Pulse-by-Pulse Optical Observation of Growth of Laser-Induced Periodic Surface Structure at Oblique IncidenceWednesday (24.06.2020) 08:40 - 09:00 Room 2
Formation of Laser-induced Periodic Surface Structure (LIPSS) attracts interest from the viewpoint of light-matter interaction as well as promising applications. In particular, the LIPSS whose period is shorter than the laser wavelength have attracted great interest in the past two decades. Many research on period have been reported as a function wavelength, pulse duration, fluence, pulse number, and so forth. Some researchers reported that the period decreases with increase in pulse number (for example, ). However, it is not clear that how the period changes, which might be a key to elucidate the mechanism of the formation of LIPSS.
In order to obtain the information about this, we attempted to observe the change in the surface morphology with increasing pulse number at the same position. There are few reports on such observation using confocal laser scanning microscopy  or scanning electron microscopy.  Compared to these techniques, simple optical observation is faster thus easy to acquire many data, but spatial resolution of optical observation is not good enough with respect to the period. In the present study, we utilized incident angle dependence of the period. LIPSS with period longer than the laser wavelength was generated by oblique incidence irradiation, and growth of the LIPSS was observed pulse-by-pulse using in-situ optical microscopy.
In the experiment, a Nd:YAG laser with a wavelength of 1,064 nm and a pulse width of 0.5 ns was used, and the laser pulses were irradiated onto austenitic stainless steel substrate (SUS304 in Japanese Industrial Standards, equivalent to X5CrNi18-10 in ISO) at oblique incidence with p-polarization. The period of LIPSS at normal incidence was almost equal to the laser wavelength. At oblique incidence, LIPSS with period longer than the wavelength appeared. The period became longer with increasing incident angle, and its maximum value was about 3 μm around the incident angle of 60°. LIPSS with period shorter than the wavelength was also observed, but LIPSS with period longer than the wavelength occupied most of the area. The growth of LIPSS was observed from a direction perpendicular to the substrate using a 50x objective lens with a numerical aperture of 0.55. We found that the way of connection of the fringes is changing with increasing pulse number.