On the Origin, Growth and Application of Ripples

The origin and growth of a self organizing rippled nanostructure have been studied on slightly etched high temperature steel using 240 fs laser pulses with a wavelength of 800 nm. SEM, AFM and confocal microscopy have been used for analyses. A relatively low energy input results in a regular structure with a wavelength in the range 300 -500 nm and with an orientation perpendicular to the polarization direction of the laser light. In twinned areas nearby on the same specimen only "pre ripples" with a much smaller wavelength of about 150 nm and an amplitude of about 10 nm are observed. Surprisingly these "pre ripples" have a different orientation. It is proposed that energy absorption within the matrix is strongly influenced by crystal orientation differences and that "pre ripples" can be observed only within a certain energy input range. With increasing energy input regular ripples initiate, here on lines of small intergranular carbides in an area with "pre ripples". When the energy input is further enhanced the regular rippled pattern is disturbed and finally the ripples transform into disordered structures. Surfaces with a microstructure due to laser ablation and a superimposed rippled texture have been used successfully for creating hydrophobic surfaces.