Influence of light scattering on the development of laser-induced ridge-cone structures on target surfaces

A new model of light-target interaction, which takes into account multiple interaction of the scattered laser beam with the surface structure of a target, is reported. The model is based on calculations of the local intensity of the electromagnetic wave, from which the local amount of ablated material can be obtained after each simulated laser pulse. The simulations of the sequence of laser pulses was employed to explain the observed development of the particular surface topography. It is shown that during laser ablation a small initial surface roughness is sufficient to initiate further deepening of the surface relief. During this preliminary development of the relief, the 1^st-order scattering plays a dominant role. But, as soon as the surface acquired a relatively deep relief, the 2^nd-order interaction leads to the appearance of "hot" valleys, where the electromagnetic energy of the light wave is significantly more concentrated and, therefore, the ablation is more efficient. As a result of this effect the depth of the valleys increases and the cone structure becomes more and more pronounced.