Ultrafast Surface Plasmonic Switch in Non-Plasmonic Metals

We demonstrate that ultrafast carrier excitation can drastically affect electronic structures and induce brief surface plasmonic response in non-plasmonic metals, potentially creating a plasmonic switch. Using first-principles molecular dynamics and Kubo-Greenwood formalism for laser-excited tungsten we show that carrier heating mobilizes d electrons into collective inter and intraband transitions leading to a sign flip in the imaginary optical conductivity, activating plasmonic properties for the initial non-plasmonic phase. The drive for the optical evolution can be visualized as an increasingly damped quasi-resonance at visible frequencies for pumping carriers across a chemical potential located in a d-band pseudo-gap with energy-dependent degree of occupation. The subsequent evolution of optical indices for the excited material is confirmed by time-resolved ultrafast ellipsometry. The large optical tunability extends the existence spectral domain of surface plasmons in ranges typically claimed in laser self-organized nanostructuring. Non-equilibrium heating is thus a strong factor for engineering optical control of evanescent excitation waves, particularly important in laser nanostructuring strategies

Ultrafast electron dynamics and orbital-dependent thermalization in photoexcited metals

International audienceWe use a time-dependent density functional theory (TDDFT) to analyze nonequilibrium dynamics of laser-excited electrons in transition metals (Ni, Cr, Cu) and shed light on the ultrafast thermalization process from a microscopical point of view. As a first result, after instant increase of the electron temperature up to 50 000 K, we observe that the dynamics of electron density of states is faster than a laser subcycle, on the attosecond timescale. This is related to an ultrafast rearrangement of excited electrons in space to accommodate strong changes in ion screening. Secondly, we show that the electron thermalization dynamics strongly depends on the electronic structure of a given metal. Excited by a 7-fs laser pulse, d-block transition metals exhibit two subsystems of electrons, each one achieving its own temperature. Due to a higher localization, electrons from d block stay cold, while excited delocalized sp electrons rapidly reach a high temperature. Electrons of each band of energy are mutually thermalized within the time of the laser pulse. Much more time is however needed to reach equilibrium of the whole electronic system. These results redraw the validity limits of current two-temperature models during the laser irradiation time, potentially impacting further material reaction

Mechanisms of Ultrashort Laser-Induced Fragmentation of Metal Nanoparticles in Liquids: Numerical Insights.

International audienceFemtosecond laser-induced fragmentation of gold nanoparticles in water is examined. Numerical calculations are performed to elucidate the roles of thermal and electrostatic effects due to electron emission in the corresponding decomposition mechanisms. The obtained results demonstrate that particles smaller than a well-defined size R* melt at smaller fluences than the ones required for electrostatic decomposition. The limiting size depends on the absorption coefficient calculated as a function of particle radius, which depends on laser wavelength and on the optical properties of the particle and the background environment. To decompose particles with radii larger than R*, a considerable increase in laser fluence is required. In this case, thermomechanical effects become prevailing. Both the calculated range of particle sizes to be decomposed by the considered laser pulses and the corresponding fluences agree with several experimental measurements