Sub-wavelength ripple formation on various materials induced by tightly focused femtosecond laser radiation

Sub-wavelength ripples (<lambda/4) are obtained by scanning a tightly focused beam (1 mu m) of femtosecond laser radiation (t (p)=100fs, lambda=800nm &400nm) over the surface of various materials. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing A on the material. New results concerning the dependence of the spacing on the wavelength are presented. Some possible models for ripple growth are discussed and conditions under which these phenomena occur are contained. In opposition to the classical ripple theory, the observed ripple spacing is dependent on the material, giving indication to understand the processes during the sub-wavelength ripple formation by femtosecond laser radiation.