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 femtosecond mid-IR pulses with wavelengths between $\lambda=2.0$ and $3.6 \; \mathrm{\mu m}$ was studied with varying angle of incidence and polarization. The period of these structures varied from $\lambda/3$ to $\lambda/8$. A modified surface-scattering model including Drude excitation and the optical Kerr effect explains spatial period scaling of HSFL across the mid-IR wavelengths. Transmission electron microscopy (TEM) shows the presence of a $30 \; \mathrm{n m}$ 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. 
Drake. R. Austin, Kyle R.P. Kafka, Yu Hang Lai, Zhou Wang, Kaikai   Zhang, Hui Li, Cosmin I. Blaga, Allen Y. Yi, Louis F. Di...
physics.optics cond-mat.mtrl-sci