Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass

We report the results of a detailed investigation into the properties of the periodic damage structure that can be produced on nominally smooth surfaces of solids when they are irradiated with a single beam of intense laser radiation. The study is primarily concerned with extracting information from the Fourier transform of the damage structure as observed via the Fraunhofer diffraction pattern produced by reflecting a cw laser beam from the surface. In particular, the patterns produced in Ge, Si, Al, and brass by pulsed 1.06- and 0.53-μm radiation are compared as a function of the angle of incidence and polarization of the beam. We find that all materials contain similar and much more intricate detailed structure than has been previously appreciated. Whereas periodic ripple patterns oriented perpendicular to the polarization at near-normal incidence are commonly reported, the diffraction patterns reveal that in fact there exists a continuous distribution of periodic structure oriented at all angles with respect to the polarization. At near-normal incidence there are two dominant sets of "fringes" running perpendicular to the polarization, while for a p-polarized beam incident at >35° there exist three dominant periodic structures; two which run perpendicular to the polarization and one which is oriented parallel to it. For s-polarized light incident at angles >35° there are two dominant patterns which form a cross-hatched pattern with axes oriented at 45° to the plane of incidence. A study of the evolution of the patterns on a shot-to-shot basis indicates that both the initial and laser-induced surface roughness play important roles in the evolution of the damage. We conclude with a comparison of our experimental results with those predicted by the theory developed in the preceding paper. Excellent agreement is found.