Improved two-temperature modeling of ultrafast thermal and optical phenomena in continuous and nanostructured metal films

Abstract

In this work, a pump-probe experiment is used to study the ultrafast dynamics of heat transfer in thin gold films and gold nanostructures on glass substrates, following local heating by ultrashort laser pulses. Full spectrotemporal differential reflectivity and transmission maps were obtained for different film thicknesses (30, 50, 80, 150, and 200 nm) and different laser fluences $(0.38 \mathrm{to} 9.5\phantom{\rule{0.16em}{0ex}}{\mathrm{Jm}}^{\ensuremath{-}2})$. For arrays of gold nanorods, the two orthogonal probe polarizations were also acquired. We propose an improved model for these phenomena based on a modified two-temperature model that integrates thermal conduction and the three-dimensional finite element method model to link the spatiotemporal temperature maps to the spectrotemporal optical response maps. The impact of an underlying titanium adhesion layer is reported. Excellent agreement between numerical and experimental data for both the gold films and the nanostructures is shown.