Plasmonic Coupling of Multipolar Edge Modes and the Formation of Gap Modes

The coupling of plasmonic resonances is an effective tool to tailor the optical properties of nanostructures. However, the coupling of higher order plasmonic resonances has not received much attention, with most studies focusing on the interaction of dipolar modes. Taking advantage of the high spatial and energy resolution of modern scanning transmission electron microscopes equipped with electron energy loss spectroscopy, we analyze the coupling of edge modes in planar nanostructures with emphasis on the interaction of high order modes and the formation of gap modes. We show that coupling of edge modes can be understood by a simple and intuitive scheme, with three regimes: First, a strong coupling through the edge of the structure resulting in bonding and antibonding gap edge modes; second, coupling through the corners of the structures resulting in bonding and antibonding corner edge modes; and a third behavior where the edge modes do not couple and behave independently of the rest of the structure. The formation of gap modes through the coupling of edge modes is analyzed and compared to the modes found in planar slot waveguides, finding that the properties of the symmetric and asymmetric modes on slot waveguides are equivalent to the antibonding and bonding gap edge modes, respectively. Our experimental and numerical analysis of the plasmon resonances in nanosquares and waveguides shows that our scheme of plasmonic coupling of edge modes can be generalized to other planar structures with straight edges and might inspire the design of more complex planar plasmonic devices based on the coupling of edge modes.