Deceleration of fast ion rarefied beam due to Cherenkov interaction with ion-acoustic waves.

The deceleration of a low-density beam of ions in plasma with developed ion-acoustic turbulence arising in strong electric field is described. The time and length of beam deceleration along and across the anisotropy axis of the wave number distribution of ion-acoustic waves are found. It is shown to what extent an increase in the strength of the electric field that generates turbulence is accompanied by a decrease in the time and length of braking. As the beam propagates along the anisotropy axis, its velocity decreases to approximately the velocity of ion sound, and the direction of propagation does not change. When the beam is decelerated with an initial velocity across the anisotropy axis, a velocity component appears along the anisotropy axis during deceleration, which results in the beam deflection from the initial direction. In this case, the modulus of the beam velocity at the end of deceleration is close to the ion sound velocity.

Terahertz sound generation at the effect of a femtosecond pulse of laser radiation on a metal.

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.

Generation of terahertz radiation under the effect of a femtosecond pulse on plasma in a magnetic field.

The generation of terahertz (THz) radiation under the effect of a femtosecond laser pulse on a transparent semibounded plasma in a constant magnetic field is studied. It is shown how the generation patterns of THz radiation depend on the laser pulse duration and the ratio between the cyclotron and plasma frequencies of electrons. In each of the modes considered, the total energy per unit area, spectral composition, and pulse shape of the generated THz radiation are found. It has been established that optimal generation conditions are realized at laser pulse duration comparable to the reciprocal frequency of plasma oscillations and at electron cyclotron frequency lower than plasma frequency. When cyclotron frequency exceeds plasma frequency, the possibility of increasing the generation frequency up to frequencies comparable to the cyclotron frequency is revealed.

Reflection and absorption of electromagnetic radiation by inhomogeneous photoionized plasma, produced by multiphoton ionization of inert gas atoms.

The interaction of electromagnetic radiation with inhomogeneous plasma formed by multiphoton ionization of inert gas atoms has been studied. In high-frequency and normal skin effects modes the field structure in plasma is described and reflection and absorption coefficients are found. It is shown that as the thickness of the plasma region, in which the photoelectron density grows linearly, increases, both the depth of field penetration and the absorption coefficient increase, too. It is found that, due to the Ramzauer-Townsend effect, there is a relative increase in the effective frequency of photoelectron collisions with atoms, which is accompanied by a significant increase in the absorption coefficient.

Controllable ablative machining of Al/Ti and Ti/Al nano-layers on a Si substrate by single-pulse femtosecond laser irradiation.

Results concerning the controllable ablation of nano-layered thin films (NLTF) by femtosecond laser pulses are presented. Investigated samples were titanium-aluminum bilayers, deposited on a silicon substrate, with the top titanium or aluminum layer of variable thickness on the surface. Irradiation was done in ambient air with single femtosecond laser pulses under standard laboratory conditions. The samples were analyzed by complementary methods of optical and scanning electron microscopy and optical profilometry, exhibiting laser-fluence-dependent ablative removal either of the top layer or the entire bilayer or even partial ablation of the underlying silicon substrate. The removal (spallation) threshold fluences for the topmost layer are scalable versus its thickness almost irrespectively of its material, being rather selective for the Ti-coated samples and much less selective for the Al-coated samples. The removal of the entire bilayers was found to be strongly influenced by electronic properties of the underlying metallic layer, dictating the NLTF-Si adhesion, heat conduction, and capacity in the NLTFs toward the NLTF-Si interface and beyond, as well as by their thermophysical characteristics, e.g., almost twice higher melting temperature and enthalpy for Ti. As a result, precise fs-laser machining of the entire NLTFs is pronounced and selective for the samples with the fusible Al at the low-adhesion Al-Si interfaces, compared with the incomplete NLTF removal from the high-adhesion and refractory Ti-Si interfaces.

Competition of quasi-cylindrical and surface waves excited at the femtosecond pulse effect on the metal.

