Probing Bright and Dark Surface-Plasmon Modes in Individual and Coupled Noble Metal Nanoparticles Using an Electron Beam

Abstract

We examine the efficacy of Dark-mode plasmonics as a platform for enhanced magneto-optics. Dark-mode of a small particle consists of two co-existing equal-intensity and mutually opposing dipolar excitations. Each of these two opposing dipoles may even resonate at or near the dark-mode frequency , but the net dipole moment vanishes due to the mutual cancelation between the opposing dipoles. We show that application of external magnetic bias may alleviate the intense destructive interference. Furthermore, under external magnetic bias the opposing dark-resonances of a plasmonic particle shift in opposite directions and create a region of extremely sensitive Faraday rotation. We show that the magnetized dark resonance in lossless Ag-like particle may provide more than 20 degrees rotation under magnetic fields of the order of 1-2 Tesla, exhibiting magneto-plasmonic activity that is 2-3 orders of magnitude larger than that observed in conventional plasmonic particle of the same material. 1 Dark modes of an open optical structure can be described as states of excitations that incorporate mutually opposing local dipoles whose far-fields interfere destructively. The net dipolar excitation then vanishes, resulting in a significant reduction of the far-field radiation, and consequently a reduction of the associated radiation damping and bandwidth. 1-8 This, in turn, may trap optical fields in a structure that is inherently coupled to a continuum. More formally, dark modes can be viewed as manifestations of discrete eigenvalues embedded within the continuous spectrum of the associated non-compact scattering operator. Dark modes were suggested as candidates for electromagnetic energy storage, enhanced biological and chemical sensors , and nanoscale waveguides. These modes can be supported by simple structures such as nano-dimers (see, e.g. the "anti-bonding" plasmons in Ref. [ 1]), trimers, 6 clustered nano-rods 3 and spheres. 7,8 In a seemingly unrelated research endeavor, nonreciprocal magneto-optics and its implementation for one-way waveguides, optical isolators and circulators, and Faraday rotators, have been under intensive study. 9-14 Currently the major drawback of nonreciprocal magneto-optics is the requirement for strong magnetic bias B 0. Efforts to reduce B 0 for various applications (e.g. Faraday polarization rotation) in plasmonic structures can be found, e.g. in. 15-17 The work in 15 reports on an experimental evidence for a 2-3 fold enhancement of magneto-optical activity in coated nano-particles. The efforts in 16,17 are limited to graphene metasurfaces. Here we study the effect of bias magnetization on plasmonic particle dark modes, and explore its potential applications as a new platform for non-reciprocal optics. As a simple and physically transparent test-case, we consider the core-shell spherical particle shown in Fig. 1, made of two plasmonic materials with close, but not identical , plasma frequencies. The structure is excited by a linearly polarized local field E L (r) = ˆ zE L e iky. When properly d...

Plasmon Excitation of Atomic Clusters and Their Assembly by Electrons and Photons

“…Figure 7A is extinction spectra using DDA model to calculate the interaction between incident light beam and two particle system. [26,27,28] When the spacing diameter ratio is 1, similar to one particle spectrum, the peak is at 3.43eV. When the spacing/diameter ratio gets smaller, the low energy mode gradually appears, and an energy mode less than 3eV arises when the ratio reaches 0.05, as well as a high order mode appears at 3.2eV and 3.4eV, similar to the black curve in Figure 6A.…”

Plasmon Spectroscopy and Imaging of Individual Gold Nanodecahedra: A Combined Optical Microscopy, Cathodoluminescence, and Electron Energy-Loss Spectroscopy Study

“…It must be stressed that both external far-field light and electron beams can excite bright plasmon modes when these have sizable dipole moments, but only electrons can excite higher-order dark modes, and weak-dipole modes (gray modes), which couple weakly to external light, but are efficiently reacting to the evanescent electromagnetic field of the electron beams. 21,22,31,32 The requirement for electron transparent samples typically below 150 nm thickness and an expensive electron detection system are the main disadvantages of EELS. Alternatively, cathodoluminescence (CL), combined with scanning electron microscopy (SEM), circumvents these disadvantages and has been successfully used to spectrally resolve and spatially image SPs on metallic nanostructures.…”

Coexistence of classical and quantum plasmonics in large plasmonic structures with subnanometer gaps

“…10 The choice of EELS for characterization of plasmonic nanostructures has become increasingly popular, due to the subnanometer spatial resolution of this technique. 11,12 Depending on the geometry of the plasmonic structure and the position of the electron beam, EELS can probe bright and/or dark dipole modes and higher order modes. 13 Therefore, the same optically active dipolar resonances, which dominate the optical response of plasmonic structures, can be excited and studied by the electron beam in EELS.…”