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

TL;DR

This study investigates the formation of high spatial frequency laser-induced periodic surface structures in germanium using mid-infrared femtosecond pulses, revealing how their period scales with wavelength and the underlying mechanisms involved.

Contribution

It introduces a modified surface-scattering model incorporating Drude excitation and Kerr effects to explain HSFL period scaling in mid-IR wavelengths.

Findings

HSFL periods range from λ/3 to λ/8.

A 30 nm amorphous layer is formed on germanium surface.

The model accurately predicts period scaling across wavelengths.

Abstract

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.