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.

Kinetic approach to THz radiation generation at femtosecond laser pulse ponderomotive effect on plasma in magnetic field.

For a semibounded plasma in a constant magnetic field and interacting with short laser pulse, a kinetic equation is derived, which makes it possible to describe the low-frequency movements of electrons. In the linear approximation in laser radiation intensity the solution of kinetic equation is obtained taking into account mirror reflection of electrons by the plasma surface. Using this solution, we derived low-frequency currents generated by low-frequency field and ponderomotive force that changes during the pulse affect. Under the assumption that characteristic spatial scales of changes in the low-frequency field and ponderomotive force exceed the Larmor radius of electrons, we studied low-frequency currents near the plasma surface. If the electron cyclotron frequency exceeds the inverse pulse duration, then low-frequency currents differ from their values in a homogeneous plasma only at a distance from the surface not exceeding several Larmor radii. Taking this fact into account, a solution to the equation for low-frequency field in the plasma was obtained. The terahertz (THz) magnetic field generated by nonlinear currents is found. It is shown that the maximum value of the generated field is attained at cyclotron frequency comparable with the product of the plasma frequency square and laser pulse duration.

Nonlinear transmission and reflection of a strong terahertz pulse by a metal film.

Nonlinear interaction of the powerful terahertz radiation pulse with metal nanofilm is investigated. Quantitative analysis of the nonlinear interaction is illustrated by an example of an aluminum film of a thickness of several tens of nanometers. Rapid heating of the conduction electrons and the lattice causes significant rise of the effective electron collision frequency. This implies a noticeable change in reflectivity and an increase of transmissivity by an order of magnitude.

Free-electron mechanisms of low-frequency radiation generation on metal surfaces.

We present a time-domain solution of the equations describing low-frequency radiation generation when a metal surface is irradiated by a non-damaging femtosecond optical pulse of s-polarized radiation. Ponderomotive force, drag current, and temperature gradient along the surface are found to be sources of the terahertz radiation. A comparison of these sources under typical experimental conditions reveals that both the drag current and the thermal gradient give similar contribution to the terahertz signal. The ponderomotive force effect is several times less than these two and exists only during the incident pulse.

Third-harmonic generation in the skin layer of a hot dense plasma.

The third-harmonic generation of a pump wave, resulting from the electron-ion collision frequency dependence on the electric field in the skin layer of a hot dense plasma is investigated. The relation of the current third harmonic with the high-frequency field in the skin layer is established for arbitrary ratios of the electron-ion collision frequency to the field frequency. For arbitrary ratios of these two frequencies, the field structure inside the skin layer is determined, and the field of the wave irradiated by the plasma at tripled frequency, too, is calculated. It has permitted us to find the explicit dependencies of the third-harmonic generation efficiency on the plasma and pump field parameters.

Anomalous transmission of an ultrashort ionizing laser pulse through a thin foil.

The formation of a highly anisotropic photoelectron velocity distribution as a result of the interaction of a powerful ultrashort laser pulse with a thin foil is found to yield a large skin-layer depth and an anomalous increase of the transmission coefficient. The physical reason for the effect is the influence of the incident wave magnetic field, through the Lorenz force, on the electron kinetics in the skin layer.

Plasma electron kinetics in a weak high-frequency field and magnetic field amplification.

We describe the linear stage of Weibel instability in a plasma heated via inverse bremsstrahlung absorption of a high-frequency, moderate intensity radiation field under conditions in which the plasma electron velocity distribution function is weakly anisotropic. We report on the possibility of a significant amplification of spontaneous magnetic fields both in the case of an electron distribution function slightly departing from a Maxwellian in the region of subthermal velocities, and in the case where the Langdon nonequilibrium distribution is formed. We show that the direct influence of collisions on the Weibel instability growth rate may be traced back to subthermal electrons, for which the effective collision frequency is large.