On the formation of laser induced self-organizing nanostructures

Laser induced self-organizing rippled nanostructures on steel are formed by femtosecond laser pulses. They are applied as hydrophobic surfaces. A low fluence results in 'regular ripples' with a spatial repetition of 300-500 nm, orientated perpendicular to the laser polarization direction. In twinned areas 'pre-ripples' with much smaller wavelength (about 150 nm) are observed, with a different orientation. We found indications that the energy absorption depends on the crystal orientation and that pre-ripples are only formed at very low fluence. Pre-ripples initiate on secondary carbides or on grain boundaries. At higher energy, regular ripples initiate in areas with pre-ripples; at even increasing fluence disordered structures are obtained.