Inelastic Light Scattering by Multiple Vibrational Modes in Individual Gold Nanodimers

Abstract

To be Raman-active (or, more generally, detectable using optical spectroscopy techniques), a vibrational mode of a nanosystem has to modulate its optical response. For small, isolated nanospheres, this is the case for only two categories of vibrational modes, namely, quadrupolar and radial ones. However, assembling nanospheres as dimers makes additional modes Raman-active, as previously demonstrated by the detection in the ultralow frequency range of a hybridized quasi-translation mode in previous measurements on single and ensembles of gold nanosphere dimers. In the present work, we use our recently developed single-particle Raman spectroscopy setup to compare inelastic light scattering by single isolated and dimerized gold nanospheres in an extended frequency range (0–40 GHz). The Raman spectra acquired on isolated nanoparticles present a single peak associated with their fundamental quadrupolar mode, consistently with previous ensemble measurements. In contrast, the spectra measured on dimers are richer and display a number of peaks increasing with decreasing interparticle distance, with all l = 2–8 Lamb modes detected in the quasi-contact case. These observations are rationalized using a recently developed classical model of inelastic light scattering by nanospheres. Importantly, our modeling approach takes into account the real electric field within the nanoparticles (computed using standard or generalized Mie theories) instead of relying on the frequently used Born and quasistatic approximations. This ingredient appears decisive for reaching a qualitative understanding of the measured spectra, explaining in particular the dominance of the l = 2 quadrupolar mode for isolated spheres and the increasing contribution of higher-order modes for increasing electromagnetic interactions in nanosphere dimers.