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

Ultrashort-pulsed laser irradiation is a more efficient approach for fabricating fine surface structures than traditional processing methods. However, it has some problems that the equipment expenses usually increase with shortening of the pulse length, and the process principle has not been clarified completely, yet the collisional relaxation time (CRT) is assumed to be a major factor. Thus in this study, a 20-ps-pulsed laser was employed to fabricate nanometer-sized periodic structures on stainless steel alloy, SUS304. The pitch length of the fabricated fine periodic structures was similar to the laser wavelength, and also results suggested that periodic structures can 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 changing the workpiece temperature and the laser wavelength. In this way, CRT was extended and EFR was expanded by cooling the workpiece and the pitch lengths were approximately equal to the irradiated laser wavelengths. Consequently, it is easier to fabricate the fine periodic structures by cooling the workpiece than at ordinary temperature and it is possible to control the pitch length of fine periodic structures by changing the laser wavelength.