One of the main methods for obtaining information about the generation of sound pulses in metals is to measure the reflection coefficient of a probe wave. Various theoretical models are used to interpret the results of measuring the contribution to reflection coefficient ΔR(t) due to sound-generated displacements of lattice atoms. The purpose of this paper is to establish the degree of accuracy of models used in the case of sound generation in thin films exposed to a femtosecond pulse. It is shown below that the assumption of uniform heating used for thin films is justified if the film thickness is less than the film heating depth and for thicker films at times greater than the film heating time over the entire thickness. For optically thick films, a relatively simple expression for the field can be used. If the film thickness is less than the skin layer depth of the pump field, then it is necessary to consider the field reflection from a substrate. In this case, depending on the optical properties of the metal and the substrate, taking into account reflection can lead to either an increase or a decrease in ΔR(t). It has been established that if the skin layer at the frequency of probe radiation is less than the film heating depth, then taking into account temperature gradients in the equation for the displacement of lattice atoms leads to small changes in ΔR(t). This makes it possible to significantly simplify calculations of the displacement of lattice atoms.

The excitation of sound by a femtosecond laser pulse in a metal layer on a dielectric substrate has been studied. The modulations of the reflection coefficient of the metal, which arise due to the propagation of sound in it, are described in detail. It is shown that, in addition to oscillations corresponding to odd harmonics of sound waves, oscillations corresponding to even harmonics can be present on the profile of the Fourier image of the reflection coefficient change. The efficiency of even harmonic generation depends on the substrate material and the metal film thickness. The dependence of the reflection coefficient change on time has been studied. It has been established that if the electron heat flux reaches the metal–dielectric interface before the energy of the electrons is transferred to the lattice, then modulations are present both due to sound that occurs at the metal–vacuum interface and at the metal–dielectric interface. The wavelength of probe radiation also affects the reflection coefficient change. If real and imaginary parts of the permittivity at the wavelength of probing radiation are comparable in magnitude, then the Fourier image of the reflection coefficient change contains a smaller number of maxima, and the change in the reflection coefficient over time is accompanied by splitting of peaks.

Sound generation by a femtosecond laser pulse in a metal layer on a dielectric substrate is studied. The excitation of sound caused by the effect of the ponderomotive force, temperature gradients of electrons, and lattice is considered. A comparison is made of these generation mechanisms for various excitation conditions and frequencies of generated sound. It is shown that in the case when effective collision frequencies in the metal are low, sound generation dominates in the terahertz frequency range due to the ponderomotive effect of the laser pulse.

The generation of low-frequency quasi-cylindrical and surface waves generated by a long pulse of laser radiation focused into a narrow strip on the metal surface is studied. Waves are generated by non-linear currents arising from inhomogeneous heating of electrons. It is shown that the total low-frequency field is mainly determined by the quasi-cylindrical wave field. The angular and frequency distributions of the generated radiation energy have been studied. The range of angles in which it is necessary to take into account the near-surface structure of quasi-cylindrical waves is established. (C) 2022 Elsevier B.V. All rights reserved.

The pulse shape of low-frequency surface and quasi-cylindrical waves arising under the effect of a femtosecond pulse of laser radiation focused into a strip on a silver surface is studied. In silver, the effective electron collisions frequencies in the laser pulse field and in the low-frequency field may differ greatly. It is shown that for a large difference in the collision frequencies, conditions are possible when the field of surface wave exceeds the field of quasi-cylindrical wave, and the generation of low-frequency fields is mainly due to the inhomogeneous heating of electrons in the skin layer.

Nonlinear generation of the quasi-cylindrical and surface waves in terahertz frequency domain under exposure of a femtosecond laser pulse focused into a strip on a metal was studied. Competition between generated waves is determined by the value of the product of electron collision frequency and laser pulse duration. Comparison of magnetic field pulses of the quasi-cylindrical and surface waves generated on the surface is given for gold, silver, and aluminum. It is shown that far from the focusing strip the surface wave pulse contains oscillations arising due to frequency dispersion.

The spatial structure of the Fourier image of a quasi-cylindrical wave field excited at the effect of a femtosecond laser pulse focused into a strip on the conductor has been studied. In all areas above the conductor surface, except for the near zone, analytical dependencies of the Fourier image of the field on the coordinates and physical characteristics of the conductor and laser pulse are established. The areas in which the quasi-cylindrical wave field dominates the surface wave field are found. For a typical metal, a comparison of quasi-cylindrical and surface wave magnetic field shapes generated on the conductor surface is performed.

The action of an s-polarised femtosecond laser pulse is found to result in nonlinear currents in a metal film, which arise due to the nonuniform heating of the conduction electrons and under the influence of the drag force. The Fourier transforms of a low-frequency magnetic field generated by nonlinear currents are calculated. It is shown that when the film thickness is less than the scale of the low-frequency field nonuniformity, the amplitude of the terahertz signal increases in inverse proportion to the film thickness. If the thickness of the film is less than the depth of the skin layer at the laser frequency, the low-frequency signal is amplified inversely proportional to the square of the film thickness.

A method is proposed for investigation of the mutual coherence function Γ (x1, x2, τ) of a laser beam on the basis of an analysis of the Fourier transform of a speckle field formed as a result of interaction of laser radiation with a random Gaussian scatterer. The proposed method was used to study the coherence of high-power laser beams at various stages of pulse amplification in the Del'fin-1 facility designed for controlled laser fusion experiments. It was found that when the radiation from a master oscillator in this facility was practically coherent throughout the whole beam, subsequent amplification of a pulse reduced the half-width of the function Γ (x1, x2) to 0.1–0.2 of the beam diameter and this was due to nonlinear effects in the active medium at power densities ~ 1 GW/cm2. A study was also made of the dependence of the function Γ (x1, x2) on the spectral composition of laser radiation.

A study was made of the dependence of the efficiency of "saturation" amplification of nanosecond laser pulses in silicate neodymium glass on the width of the spectral distribution of the radiation. It was found experimentally that when the flux density was ~ 1 GW/cm2, an increase in the width of the radiation spectrum from 7 to 10.6 nm increased the gain of an amplifier with a GLS-1 glass active element by 25% and reduced the divergence by 30%.

The energy balance of a laser thermonuclear reactor is considered and it is shown that the energy of the reaction products absorbed by the focusing optics of the reactor considerably exceeds the energy absorbed from the laser pulse. The x-ray spectrum of the target emission is calculated and used to find the minimum distances from the center of the chamber to the optical system. Variants of the gas shield are analyzed. The laser radiation brightness required to ignite the reaction is used to obtain a relationship between the minimum focal length and the optical strength of the material employed for the optics. It is shown that thermonuclear neutrons incident on the focusing system generate considerable thermal stresses and deformations.

Investigations were made of the energy balance, anisotropy of plasma expansion, and laser radiation scattering on spherical targets heated in the Delfin-1 facility at energies of 0.1--1 kJ incident on the target. The efficiency of the absorption of laser radiation by high aspect-ratio targets of diameter 300 ..mu.. or more was 40--45% for a surface power density of (0.1--1) x 10/sup 14/ W/cm/sup 2/. A correlation was observed between the uniformity of irradiation of the target, the homogeneity of heating, and the anisotropy of plasma expansion.