Laser wavelength effects in ultrafast near-field laser nanostructuring of Si

We study the effect of laser wavelength (400 and 800 nm) on the near-field processing of crystalline silicon (Si) in the femtosecond pulse duration regime through subwavelength apertures. Distinct differences in the obtained nanostructures are found in each case both in terms of their physical sizes as well as their structure, which can be tuned between craters and protrusions. A single or a few femtosecond pulses can deliver enough energy on the substrate to induce subdiffraction limited surface modification, which is among the smallest ever reported in subwavelength apertured near-field scanning optical microscope schemes.