions show a high degree of correlation.

Excitation of tantalum-181 nuclei in a high-temperature femtosecond laser plasma

The excitation of nuclei in a laser plasma is observed. Gamma rays from the radiative decay of the isomeric level 9/2− (6.238 keV) 181Ta in a high-temperature femtosecond Ta laser plasma are detected.

Evolution of a femtosecond laser-induced plasma and energy transfer processes in a SiO2 microvolume detected by the third harmonic generation technique

A nonlinear optical method has been proposed for probing the evolution of a plasma and energy transfer processes in the microvolume of a transparent dielectric using the third-harmonic probe signal. The electron self-trapping time has been estimated as t = 150 ± 80 fs. A decrease in the third-harmonic probe signal has been detected at the picosecond time scale; this decrease can be attributed to a change in the third-order susceptibility χ(3) of a fused silica sample modified by laser radiation.

Nonlinear rotation of the polarization of the intense femtosecond laser radiation in BaF2

The dependence of the polarization plane rotation angle in the BaF2 crystal on the intensity of the femtosecond laser radiation with 0.62 μm wavelength in the ω = ω + ω − ω process on cubical nonlinearity χ(3) is investigated. An anomalous increase in the efficiency of the cross-polarized radiation generation at I > 2 × 1012 W cm2 is observed for the first time. The 10% efficiency of the orthogonal component generation at the ∼3 × 1012 W cm2 intensity is obtained.

Generation of the third harmonic of near IR femtosecond laser radiation tightly focused into the bulk of a transparent dielectric in the regime of plasma formation

The generation of the third harmonic of femtosecond laser radiation with a wavelength of 1.24 μm tightly focused into the bulk of fused silica was simulated numerically for the regime with light intensity of 3 × 1013 W/cm2, which is extreme for solid bodies. The efficiency of third harmonic generation (THG) was found to be restricted to 0.1% in the regime of plasma formation. This is determined by two competitive processes: a decrease of THG efficiency due to an increase of wave detuning and an increase of THG efficiency due to a growth of focusing asymmetry. In an isotropic medium, determination of the threshold of plasma formation by use of the third harmonic signal is shown to be a more sensitive method as compared with the standard scheme of nonlinear transmittance detection.

Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 1013 W/cm2 in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF2 crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF2 and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.

Self-channeling of femtosecond laser radiation in transparent two-component condensed medium

We have observed self-guiding of a single femtosecond visible laser pulse in the bulk of transparent nonlinear media (SiO2, KDP) and in the water. The dependence of filament length on laser pulse energy was measured. Continuous open-ended channels and frozen modifications of the matter were observed in transparent two-component condensed medium (thin glass plate placed in water).

Resonant laser-plasma excitation of coherent THz phonons under extreme conditions of femtosecond plasma formation in a bulk of fluorine-containing crystals

The dynamics of coherent phonons in fluorine-containing crystals under plasma formation were studied using a nonlinear pump?probe technique based on third harmonic generation. In LiF crystal more than one phonon mode was observed. The modes are the overtones of a fundamental wave with a frequency of 0.38?THz. In CaF2 crystal phonons with frequencies of 1 and 0.1?THz were observed. In BaF2 crystal, in addition to coherent phonons with frequencies of 1?THz and 67?GHz, a significant increase of amplitude in the phonon modes with a time delay of 15?ps was detected.