CC BY
14HiLASE-I-9
Modeling of femtosecond laser induced out of equilibrium electron transport in metals
S. Coudert1, G. Duchateau1, P. Lalanne2, S. Dilhaire3 1CELIA- Universite de Bordeaux, ifcia, Talence, France 2LP2N-Universite de Bordeaux, plasmonics, Talence, France 3LOMA - Universite de Bordeaux, Laser, Talence, France
Plasmonic devices enables to manipulate light at extremely narrow space scales, down to tenth of the optical free wavelength of light, and even less. Applications of such devices include the enhanced Raman spectroscopy, sensing, and photonic nanoswitches. These developments lead to the emergence of new field of investigation: managing hot electrons production and transport which promise further significant advantages as unprecedent efficiency for water photocatalysis and photovoltaic devices for example. It is thus crucial to understand the field induced electron dynamics at short time and space scales.
Such conditions are fulfilled by irradiating a nanometric film by femtosecond laser pulses. In that case, the heated volume can be significantly smaller than the electron mean free path, and the averaged electronic relaxation time is of the order of the pulse duration. More precisely, for a laser wavelength in the visible range, the photon energy is much larger than the electron temperature, leading to an electron energy distribution far from the equilibrated Fermi-Dirac statistics on the shortest timescales. The so-called hot electrons then may lead to an energy transport departing from the standard diffusive (Fourier) behavior.
To model the laser induced out of equilibrium electron transport in metals, we have developed an approach relying on the resolution of the kinetic Boltzmann equation which provides the evolution of the electron energy distribution in time and in one dimension of space. This approach is based on a decomposition of the distribution function on a Legendre polynomial basis set, making the numerical scheme efficient. The results show that the out of equilibrium energy distribution affects the energy transport, highlighting a non-trivial behavior due to the simultaneous contribution of both diffusive and ballistic electronic transport. The numerical results are compared to data obtained with a pump-probe thermo-reflectance setup. The results are in a good agreement, validating the present theoretical development.
