Particle assisted structuring on metallic substrate: Anomaly when particle size exceeds irradiation wavelength

We present here results of our study on particle assisted micro-structuring on a metallic surface. For this, dielectric polystyrene spherical particulates were deposited on an optical grade stainless steel (SS) surface, and the fundamental emission from a Nd:YAG laser and its second harmonic, to which the particulates are transparent, were used as the coherent sources for these experiments. The diameters of the particulates were chosen so as to encompass all the possibilities, viz., particulate size ≈ wavelength, particulate size < wavelength, and particulate size > wavelength. While the experimentally measured fluence values for the onset of pitting are in agreement with the analytically estimated field enhancement factors in the former two cases, there is a strong departure noted in the latter case. Percentile absorption of the incident radiation by the SS substrates as measured by an integrating sphere bore testimony to this anomalous behavior. Toward gaining a deeper insight into this phenomenon, atomic force microscopy based topographic analysis of the laser exposed surface was undertaken. This study clearly reveals the formation of double pits underneath the particulate when its size exceeds the wavelength of exposure, thus pointing to the fact that the incident energy is being redistributed, giving rise to the observed anomaly.