The effect of geometric anisotropy on the heating of gold nanoparticles under a femtosecond pulse

Gold nanospheroids and cylinders exhibit two surface plasmon resonances, transverse (T mode) and longitudinal (L mode), in the infrared band. In accordance with the dipolar plasmonic response of spheroids, an increase in the aspect ratio (η) causes the two resonances to split further apart spectrally, with the T mode undergoing a blue shift while the L mode moves towards longer wavelengths.
We present in this work a numerical approach to studying the effect of anisotropy on the heating of prolate gold nanoparticles (GNs) interacting with a Ti–Sapphire laser oscillating at a wavelength of 800 nm. The GNs are cooled in water, and the heat transfer from the particles to the water is assumed to occur without mass transfer. The effect of η on the GN temperatures is investigated under 100-femtosecond laser pulse irradiation and 1 J/m2 fluence, and several values of η are considered: 2, 3, 4, and 5. First, the extinction cross-section of randomly oriented GNs was computed in the quasi-static limit using the Rayleigh–Gans formulae. Second, the ultrafast dynamics of the heat inside the GNs and at the GN/water interface were modeled through two temperature equations and Fourier’s law, respectively. The numerical simulation was carried out by a code written in C++ language. It was found that the maximum temperature at the GN/water interface increases as a function of η but does not exceed a certain value corresponding to η= 4.5, and this is independent of the size of the GNs.