Ultrafast destructuring of laser-irradiated tungsten: Thermal or nonthermal process
Résumé
The time needed for metals to respond structurally to electronic excitation is usually considered to be set by
picosecond-long electron-phonon coupling, hence limiting the rapid achievement of structural phase changes. Via
time-resolved ellipsometry, we show that fs laser excitation of tungsten determines unexpectedly fast optical and
structural transformations, almost on the time scale of the laser pulse, with sub-ps destructuring of matter. If at
low energies, below the damage threshold, Fermi redistribution within the d-band pseudogap populates localized
states that screen ions, in ablative ranges, at electronic energies above 4 eV, a charge deficit appears on bonding
orbitals generating bond-softening and ionic repulsion in spite of the metallic environment. Along pressure
gradients, this rapidly destabilizes the structure. Sub-ps ablation occurs, challenging thermally driven scenarios
of phonon activation. First-principles and hydrodynamic models show that ablation interrelates nonclassical
charge distortions, electronic stress, and classical electron-phonon dynamics, with the coexistence of thermal and
nonthermal effects on scales believed to be prohibitive for the former.