The mechanical behavior of thin films which were subjected to laser irradiation was described by using a dynamic model that was based upon coupled evolution equations for the deformation and vacancy density fields. Lattice vacancies were generated in a thin layer, as the result of shallow absorption of electromagnetic laser radiation. The strain field which was associated with lattice dilatation due to vacancies was shown to be coupled to bending and stretching mechanical deformation fields. The present dynamic model was an extension of the work of Emelyanov, in that the coupling between diffusion and mechanical deformation fields was rigorously combined with additional cross-field contributions. Also, new equations for reduced dynamics were derived and were used to analyze the physical conditions required for the onset of a deformational instability. For a given material, the threshold for this instability was related mainly to the laser power. It was also shown that, although the instability threshold and critical wavelengths were given by the linear part of the dynamics, the selection and type of deformation pattern which was introduced by this instability required a non-linear formulation. According to the relative importance of non-linearities which arose from the defect or from the bending dynamics, square or hexagonal planforms were selected. It appeared that 1-dimensional gratings were always unstable in isotropic systems. The results for square patterns were consistent with experimental observations, while those for hexagonal and 1-dimensional gratings demonstrated the importance of anisotropy in the final selection.

D.Walgraef, N.M.Ghoniem, J.Lauzeral: Physical Review B, 1997, 56[23], 15361-77