Discussion of the possible formation mechanism of near-wavelength ripples on silicon induced by femtosecond laser

In this paper, the effect of the irradiation pulse number on the morphology of the near-wavelength ripples perpendicular to the laser polarization was investigated. The results show that the ripple spacing first fast decreases with the increased irradiation pulse number and finally reaches a stable value. The change of ripple morphology with different incidence angles was also studied. The ripples are found keeping always perpendicular to the laser polarization and have respective ripple spacings of 650, 480 and 390 nm upon the laser irradiation with pulse number N = 16 and fluence of 1.33 J/cm2 at the incidence angles of 0°, 30° and 60°. The change of the ripple spacing with different excitation levels was also studied. The theory for interference of the incident wave with the surface plasmon polariton wave (SPP) and the Sipe–Drude theory are both employed to analyze the experimental results. Detailed calculations show that the Sipe–Drude theory works better for explaining the main features of the ripples on silicon wafer surface. We suggest that the interaction of the incident wave with some form of surface electromagnetic wave except SPP is most possibly responsible for the ripple formation on silicon surface.