Femtosecond-laser induced ablation of silicate glasses and the intrinsic dissociation energy

Abstract

The relation between ablation threshold fluence upon femtosecond laser pulse irradiation and the average dissociation energy density of silicate based multicomponent glass is studied. A simple model based on multiphoton absorption quantifies the absorbed energy density at the ablation threshold fluence. This energy density is compared to a calculated energy density which is necessary to decompose the glass compound into its atomic constituents. The results confirm that this energy density is a crucial intrinsic material parameter for the description of the femtosecond laser ablation threshold fluence of dielectrics

Micro additive manufacturing using ultra short laser pulses

“…In metals, as already briefly explained in the introduction, the laser pulse is absorbed by an abundance of free electrons. In dielectric materials, nonlinear processes such as multi-photon ionization lead to the transition of electrons from the valence to the conduction band, which enables the direct interaction of the laser with the generated free charge carriers in the conduction band [41,42]. Various modelling approaches for the rate of the generation of free charge carriers by strong electric field ionization (SFI) are based on the so-called Keldysh theory, which calculates an ionization rate depending on the band gap and the reduced effective mass of the electron-hole pairs [42,43].…”

“…When the laser pulse reaches higher intensities, the electric field starts to distort the structure of the material's electric bands such that the term tunnel ionization describes the physical picture of the rate of the carrier generation [42].…”

“…For pulse durations longer than 10 fs, the nonlinearly excited electrons can be excited further by phonon-mediated linear absorption, denoted as free-carrier absorption, until their kinetic energy is sufficient to excite other bound electrons from the valence band into the conduction band [42,44]. Due to its rapidly expanding nature, this process is often referred to as avalanche ionization [44].…”

“…This means that it scales with the number of simultaneously absorbed photons needed to overcome the band gap energy to reach the conduction band [42,45]. The definition of an ablation threshold for laser-processing of dielectrics turned out to be a very controversial issue.…”

Commentary: Comments regarding Becker et al. (Analytica Chimica Acta, 835, 2014, 1–18)

“…As is clear from the paper in question, experimental parameters for all imaging experiments were identical, regardless of treatment group. Additionally, the energy fluence of 0.8 J cm À2 is well below the threshold required to ablate silicate glass (minimum threshold ¼ 2.4 J cm À2 [8]). Considering the authors' reputation in laser ablation, it is surprising that they included this clearly incorrect statement.…”