Laser induced ripples that are also known as Laser Induced Periodic Surface Structures (LIPSS) have gained considerable attention by researchers and industry due to their surface functionalization applications. Surface modification with LIPSS takes place in metals, semiconductors or dielectrics [1]. Their morphology depends strongly on many parameters, both on material properties and irradiation conditions (pulse fluence, wavelength, light polarization, pulse repetition rate, and processing speed). Two types of LIPSS are distinguished by their spatial periodicity, namely high-spatial frequency LIPSS (HSFL) having a periodicity significantly smaller than the laser wavelength and low-spatial frequency LIPSS (LSFL) having a periodicity close to the laser wavelength. In this work, we have investigated the LIPSS formation on germanium by using a Ti:Sapphire (800nm, 120 fs) laser. The results show different scenarios depending on the irradiation conditions. One of these scenarios is presented in Figure 1, showing images of LIPSS formed by the same fluence but different number of pulses. For a low number of pulses, LIPSS of type HSFL were formed parallel to polarization, whereas for larger number of pulses they were of type LSFL and perpendicular to polarization. This evolution of spatial periodicity by increasing the number of pulses is fundamentally different regarding the behaviour in silicon [2], which will be discussed for comparison. On the other hand, since the spatial intensity distribution of the laser beam is Gaussian, the central disk of the irradiated spot has been exposed to the peak fluence. Suprisingly, this central disk did not present ablation but LIPSS appeared on its surroundings, as can be seen in the images. This suggests that a phase change from the crystalline phase into the amorphous one could be the origin of these LIPSS. References 1 Bonse, J., Höhm, S., Kirner, S.V., Rosenfeld, A. & Krüger, J. IEEE J. Sel. Top. Quantum Electron 2017, 23, 9000615 2 Fuentes-Edfuf, Y., Garcia-Lechuga, M., Puerto, D., Florian, C., Garcia-Leis, A., Sanchez-Cortes, S., Solís, J., Siegel, J. Scientific Reports 2017, 7, 4594

USTS 2019 Meeting, Madrid, November 6th to 8th, 2019