29 Mai – Thesis defense - Stéphane Coudert

13 h30 Council room - Laboratory CELIA (Talence)

Modelling plasmon-electron-phonon dynamics in confining metallic nano-structures.

Recent developments of metallic nanostructures have led to light focusing towards nanometric size scale. Applications have emerged from these new structures including nanophotonics, photocatalysis, cancer therapy, and photovoltaïc devices. The efficiency of the photo-excitation processes required for such applications is quite low (close to 1%4). A better understanding of the processes involved in such systems is thus required, from both theoretical and experimental points of view to improve the laser-matter coupling. At LOMA laboratory, Stefan Dilhaire team develops new methods and experimental set-ups based on pump probe experiments to study such devices. This enables to get information on electronic temperature on the optical spatial scale and 100 fs timescale. Set-ups and protocols elaborated in LOMA gave a demonstration of adiabatic plasmon focusing5 and hot carriers generation in confinement area.
In the present work, we present a theoretical study aiming at understanding ultra-fast generation, relaxation and transport processes of hot carriers in metals. We have developed a numerical code solving the Boltzmann equation for both phonons and electrons which enables to model these ultrafast out of equilibrium processes. The importance of Umklapp processes in absorption mechanisms for electron-electron and electron-phonon scattering is shown. By using the Rosei model, experimental observable are extracted from microscopic calculations as the thermoreflectance signal. Numerical results are compared to experimental data. In general a good agreement is obtained. By coupling the present approach to experimental data, absolute thermoreflectance measurements can be carried out.
Finally, by decomposing the electron distribution function over a Legendre polynomials basis set, the Boltzmann equation for electrons with one spatial dimensions and three dimensions in momentum space is numerically solved. This enables to model ultrafast transport from ballistic spatial (~10 nm) and temporal time scale (~10 fs), beyond Fourier transport where the temperature is no longer defined, to macrocopic scales. The importance of describing the ultrafast transport of hot carriers is highlighted. The numerical predictions have been compared successfully with experimental results obtained in LOMA and in the litterature.

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