| Abstract [eng] |
This Master's thesis provides a comprehensive investigation into the surface modification of BF33 glass using ultrashort femtosecond laser pulses, aiming to establish optimal conditions for the high-precision fabrication of diffractive optical elements (DOEs). Given that BF33 glass features a wide bandgap (3.7–4.0 eV), interaction with 1030 nm radiation (1.2 eV photon energy) relies on non-linear ionization processes, where multi-photon absorption and tunneling become the primary mechanisms for electron excitation into the conduction band. The research centers on accumulation phenomena, specifically the incubation effect, which manifests as a systematic reduction in the ablation threshold fluence as the number of pulses applied to a single spot increases. The experimental phase was conducted using an Yb:KGW solid-state laser system and high-precision positioning stages, with surface topography analyzed via confocal microscopy to achieve sub-micron resolution. By applying the Liu method, ablation thresholds for various pulse numbers were precisely defined. The results demonstrate that sub-threshold irradiation on pristine, polished surfaces allows for exceptional depth control, where the removed layer thickness depends linearly on the number of scanning cycles, achieving a minimum structure depth of just 8 nm. Crucially, the surface roughness S_a remains at the nanometric level (1–2 nm) throughout this process, which is a vital requirement for phase-modulating optical elements that demand minimal light scattering and high optical transparency. Despite the success in precision depth control, the study identified critical physical limitations where surface quality degrades rapidly. It was found that exceeding a critical fluence threshold (3.1 J/cm^2) triggers the formation of periodic surface structures related to the scan step, driven by the Marangoni effect - thermocapillary movement of the melt phase due to local temperature gradients. Furthermore, when the number of scanning cycles exceeds 1000, accumulation processes lead to surface degradation characterized by the formation of pyramid-shaped basins and a dramatic increase in roughness to hundreds of nanometers. Additional experiments on pre-damaged surfaces revealed that sub-threshold pulses cannot be used for "smoothing," as existing surface defects act as positive feedback, increasing ablation efficiency by up to three orders of magnitude and further exacerbating surface non-uniformity. In conclusion, the utilization of the incubation effect in the sub-threshold regime is a promising tool for creating smooth surfaces with controlled phase delay, provided that parameters are strictly optimized. For the fabrication of diffractive elements, such as 4-level DOEs, a single-step process on pristine surfaces is recommended, avoiding the re-processing of rough areas and maintaining a fluence that does not provoke unwanted thermocapillary transport. Additionally, thermal annealing is suggested following the laser treatment to relieve residual mechanical stresses caused by localized heating, ensuring the long-term stability and functionality of the optical component. This research provides a methodological framework for more accurate laser micro-machining in dielectric materials, with nanometric surface quality. |
