CC BY
9LMI-I-5
Ultrashort laser heating of Al and W metals: learning from self-reflectivity and ablation threshold measurements
Thibault Genieys1, George Tsibidis2, Marc Sentis1, Olivier Uteza1
1- Aix-Marseille University, CNRS, LP3 UMR 7341, F-13288 Marseille, France 2- Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology (FORTH) N. Plastira 100, Vassilika Vouton, 70013, Heraklion, Crete, Greece
Main author's email address: uteza@lp3. univ-mrs. fr
Laser energy coupling and redistribution in materials depends on their electronic structure and of the excitation characteristics (intensity, wavelength) [1]. For instance, excitation of nonthermal electron population has been observed or postulated with consequences on optical transient properties and on energy redistribution and relaxation [2-5]. In order to explore those fundamental mechanisms in the energetic regime of ablation and to progress into knowledge of laser excitation of metals, we use Gaussian ultrashort pulses of different pulse duration (15 - 100 fs @ 800 nm central wavelength) and incident energy (0.1 - 30 Fth, with Fth: ablation threshold fluence). This range of laser parameters gives a convenient tool for probing laser heating of metals in strong non-equilibrium conditions and under various situations of energy coupling and relaxation in the material. It also provides valuable experimental data for evaluating ablation characteristics and reliability of modeling approach, based on Drude-Lorentz and two-temperature approach, in a range of pulse duration little investigated yet. The study is applied to posttransition (Al) and transition (W) metals with diverse electronic structure, from free-electron gas Distribution of State (DOS) structure to more complex DOS distribution with possibility of excitation of bound d-band transitions. To provide quantitative details for further interpretation, we carefully measure the reflectivity integrated over the pump pulse below and above ablation threshold (see figure 1). Using post-mortem diagnostic (confocal microscopy) and classical diameter-regression technique, we also determine the ablation threshold fluence as a function of ultrashort pulse duration. The results obtained are detailed and discussed in view of bringing useful information for understanding laser energy deposition and transformation of metals in the ultrashort irradiation regime [6,7]. In particular, we retrieve the effective electron collision rate as a function of excitation which is an important marker to benchmark laser - matter interaction.
Fig. 1. Evolution of reflectivity as a function of incident fluence and for different pulse durations (15 - 100 fs) for two cases: Al (left) and W (right). R0 indicates the value of unperturbed reflectivity (F << Fth). The red and grey lines indicate the energetic level above which nonlinear effects develop in our experiments. Note that Fth (dot line) was found identical for all pulse duration tested (15 - 100 fs).
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