In situ Observation of LIPSS Formation on Si Wafers under THz-FEL Irradiation

In order to investigate the mechanism of the formation of laser induced periodic surface structures (LIPSS), in situ observation of LIPSS formation on Si wafers under the irradiation of intense THz pulses with a center wavelength of ~72 μm emitted from a free electron laser (THz-FEL) was performed. When the number N of the irradiated THz pulse is less than 50~70, fine LIPSS with a period of 3~4μm almost parallel to the polarization of the THz pulses were observed. For N>250, coarse structures with a period of 30~40μm emerged.