Efficient Excitation of Higher Order Modes in the Plasmonic Response of Individual Concave Gold Nanocubes

Abstract

Recently, concave nanocube (CNC) shaped metal nanoparticles (MNPs) with high index facets have drawn special attention due to their high chemical activity and large electromagnetic (EM) field enhancements, making them good candidates for multifunctional platforms. However, most of the previously published works focused on the plasmonic properties of silver simple nanocubes of smaller dimension, i.e., within the quasi-static limit, hardly supporting efficient excitation of high-order plasmonic modes. Site-selective electron beam excitation of individual Au CNC particles gives rise to simultaneous excitation of edge and corner localized surface plasmon (LSP) modes. We show that spatial variation of the radiative modes is strongly localized at the corners and extended along the edges of the top surface of the CNCs. Extensive finite-difference time-domain (FDTD) numerical analysis reveals that the substrate-induced plasmon hybridization leads to the activation of corner octupolar and corner quadrupolar LSP modes, in agreement with the cathodoluminescence (CL) measurements. Remarkably, the strength of the hybridization is shown to depend on the CNC size. Furthermore, we show that the edge quadrupolar mode becomes prominent with increasing concaveness, thus opening up a new way of engineering the LSP modes.

Study on Electron-Induced Surface Plasmon Coupling with Quantum Well Using a Perturbation Method

“…At Point A in Figure 2a, the e-beam can hardly penetrate through the Ag NP with the diameter of 200 nm [30]. The strong CL emissions mostly come from the LSP excited QW radiators under the Ag NP because the direct emission from the LSP is very weak [39]. Furthermore, it can be evidenced by the CL emission at Point A in Figure 2b, which is two orders less than that from QWs in Figure 2a.…”

“…The spacing between the q-dipole and the bottom surface of the Ag NP is set to be 20 nm, which is consistent with the experimental condition. In FDTD simulations, the e-beam is usually modeled as a series of point dipoles with phase delay related to the e-beam velocity [35,39,41,42]. Since the e-beam velocity only modifies the phase delay through a cosine function, this allows one to split these dipoles into smaller sub-simulations.…”

Effect of dipole polarization orientation on surface plasmon coupling with green emitting quantum wells by cathodoluminescence

“…As the e-beam impinges on the Ag NP, the LSP in the Ag NP is strongly excited by high-energy e-beam 21 rather than by the electron-hole pair in QWs. Since the excitation by the ebeam is broadband optical sensitive and can induce both the LSP dipole and high order modes, [21][22][23] the evanescent eld is a superposed result of several modes and very strong. The dipoles with different polarization orientation and position coupled to LSP are simulated by Kuo et al 20 When the polarization orientation difference of the two dipoles varied from 10 to 180 , the radiated power enhancement changes smoothly.…”

“…On the other hand, according to the recent reports, cathodoluminescence (CL) has been performed on the Ag NPs in a scanning electron microscope (SEM) setup. [21][22][23] With the combination of the ultrahigh spatial resolution of the electron beam (e-beam) and broadband optical sensitivity, optical process and nanometer-sized features of the Ag NPs can be resolved. The octupolar LSP mode can be excited via highenergy e-beam.…”

“…The octupolar LSP mode can be excited via highenergy e-beam. 22 There are also some reports on the Ag NPs-QW structure excited by e-beam. 24,25 However, they did not concern the SP modes induced by e-beam and its effect on the QWs under the metallic NPs.…”