In modern life, technical glass is an indispensable material for window panes, containers, optical applications or transparent protective layers such as displays. The main focus is on its properties such as transparency, chemical resistance, refractive index or mechanical stability. In the last decade, various methods of laser processing of glass have become an established standard in the industry, such as selective structuring or especially laser cutting of displays. All methods are based on a local modification of the thermal, mechanical or chemical properties of transparent media. Although various laser processing methods are well established in various industrial applications, the direct impact of these laser applications and their modification effect on the glass network structure is still not fully understood. Depending on the type of glass, its properties and the intended application, the necessary processing parameters of the modifying laser vary considerably. In particular, the individual effects of the various process parameters during ultra-short pulsed laser irradiation and their relationship to the modification effects on the glass network still show some gaps. To study the modification effects of laser irradiation, the response of the glass network to a combination of external factors such as thermal influences, mechanical effects, and chemical changes must be characterized. This can be done non-destructively using common vibrational spectroscopy techniques such as Raman scattering, but also less common photoluminescence or Brillouin spectroscopy techniques as well as optical high-speed pump-probe observations. The locally observed response of the glass network to different laser irradiation parameters can then be compared with known influencing factors such as cooling rate or residual stress. From these fundamental properties of the glass network, models can be established that trace certain influences back to specific laser parameters during the irradiation process. The following work follows the described approach to investigate ultra-short picosecond pulsed laser modifications within the volume of fused silica and soda-lime silicate. This requires several sequential steps: First, for each observation technique, the response of the glass network to either a thermal (change in cooling rate) or mechanical (residual stress) influence must be characterized. Since none of the observation techniques are quantitative approaches, a set of calibrations with known thermal or mechanical influence is generated. Therefore, suitable observables are derived from the spectroscopic observations, which show a linear dependence on cooling rate and residual stress within the observed ranges and in good agreement with literature. It was found that the glass network rearrangements due to thermal and mechanical influence have similar effects on the spectroscopic observables. However, by intercorrelating the set of calibrations, response models are derived that can discriminate between the two influences, thermal and mechanical, and further analyze them quantitatively. Second, multiple samples are laser modified with different parameters in terms of pulse energy, repetition rate and speed of sample movement through the laser focus (feeding speed). The intended parameter matrix is based, on the one hand, on the physical limits, such as the pulse energy, which is modulated between the minimum, i.e. the ionization energy of the glass network, and the maximum, i.e. the vaporization of the sample. On the other hand, a reasonable range centered around the industry standard is chosen, such as for the feeding speed. Third, the response of the glass network for different laser parameter combinations is evaluated according to the developed thermal and mechanical influence models. In general, an increase in the thermal influence (cooling rate) is observed with increasing pulse energy, with a maximum in the center of the modifications and a radial decrease towards the surrounding unmodified glass. For the mechanical influence (residual stress), more differentiated effects are observed depending on the glass type and specific combinations of repetition rate and feeding speed (line energy), resulting in differently shaped modified zones. Last, the response of the glass network is related to the laser parameters of the picosecond pulsed laser modification. For fused silica, no direct effect of pulse energy on thermal or mechanical influence could be inferred. However, the thermal influence is highly dependent on the line energy, which affects the size and cooling rate distribution within the modified zone. The mechanical influence depends on the formation of gas bubbles within the modified zone, which affect the plasma formation and thus the modification process. For soda-lime silicates, a strong dependence of the thermal influence on the line energy is observed. In contrast to fused silica, the mechanical influence is only related to the thermal influence and not to any additional effects. Finally, due to the novelty of the described spectroscopic approach, the obtained models to estimate the thermal and mechanical influence are correlated with a comparable approach, i.e. high-speed pump-probe interferometry, to evaluate the reliability of the models. By relating an observed phase change of the probe beam to the temperature-dependent optical properties of the glass, a time-dependent temperature evolution is derived, which provides the in-situ thermal influence and the cooling rate. Despite the different approaches, spectroscopic and interferometric, remarkably similar thermal influences are observed, which conclude a good reliability and quality of both approaches.
Michael Bergler (Thu,) studied this question.