Control parameters in pattern formation upon femtosecond laser ablation

Femtosecond laser ablation experiments on different materials have revealed a great diversity of self-organized nano-structures on the bottom of the ablated area. They range from arrays of nano-spheres to bifurcating longer lines, with size and shape depending on the irradiation dose and number of laser pulses. In this contribution, laser beam polarization and surface charge distribution, following surface relaxation, are investigated as control parameters for this nano-structures formation. Experiments with linear, circular and, for the first time, elliptical polarized light indicate that the laser polarization has a great influence on the orientation of the ripples. Investigations with laser beams of elliptical polarization show that, whereas the ripples orientation is defined by the major axis of the polarization ellipse, the ripple morphology is sensitive to the ellipticity. For silicon samples electrical measurements with a scanning-probe microscope on the ablated area reveal a spatial variation of the surface potential, possibly generated by a local change in dopant density, correlated with the patterns' modulation on the crater bottom.