How dark are dark plasmon modes ‐ a correlative EELS and CL study on lithographed silver nanodisks

Plasmonic nanostructures enable the concentration of light to the deep subwavelength regime and, thus, are the topic of intense fundamental and application oriented research. Nanoparticles acting as optical antennas are well known for their nanoscale mode volumes, due to the excitation of localized plasmon modes. In this context electron energy‐loss spectroscopy (EELS) in a transmission electron microscope (TEM) became a powerful technique as it enables to map the full modal spectrum of plasmon eigenmodes with unprecedented high spatial resolution [1–3]. Beside EELS, Cathodoluminescence (CL) has also recently been used to gain information about the optical response taking advantage of the same high spatial resolution in a TEM [4]. While it is stated that EELS is linked to the full photonic local density of states (LDOS), the CL signal is related to the radiative LDOS [5]. In this work we present a combined EELS/CL study of plasmon eigenmodes on silver nanodisks, using fast electrons in a TEM. In particular we compare the differences of the EELS and CL response using experimental and simulated data. Precise variation of the disk size is achieved by means of electron beam lithography (Figure 1a), enabling a comprehensive study of plasmon excitations on silver nanodisks. From theoretical considerations it is known, that for certain particle geometries (and therefore for specific surface charge distributions) there exist so called dark modes, which are “invisible” to photons but “visible” to electrons, and therefore can be measured with EELS but not with light [6]. Here we discuss how dark these dark modes are comparing EELS and CL (figure 1b+c). In particular, radial breathing modes (C in figure 1b+c) were predicted to be dark modes [6], although we will show how comparison between EELS and CL can mitigate this statement. Additionally, limitations for the theoretical predictions will be discussed, when the particle size is increased and therefore retardation effects become more important. In this case we show that dark modes are getting brighter. Furthermore symmetry breaking by the excitation source itself, a focused fast electron beam, will be discussed.