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

Abstract

Formation of high spatial frequency laser induced periodic surface structures (HSFL) in germanium by femtosecond mid-IR pulses with wavelengths between $\hat{I}»=2.0$ and $3.6\hat{\mathrm{I}}¼\mathrm{m}$ was studied with varying angle of incidence and polarization. The period of these structures varied from $\hat{I}»/3$ to $\hat{I}»/8$. 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 $30\mathrm{nm}$ 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.

7 pages total (1 page of supplementary material)