Femtosecond-laser ablation dynamics of dielectrics: basics and applications for thin films

Abstract

Laser ablation of dielectrics by ultrashort laser pulses is reviewed. The basic interaction between ultrashort light pulses and the dielectric material is described, and different approaches to the modeling of the femtosecond ablation dynamics are reviewed. Material excitation by ultrashort laser pulses is induced by a combination of strong-field excitation (multi-photon and tunnel excitation), collisional excitation (potentially leading to an avalanche process), and absorption in the plasma consisting of the electrons excited to the conduction band. It is discussed how these excitation processes can be described by various rate-equation models in combination with different descriptions of the excited electrons. The optical properties of the highly excited dielectric undergo a rapid change during the laser pulse, which must be included in a detailed modeling of the excitations. The material ejected from the dielectric following the femtosecond-laser excitation can potentially be used for thin-film deposition. The deposition rate is typically much smaller than that for nanosecond lasers, but film production by femtosecond lasers does possess several attractive features. First, the strong-field excitation makes it possible to produce films of materials that are transparent to the laser light. Second, the highly localized excitation reduces the emission of larger material particulates. Third, lasers with ultrashort pulses are shown to be particularly useful tools for the production of nanocluster films. The important question of the film stoichiometry relative to that of the target will be thoroughly discussed in relation to the films reported in the literature.

Femtosecond ultraviolet laser ablation of silver and comparison with nanosecond ablation

“…However, it can be noted that since the ns pulse is much longer than the electron-phonon relaxation time, which determines the time for heat conduction in the fs case, the heat diffusion depth is smaller in the fs case. In fs ablation most of the ablated material is removed by nanofragmentation of superheated material, 15,17,34,35 while in the ns ablation material removal is limited by surface evaporation from the laser heated surface. In ns ablation plume material evolved during the laser pulse absorbs the laser light leading to a reduction of the energy coupled to the target.…”

“…However, one should note that the strong-field excitation induced by fs-lasers makes it possible to ablate materials using longer wavelengths where the materials are transparent to the laser light. 15 Plume studies of fs laser ablation are often based on timeand space-resolved spectroscopy from which it can be difficult to extract the velocity and angular distributions of the ablated particles. Spectroscopic measurements of emission from excited atomic and ionic species may not give a representative picture of all the ablated material.…”

“…In particular, it is known that in fs irradiation a significant part of the ablation occurs via emission of nanoparticles (NPs), which leave the target at lower velocity than atoms or ions and emit a continuous light spectrum. [15][16][17] There are some reports of ion energy distribution measurements using Langmuir probes 7 or ion energy analyzers, 2,4,7,11 but these measurements have been made in a direction close to the normal of the target surface, and do not give information about the shape of the ablation plume. Donnelly et al 8,9 have used ion probes to measure the plume shape and the ion energy distribution in fs ablation of nickel.…”

Front-surface fabrication of moderate aspect ratio micro-channels in fused silica by single picosecond Gaussian–Bessel laser pulse

“…Under intense irradiation of short pulse, dielectrics first experience moderate excitation via nonlinear photoionization; then, the generated conduction band electrons undergo rapid multiplication by repeated process of laser heating (inverse Bremsstrahlung) and impact ionization. As the free electron density goes above a critical value, plasma gradually turns highly absorbing yielding to optical breakdown and further to material ablation when the deposited energy is sufficiently high [4]. In surface ablation regime, for Gaussian beam with pulse duration especially below 100 fs, a plasma shielding effect (solid density plasma mirror) may take place transiently, which contributes to self-limiting the ablation depth to hundreds of nanometers at most [5].…”

Cold ablation driven by localized forces in alkali halides

“…If E g largely exceeds the photon energy (hn), absorption is frequently dominated by highly nonlinear processes that lead to ionization and avalanche effects [8][9][10] . The investigation of elementary electron-hole pairs (excitons) in ionic dielectric materials, such as alkali halides, has therefore required the use of hv in the ultraviolet (UV) range to mitigate the aforementioned competing channels.…”