Sub-wavelength ripple formation on dielectric and metallic materials induced by tightly focused femto-second laser radiation

摘要

Sub-wavelength ripples are obtained by scanning a tightly focused beam (～1μm) of femtosecond laser radiation (λ=266, 400 and 800 nm, τ_p=100 fs) over the surface of various dielectrics, metals and silicon. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the lateral and temporal spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N=1 to N=20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. The ripples have been continued not only in one dimension but also in two dimensions on the surface by several scans with an offset, resulting in the formation of an extended grating.