High spatial frequency laser induced periodic surface structure formation in germanium by mid-IR femtosecond pulses

Formation of high spatial frequency laser induced periodic surface structures (HSFL) in germanium by 90 fs mid-IR pulses at a 1 kHz repetition rate with wavelengths between λ = 2 and 3.6 μm was studied with varying angle of incidence and polarization. The period of these structures varied from λ/3 to λ/8. A modified surface-scattering model including Drude excitation and the optical Kerr effect explains the spatial period scaling of HSFL across the mid-IR wavelengths. Transmission electron microscopy shows the presence of a 30 nm amorphous layer above the structure of crystalline germanium. Various mechanisms including two photon absorption and defect-induced amorphization are discussed as probable causes for the formation of this layer.