Modeling of Surface Manipulation by Femtosecond Laser Pulses
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laser technologies. Experimental investigation of this problem is often very difficult due to many uncontrolled parameters and factors influencing experimental results. Theoretical approaches to the problem are still not effective because till now there is no clear understanding of nature and regularities of femtosecond laser-matter interactions. Main characteristic features of femtosecond interactions with transparent materials are 1) high intensity of radiation within focal spot which can exceed 1013 W/cm 2 [2, 3]; 2) little pulse duration which is below relaxation time for electronic and phonon sub-systems. The first of them points at critical role of nonlinear electrodynamic effects in femtosecond interactions because laser-induced variations of refraction in focal area are about 0.01 - 0.001 and nonlinear distortions of laser beam are enough to change its space structure sufficiently. Thus, the first question is as follows: what processes can result such distortions in? The second feature of femtosecond pulses points at need to look for fast mechanisms of laser-matter interactions because traditional models of laser-induced heating, cannot explain rather large ablation rates [2], formation of surface ripple patterns and other peculiarities of laser-surface interaction for femtosecond pulses. Recent attempt of the authors to generalize two-temperature model for the case of dielectrics and semiconductors was not correct as it was pointed out by Prof. M.N.Libenson [5]. In this connection general problem arises - how is radiation energy transported to crystal lattice to damage it? The mechanism of energy transport must be fast and can be connected with nonlinear wave propagation. 163 Mat. Res. Soc. Symp. Proc. Vol. 573 01999 Materials Research Society
Here we present a new model for description of nonthermal femtosecond laser interaction with surface of low-absorbing materials (e.g., dielectric or wide-bandgap semiconductor). It is based on results of correct calculations of nonlinear wave propagation in transparent media. MODELING OF FEMTOSECOND LASER-MATTER INTERACTION We start with modeling of nonlinear electrodynamical processes because they are the most suitable candidates for being a mechanism of femtosecond ablation and surface manipulation. Nonlinear radiation propagation is highly complicated process described by nonlinear wave equation that it difficult for analytical solving. Numerical methods, for example, finite-difference time domain technique seems to be the best tool to attack this problem. In this work field evolution was simulated according to developed by the authors technique [6, 7] only for the case of electric-field vector of incident wave parallel to surface ripples (TE polarization in Fig. 1). This allows reducing general nonlinear wave equation to simpler one for z-projection of electric field vector Y2E + 2E _ .- 2E 41r L9 PNL 2 (d) d, 212 c y y e3X2 where co is light speed in vacuum, Fo is constant part of dielectric function.
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