Polarization charge model for laser-induced ripple patterns in dielectric materials

An explanation of laser-induced ripples in dielectric surfaces is given. The model takes into account the polarization charge which is induced on the boundaries of defects by the applied laser field. It is shown that this charge results in a sinusoidally varying perturbation to the applied laser field in the vicinity of the defect, and that the perturbation field has a period of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda/n</tex> , where λ is the laser wavelength and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> is the index of the dielectric. It is further shown that this perturbation is greatest along the direction of laser polarization. At sufficiently high laser field intensities, the sum of the applied laser field and the maxima in the perturbation field will exceed the damage threshold, and a permanent ripple pattern in the surface will result. The predicted spacing and orientation of these ripples is in agreement with the experimentally observed data. Finally, it is shown how bootstrapping occurs to enhance the ripple pattern, once begun, and what roles propagation delays, defect size, and laser polarization state play in the process.