Probing ultrafast heat transfer mechanisms in plasmonic gold nanostructures: FEM analysis of core–shell configurations under femtosecond laser irradiation

Abstract

This study presents a comprehensive numerical investigation of the photothermal response of core–shell gold nanoshell (CGNS) and gold nanorod (CGNR) under femtosecond (fs) laser pulse irradiation. Using the two-temperature model (TTM) integrated with finite element modeling in COMSOL Multiphysics, we simulated the optical and thermal dynamics of these nanostructures. A key innovation in our approach is incorporating the temperature dependencies of electron heat capacity and electron–phonon coupling, allowing us to capture the non-linear thermal response at elevated electron temperatures. Our analysis showed that, while lattice temperatures increased linearly with laser fluence, electron temperatures exhibited a more complex non-linear trend, emphasizing the need for advanced modeling in high-fluence regimes. We evaluated how variations in key parameters, including aspect ratio, shell thickness, pulse duration, and refractive index, influence the optical and thermal properties of the nanostructures. Results revealed that CGNRs with higher aspect ratios exhibited significant red-shifts into the near-infrared (NIR) region, making them ideal for deep-tissue imaging and photothermal therapy (PTT), while thicker CGNS nanostructures demonstrated blue-shifts with reduced energy absorption. Shorter pulse durations led to higher peak electron temperatures, with CGNRs displaying faster heat dissipation than CGNS due to their elongated geometry. Furthermore, CGNRs demonstrated enhanced sensitivity to changes in the refractive index of the surrounding medium, making them particularly suited for sensing applications in the NIR-II region. This study provides key insights into optimizing core–shell nanostructures for advanced PTT and sensing technologies, laying the groundwork for the development of tailored nanomaterials for biomedical applications.