Large-area high-spatial-frequency patterns (HSFLs) of periodicity have been generated by a nanojoule-femtosecond laser scanning technique (, , ) at the silicon–air interface. The excellent large-area uniformity allowed reproducible and accurate measurements of the periodicity. Variation of experimental param eters 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-armonic 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.
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