Efficient excitation of higher order modes in the plasmonic response of individual concave gold nanocubes

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.