Interface and Bulk Standing Waves Drive the Coupling of Plasmonic Nanostar Antennas

Abstract

Finite element simulations of the optical behavior of gold nanostars in water reveal a new view of collective electron cloud oscillations, where localized surface plasmon resonances coexist with coherent delocalized interface waves, i.e., propagating surface plasmons. Gold nanostar spikes long enough to allow propagating polaritons and short enough to resonate with the spherical core serve as the substrate for the observed overlap between propagating modes and localized hot spots. Transverse plane plots reveal bulk polaritons coupled to surface ones. In light of these observations, we explore the mechanisms that drive plasmonic coupling in nanostars from the single spike level to multispiked and multiparticle systems.

Intracellular optical probing with gold nanostars

“…With shorter spikes, the field intensity at the tips of the spikes is in comparison smaller, in full agreement with previous reports. 10,12,13 There is also higher enhancement at the base between the longer spikes, close to the core ( Figure 4B). This supports the experimental data shown in Figure 3, where the higher signal is observed for the nanostars with longer tips.…”

Multipolar and bulk modes: fundamentals of single-particle plasmonics through the advances in electron and photon techniques

“…Apart from the impressive capabilities of both techniques in mapping the plasmons in 3D, we would like to discuss the opportunity to extend these techniques to probe the bulk of the particles. According to what we have recently theoretically proposed [9], it is possible in some plasmonic metals (Ag, Au, and Cu), and especially in gold, for the bulk of the metal to sustain image modes of the surface ones, given some morphological constraints. These bulk resonances have been previously shown in various systems [84,85], and it should be possible to map them in comparison to the surface ones, given the latest advances in EELS methods.…”

“…Combined with the energy resolution and time resolution limits achievable today (vide infra), it will be groundbreaking to measure the coupling between plasmonic and nonplasmonic phenomena and to understand how these interactions jointly resonate with LSPs. The concept we proposed [9] of a dual-cavity system, with one cavity being the bulk and the other cavity being the interface of a plasmonic particle, could be realized by employing the capabilities of the methods highlighted here. The development of a unifying numerical model addressing both the electromagnetic interaction from a standing wave approach and the local charge density changes from, for instance, a density functional theory approach, can be beneficial towards this goal.…”

“…The analytical toolbox for the study of plasmons has grown steadily and significantly since the 1980s; the focus of these studies has been however directed toward electronbased techniques. Driven by our extensive work on gold nanostars with high shape anisotropy [6][7][8][9], and motivated by the numerous fundamental questions that have arisen during these studies, we focus here on recapitulating the most cutting-edge investigative tools available for single-particle plasmonics, classifying them in two main classes: on one side those that allow the study of the local nature of surface-localized and bulk plasmon resonances (electron-based methods) [9], and on the other side the ones that reveal the collective nature of plasmonic resonances (photon-based methods). In this critical review, we bring together a decades-long progress in both classes, focusing on the most important results achieved with each technique and comparatively assessing the benefits and drawbacks of each of them.…”

“…Pumping and probing with photons (optical techniques), pumping and probing with electrons (electron energy loss spectroscopy [EELS]), or with combinations of both (cathodoluminescence [CL] and photoninduced near-field electron microscopy [PINEM]). We then focus on the description of a particular class of nanostructures, that is, those encompassing elongated, anisotropic features (e.g., gold nanostars), since our recent theoretical work appears to support the hypothesis that within them both propagating and localized plasmons can coexist [9]. In particular, these particles possess features (e.g., the spikes) that are long enough to allow the propagation of surface plasmon polaritons (SPPs) while being small enough as a whole to allow the formation of localized surface plasmons (LSPs).…”