Large-area, uniform, high-spatial-frequency ripples generated on silicon using a nanojoule-femtosecond laser at high repetition rate.

Large-area high-spatial-frequency patterns (HSFLs) of λ/6 periodicity have been generated by a nanojoule-femtosecond laser scanning technique (80 MHz, 170 fs, 700-950 nm) at the silicon-air interface. The excellent large-area uniformity allowed reproducible and accurate measurements of the periodicity. Variation of experimental parameters as illumination geometry, and pulse energy and number showed no influence on the ripple spacing. A wavelength dependence was observed and compared to current models of HSFL formation. A particular second-harmonic model was found to match the results best but needs to take into account transient changes in the refractive index under laser exposure. A second-harmonic mechanism is further supported by direct spectroscopic observation.