It is shown that heating of electrons due to the inverse bremsstrahlung absorption of high-power short-pulse laser radiation results in parametric generation of ion-acoustic waves. The range of wave numbers where the amplitude of the ion-acoustic oscillations increases by more than an order of magnitude is determined.
Nonlinear plasma effects important for femtosecond-scale heating dynamics are considered. It is shown that heating of electrons due to the inverse bremsstrahlung absorption of high-power short-pulse laser radiation results in parametric generation of ion-acoustic waves. The range of wave numbers where the amplitude of the ion-acoustic oscillations in-creases by more than an order of magnitude is determined. A self-similar description of electron and phonon kinetics in metals with a sharp gradient of the electron temperature is developed. It is shown that the Cherenkov generation of nonequilibrium phonons results in suppression of the electron heat flux and rapid disintegration of the metal lattice.
The results of theoretical studies are reported for threshold characteristics of a metal ablation by picosecond and femtosecond laser pulse. Two possible mechanisms of the laser ablation at laser fluence F ≤ F<SUB>th</SUB> are considered: thermal mechanism of ablation connected with a kinetics of a metal-vacuum surface evaporation and the mechanism of ablation connected with a hydrodynamics of a dense matter. The analysis has been made within the framework of a two-temperature model of metals for femtosecond and picosecond region of laser pulse duration and the extended of a two-temperature model of the metal in the case when the surface temperature T<SUB>i</SUB> more than the critical temperature of metals. Analytical expressions for the ablation-threshold fluency F<SUB>th</SUB> as well as the threshold values of the lattice temperature and the characteristic time of lattice temperature decay t<SUB>d</SUB>(F<SUB>th</SUB>) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.
The applicability of hydrodynamic models for theoretical description of UV laser ablation of polymers is studied. The plume formation is considered as a first-kind phase transition. In case of strongly absorbing polymers this phase transition occurs as a surface evaporation, and in case of weakly absorbing polymers as a bulk evaporation. The vapor plume is assumed to be transparent for laser radiation, and its expansion is described by the isoentropic hydrodynamic equations. New analytical expressions for ablation (etch) depths per pulse are obtained, which are in a good agreement with the available experimental data.
Metal ablation taking into account the hydrodynamics of a dense ablated material with ion temperature close to critical is considered. An extended two-temperature model taking into account hydrodynamic plasma expansion and degeneracy of the electron gas is developed. The new version of the RAPID code is used to perform calculations of ablation rates for several metal targets under conditions where the electron degeneracy is important.
Different regimes of heat propagation in metals irradiated by subpicosecond laser pulses are studied on the basis of two-temperature diffusion model. New analytical solutions for the heat conduction equation, corresponding to the different temperature dependences of the electron thermal conductivity (formula available n paper), are found. It is shown that in case of a strong electron-lattice nonequilibrium, the heat penetration depth grows linearly with time, l<SUB>T</SUB> varies direct as t, in opposite to the ordinary diffusionlike behavior, l<SUB>T</SUB> varies direct as t<SUP>1/2</SUP>. Moreover, the heat propagation velocity decreases with increasing laser fluence.
Laser ablation of metals by femto- and picosecond pulses is analytically and numerically studied within the framework of different models for the ablated material. Within the plasma model ablation is initiated by high-power thermal and hydrodynamic waves which propagate into the irradiated material. Analytical expressions for the thermal ablation and for the ablation by the shock wave are obtained. Numerical simulations with the computer code RAPID are in a good agreement with analytical results.
Metal ablation taking into account hydrodynamics of a dense ablated material with ion temperature close to the critical one is considered. The extended two-temperature model taking into account the hydrodynamic plasma expansion, degeneracy of the electron gas, cold pressure of ions and interaction between the electron and ion subsystems (nonideality of the metal plasma) is developed. The new version of the RAPID code is used to perform calculations of ablation rates for several metal targets under conditions where the electron degeneracy is important.
Thermal ablation of a metal surface by low-energy ultrashort laser pulses is considered theoretically. The temporal dynamics of the surface electron and lattice temperatures is studied within the framework of the two-temperature model and for different temperature dependencies of the characteristics of the metal (electron-relaxation time, heat capacity, thermal conductivity). The approximation of evaporation into a vacuum is used to determine the ablation depth. Analytical expressions for the ablation-threshold fluence F<SUB>th</SUB> as well as the threshold values of the lattice temperature T<SUB>th</SUB>(F<SUB>th</SUB>) and the characteristic time of lattice temperature decay t<SUB>d</SUB>(F<SUB>th</SUB>) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.
The generation of ion-acoustic waves in metals irradiated by ultrashort laser pulses is studied. It is shown that non- equilibrium ion-acoustic oscillations lead to an anomalous increase of the effective electron collision frequency and fast melting of metals.
The propagation of an intense femtosecond pulse in a Raman-active medium is analyzed. An analytic solution which describes in explicit form the evolution of the light pulse is derived. The field of an intense light wave undergoes a substantial transformation as the wave propagates through the medium. The nature of this transformation can change over time scales comparable to the period of the optical oscillations. As a result, the pulse of sufficiently high energy divides into stretched and compressed domains where the field decreases and increases respectively.
The results of analytical and numerical investigation of the propagation of space-limited ultrashort (pulse duration is shorter than all the relaxation times and the time scale of the medium response) light pulse in Raman-active media are represented. This process shown reveals a number of novel specific features. It is found that at definite pulse energy the self- induced focusing is replaced by the self-induced defocusing which is again followed by the selffocusing. As a result a light channel which consists of a train of moving focuses occurs on the light beam axis. The rate of self-induced focusing and defocusing processes for this propagation regime is inversely proportional to the pulse field intensity, which is contrary to the conventional selffocusing theory.
A novel mechanism for efficient high harmonics generation is proposed based on the self- scattering of powerful femtosecond light pulse in the Raman-active medium. Within the framework of the closed self-consistent model the pulse field electrodynamics are investigated, a simple analytical formula for the spectrum generated is derived. The 'plateau' effect and the sharp cutoff of harmonic spectrum are predicted. Unlike the case of rare gases, the spectrum generated is found to be tuned by the variation of input ultrashort pulse intensity.