Single-shot femtosecond laser ablation on the nanoscale

Abstract

The use of femtosecond lasers as a tool for precise machining of nano-structures in materials has been steadily growing in recent years. In particular, It has been demonstrated that direct femtosecond laser ablation can be used to rapidly prototype photonic waveguide devices operating at optical telecommunication wavelengths. To take the best advantage of femtosecond laser structuring of materials, a good understanding of the energy deposition mechanisms is required. A simple and important technique to study the ultra-fast carrier generation process and energy deposition mechanisms into the material is to measure the self-reflectivity of incident pulses. In spite of the demonstrated applications of femtosecond laser ablation for nano-structuring of materials where strongly foucsed beams are used, up to now, there have been no self-reflectivity studies in the ablation regime with strongly focused beams. Furthermore, for an application point of view, the surface morphology of the ablated nano-holes such as the diameter, the depth, the roughness etc. are of course very important to be used as controlling parameters in device fabrication. Up to now, there have been little work on a symmetric study in the morphology of the ablated holes. In this thesis, we study the energy deposition mechanisms and the surface morphology of the ablated nano-holes of femtosecond laser pulses. Experimentally we measure the self-reflectivity on silicon-on-insulator (SOI) samples and a bulk silicon sample, illuminated by tightly focused femtosecond laser pulses of energies around the ablation threshold. The surface morphology of the resulting nano-sized holes are measured using an atomic force microscope (AFM). In order to understand and reproduce the measured self-reflectivity data, we developed a model based on the Finite Difference Time Domain (FDTD) method. In this model, we incorporate the generation of a plasma by the laser pulse and the interaction between the pulse and this induced plasma. We show that by taking the dynamically changing Drude response of the laser-induced plasma into account, the model excellently describes the behavior of the self-reflectivity under nano-ablation conditions. It is shown that impact ionization is the dominating carrier generation process in silicon excited by a single femtosecond laser pulse with a wavelength of 800 nm. Furthermore, we find, to our knowledge for the first time, that part of the incident pulse is scattered by the laser-induced plasma into the guided modes of the silicon device layer, an effect that we refer to as self-scattering. In addition, we discovered a pronounced peak in the ablation depth versus incident fluence data which in practice may well turn out to be very beneficial for laser drilling of high aspect-ratio nano-holes. It is shown qualitatively by the model that optical interference effects in laser ablation of layered structures such as the SOI contributes to the appearance of this pronounced peak. In the final part of the thesis, we present ablation of materials by using a femtosecond laser beam with a ring-shaped intensity distribution. We propose material ejection purely caused by laser-induced converging shock wave.