Calculation of the temperature field of the material in the thermal model of laser ablation

The mechanisms of laser ablation of materials under the interaction of powerful pulsed laser radiation with a material coated with a thin film system were studied. One of the first models of such interaction of high-power laser radiation with matter was the thermal model. A onetemperature thermal model was used when considering the interaction of laser radiation with dielectrics, and a two-temperature model was used when considering the interaction of laser radiation with metals. When studying the processes of laser ablation within the framework of the thermal model, the method of moments with a special selection of the test function was chosen. Computer simulation was conducted to study the dependence of the surface temperature and the characteristic thermal length on the radiant flux. Variants with both constant and variable optophysical characteristics of the phenomenon were considered. Calculations were performed for different time ranges under different initial conditions. The results of the numerical experiment agree well with the data of other authors.

Radiation from asymmetric laser-induced plasmas collected by a lens or optical fiber

A method of calculating radiation spectra of an asymmetric (ellipsoidal) laser-induced plasma plume is developed for two cases, when the radiation is collected by a lens and by an optical fiber. The lens receives the radiation coming from the entire plasma plume, while the view sight of an optical fiber is restricted to an acceptance cone so that only the radiation coming with an incident angle smaller than the cone angle is collected. The method incorporates the solution of the radiative transfer equation along the line of sight. An optimal number of lines is found to achieve the numerical convergence with a relative error <1%. Several practical simulations are carried out that include different placements and orientations of the lens and optic fiber. The effect of a motion of the center of the mass of the plasma plume on the radiation spectra is also investigated.

Transient optical response of ultrafast nonequilibrium excited metals: effects of electron-electron contribution to collisional absorption

Approaching energy coupling in laser-irradiated metals, we point out the role of electron-electron collision as an efficient control factor for ultrafast optical absorption. The high degree of laser-induced electron-ion nonequilibrium drives a complex absorption pattern with consequences on the transient optical properties. Consequently, high electronic temperatures determine largely the collision frequency and establish a transition between absorptive regimes in solid and plasma phases. In particular, taking into account umklapp electron-electron collisions, we performed hydrodynamic simulations of the laser-matter interaction to calculate laser energy deposition during the electron-ion nonequilibrium stage and subsequent matter transformation phases. We observe strong correlations between optical and thermodynamic properties according to the experimental situations. A suitable connection between solid and plasma regimes is chosen in accordance with models that describe the behavior in extreme, asymptotic regimes. The proposed approach describes as well situations encountered in pump-probe types of experiments, where the state of matter is probed after initial excitation. Comparison with experimental measurements shows simulation results which are sufficiently accurate to interpret the observed material behavior. A numerical probe is proposed to analyze the transient optical properties of matter exposed to ultrashort pulsed laser irradiation at moderate and high intensities. Various thermodynamic states are assigned to the observed optical variation. Qualitative indications of the amount of energy coupled in the irradiated targets are obtained.