DEFORMATION PATTERNS IN THIN-FILMS UNDER UNIFORM LASER IRRADIATION

The mechanical behavior of thin films subjected to laser irradiation i s described by a dynamical model that is based on coupled evolution eq uations for the deformation and Vacancy density fields. Lattice vacanc ies are generated in a thin layer as a result of shallow absorption of electromagnetic laser radiation. The strain field associated with lat tice dilatation due to vacancies is shown to couple with bending and s tretching mechanical deformation fields. The dynamical model developed here is an extension of the work of Emel'yanov in two respects: (1) t he coupling between the diffusion and mechanical deformation fields is rigorously developed with additional cross-field contributions; (2) n ew equations for reduced dynamics are derived from this model, and are used to analyze the physical conditions for the onset of a deformatio nal instability. For a given material, the threshold for this instabil ity is correlated mainly with laser power. We also show that, although the instability threshold and critical wavelengths are given by the l inear part of the dynamics, the selection and type of deformation patt erns induced by this instability require a nonlinear formulation. Both numerical and analytical analysis are performed here. According to th e relative importance of nonlinearities arising from the defect or fro m the bending dynamics, square or hexagonal planforms are shown to be selected. Furthermore, it appears that one-dimensional gratings are al ways unstable in isotropic systems. The results for square patterns ar e consistent with experimental observations, while those for hexagonal and one-dimensional gratings show the importance of anisotropies on t heir final selection.