Glass Cutting

Abstract

The state of the technology of ultrashort pulse laser applications such as glass cutting is dominated by direct ablation of a dielectric material, however the first installations used in-volume filament-like modifications. The variety of intriguing physical phenomena range from numerous nonlinear effects of ionisation to propagation of radiation strongly coupled to electron dynamics and include the formation of filaments. However, the potential as well as the challenge with respect to glass cutting is to tailor the combination of material composition and the laser radiation, which enables the suppression of unwanted damage and stable propagation of an optical and electronic channel; both might be called filaments. Ultrashort laser pulses interacting with the dielectric material generate free electrons dominantly via multiphoton ionisation (MPI) and cascade ionisation (CI). The dense plasma produced results in great changes of the refractive index and the surface reflectivity. When laser-induced plasma density reaches the well-known critical value $\rho_{crit}$ = $\omega^{2} \varepsilon_{0} m_{e} \,/\,e^{2}$ dependent on the laser frequency ω, the material gets highly absorbing. Laser ablation induced by relaxation of electron energy to the atoms takes place after the laser pulse has ceased. This ablation mechanism allows the use of the critical free-electron density $\rho_{crit}$ as the criterion $\rho_{ablation}$ = $\rho_{crit}$ for modelling ablation. The material near the ablated wall is characterised by a free electron density $\rho < \rho_{crit}$. Here indeed the material is not ablated but will be modified or damaged due to the energy released by high-density free-electrons. Once more, a threshold value $\rho_{damage}$ for the free electron density can be identified. As result, the shape of the ablation front as well as the morphology of a damaged region is described nearly quantitatively.