Fabrication and control of fine periodic surface structures by short pulsed laser

Ultrashort-pulsed laser irradiation is a more efficient approach to the fabrication of fine surface structures than traditional processing methods. However, it has some problems: the equipment expenses usually increase as the pulse shortens, and the process principle has not been clarified completely, although the collisional relaxation time (CRT) is assumed to be a major factor. In this study, a 20-ps pulsed laser was employed to fabricate nanometer-sized periodic structures on a stainless steel alloy, SUS304. The pitch length of the fabricated fine periodic structures was similar to the laser wavelength, and the results suggested that periodic structures could be fabricated within a limited range of the laser fluence. In order to expand the effective fluence range (EFR) and to control the pitch length, laser irradiation was carried out with different workpiece temperatures and the laser wavelengths. In this way, CRT was extended and EFR was expanded by cooling the workpiece, and the pitch lengths were approximately equal to the laser wavelengths. As a result, two things were found: it is easier to fabricate the fine periodic structures by cooling the workpiece, and it is possible to control the pitch length of the fine periodic structures by changing the laser wavelength.