Abstract
In this paper, we establish connections between the thresholds and mechanisms of the damage and white-light generation upon femtosecond laser irradiation of wide-bandgap transparent materials. On the example of Corning Willow glass, evolution of ablation craters, their quality, and white-light emission were studied experimentally for 130-fs, 800-nm laser pulses. The experimental results indicate co-existence of several ablation mechanisms which can be separated in time. Suppression of the phase explosion mechanism of ablation was revealed at the middle of the irradiation spots. At high laser fluences, air ionization was found to strongly influence ablation rate and quality and the main mechanisms of the influence are analysed. To gain insight into the processes triggered by laser radiation in glass, numerical simulations have been performed with accounting for the balance of laser energy absorption and its distribution/redistribution in the sample, including bremsstrahlung emission from excited free-electron plasma. The simulations have shown an insignificant role of avalanche ionization at such short durations of laser pulses while pointing to high average energy of electrons up to several dozens of eV. At multi-pulse ablation regimes, improvement of crater quality was found as compared to single/few pulses.
Highlights
Years, several groups made attempts to connect free-electron plasma lightning upon laser excitation with the damage threshold[12,13,14]
Varel et al.[14] found somewhat higher FOB compared to the damage threshold at τL in the range of 500 fs–4.5 ps while at shorter pulses they measured the same values for FOB and Fth
We explore the “cold ablation” regime for another glass material, Corning Willow glass[22]
Summary
Years, several groups made attempts to connect free-electron plasma lightning upon laser excitation with the damage threshold[12,13,14]. Varel et al.[14] found somewhat higher FOB compared to the damage threshold at τL in the range of 500 fs–4.5 ps while at shorter pulses they measured the same values for FOB and Fth. Note that in ref. Shorter pulses (20 fs, Fig. 1) yield in better crater quality, culminating in the perfect crater produced by 5-fs laser pulse As for such pulses FOB values have not been measured, it can be speculated that the generated free-electron population has no time to become hot enough for transferring much energy to the lattice. Crater formation happens in a “cold ablation” regime without generation of high pressure gradients damaging the crater edges. Evidences for coexistence of several mechanisms of ablation, including air plasma effects, are discussed
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