An investigation into the micro/nano machining process for germanium substrates using femtosecond lasers

A comprehensive literature review of the laser machining technologies for difficult-to-machine materials has been presented. It is shown that the research on the investigation of femtosecond (FS) laser micro/nano machining on Ge substrates is relatively limited, and more efforts are necessary for better understanding of the mechanisms behind the process. An experimental study on the FS laser micro-grooving process on single crystalline Ge substrates is performed. The effects of different microgrooving performance measures, such as groove characteristics, material removal rate and heat affected zone, with respect to the process parameters, have been presented. Recommendations have been made to guide the selection of process parameters. Various micro/nano-structural arrays have been created on Ge substrates using a single beam FS laser, without using any multi-scanning process, multi-beam interference or auxiliary microlens arrays. The surface textures can be varied from micro/nano-structural arrays to V-shaped microgrooves by appropriately controlling the process parameters, and the laser polarisation has been found to be able to change the array patterns. The experimental findings agree well with the self-organisation theory based on the laser-induced surface instabilities. A molecular dynamics (MD) model for the FS laser ablation process has been developed, in which the two-temperature model and the effect of the laser-induced plasma, are incorporated. The calculated ablation depth from the MD model considering the plasma effect agrees well with the experimental results. The developed MD model is then used in a simulation study to understand the FS laser ablation process. The effects of laser pulse width and laser fluence on the absorption process of laser energy have been investigated. Ultrafast nonthermal response and homogeneous melting in the FS laser ablation process have been discussed. The material removal mechanisms are influenced by laser fluence regimes. The surface atoms are removed from the material in the form of phase explosions under sufficiently high laser fluence, while the photomechanical disruption plays a dominant role in low laser fluence regimes. These two mechanisms are believed to co-exist in the ablation process due to the non-uniform distribution of the laser fluence across the laser beam