Surface plasmon coupling revisited with electron energy loss spectroscopy

Abstract

In the last fifteen years or so, a significant amount of research activities took place in the field of metal nanoparticle plasmonics probed by fast electron beam. Local electron probe techniques like electron energy‐loss spectroscopy (EELS) and cathodoluminescence (CL) have advanced very fast during this time. Both these techniques helped us to gain considerable insight into the plasmonic properties of metallic nanostructures [1,2]. One of the many interesting nanostructures are the metal nanoparticle dimers. They can confine a huge amount of field in the gap. As the modes can be tuned precisely by changing the separation between the nanoparticles, they bear the promise to be used as sensors. It is well known that as the two individual particles approach each other, bonding and antibonding modes appear. Bonding and antibonding corresponds to in‐phase and out‐of‐phase interaction of the individual particle plasmons. Analogous to the molecular orbital theory, the bonding mode appears at lower energy and the antibonding mode at higher energy with respect to the individual particle plasmon mode [3]. With electron beam we can probe the precise location of the particle dimer to see which mode is excited at what location. Even though there is a wealth of literature on the plasmonic modes of individual metallic nanostructure, a systematic experimental study of coupled plasmons is deficient. The central idea of the current work is to develop a deeper understanding on the coupling of the nanoparticle plasmons when they are brought close to each other (a few nanometers). For this purpose, we have chosen a cross shaped metal nanoparticle (Figure 1). We choose this structure for easy tuning of plasmon modes along the length of the rods by changing the rod length and because it is easy to model numerically. To have a detailed idea on the underlying physics, we start with the basic building block of the cross, i.e. a single nano rod. Then we increase the complexity of the structure by adding two rods perpendicularly to make a cross, and finally bringing two crosses close together to make a dimer. We make EELS on all of these structures and see the evolution of the modes. In this way, we will be able to explore the exact formation of different modes and the coupling between them. To realize our idea, we have performed electron beam lithography to make silver nanostructures on Silicon Nitride substrate. EELS experiments are performed using a scanning transmission electron microscope (STEM) fitted with homemade EELS detection system [4]. To gain insight about the plasmonic modes, we perform 3D boundary element method (BEM) simulations [5] and compare with the experimental data. A representative EELS spectrum and the 2D plasmon maps has been shown in Figure 2. In the conference, we will present and discuss our experimental and simulation results. This will provide new insight into the physics of plasmon coupling.