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

摘要

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 ofincidence and polarization. The period of these structures varied from$\lambda/3$ to $\lambda/8$. A modified surface-scattering model including Drudeexcitation and the optical Kerr effect explains spatial period scaling of HSFLacross the mid-IR wavelengths. Transmission electron microscopy (TEM) shows thepresence of a $30 \; \mathrm{n m}$ amorphous layer above the structure ofcrystalline germanium. Various mechanisms including two photon absorption anddefect-induced amorphization are discussed as probable causes for the formationof this layer.