Mode Imaging and Selection in Strongly Coupled Nanoantennas

Abstract

The number of eigenmodes in plasmonic nanostructures increases with complexity due to mode hybridization, raising the need for efficient mode characterization and selection. Here we experimentally demonstrate direct imaging and selective excitation of the "bonding" and "antibonding" plasmon mode in symmetric dipole nanoantennas using confocal two-photon photoluminescence mapping. Excitation of a high-qualityfactor antibonding resonance manifests itself as a two-lobed pattern instead of the single spot observed for the broad "bonding" resonance, in accordance with numerical simulations. The two-lobed pattern is observed due to the fact that excitation of the antibonding mode is forbidden for symmetric excitation at the feedgap, while concomitantly the mode energy splitting is large enough to suppress excitation of the "bonding" mode. The controlled excitation of modes in strongly coupled plasmonic nanostructures is mandatory for efficient sensors, in coherent control as well as for implementing well-defined functionalities in complex plasmonic devices. IntroductionPlasmonic nanostructures consisting of particular arrangements of closely spaced resonant particles are of great interest since they offer a variety of eigenmodes that evolve due to mode hybridization [1]. Characterization and well-defined excitation of such eigenmodes is important in order to achieve welldefined functionality in devices [2,3] and to successfully apply techniques of coherent control [4][5][6][7]. Nanoantennas consisting of two strongly coupled particles can serve as a model system to study the impact of mode selectivity [6,8,9]. Upon illumination nanoantennas confine and enhance optical fields [10,11] and can therefore be used to tailor the interaction of light with nanomatter [12]. Various applications of nanoantennas have been proposed and experimentally demonstrated, including enhanced single-emitter fluorescence [13][14][15], enhanced Raman scattering [16,17], near-field polarization engineering [18][19][20], high-harmonic generation [21,22], as well as applications in integrated optical nanocircuitry [23,24]. The longitudinal resonances of a symmetric dipole antenna can be understood in terms of hybridization of the longitudinal resonances of individual antenna arms, caused by the coupling over the narrow feedgap [25,26]. Such coupling causes a mode splitting into a lower-energy "bonding" mode and a higher-energy "antibonding" mode, respectively (Fig. 1). Limited by the uncertainty of conventional nanofabrication, it is often difficult to fabricate antenna arrays with a reproducible gap size below 20 nm, which is necessary to achieve significant energy splitting between the bonding and the antibonding mode. As a result, the existence of the antibonding antenna resonance has hardly been considered, although it may offer interesting opportunities, such as impedance tunability, a high quality factor due to its weakly radiative nature, the launching of propagating plasmon modes with increased propagation lengths [27] and spatial select...

Atomic-Scale Confinement of Resonant Optical Fields

“…We therefore conclude that an experimental realization of atomic-scale concentration of electromagnetic fields at visible frequencies is possible but it requires atomic-scale shape control of the field-confining structure, i.e. the gap, as well as a careful assignment and selection of suitable optical modes.Here we achieve atomic-scale confinement of electromagnetic fields at visible frequencies by combining for the first time both atomic-scale shape control of the field confining structure 15 as well as a careful selection and assignment of suitable optical modes [16][17][18] . We study single-crystalline nanorods which self-assemble into side-by-side aligned dimers with gap widths below 0.5 nm.…”

“…Here we achieve atomic-scale confinement of electromagnetic fields at visible frequencies by combining for the first time both atomic-scale shape control of the field confining structure 15 as well as a careful selection and assignment of suitable optical modes [16][17][18] . We study single-crystalline nanorods which self-assemble into side-by-side aligned dimers with gap widths below 0.5 nm.…”

Plasmonic circuits for manipulating optical information

“…As with radio engineering, it is possible to create antenna arrays [173] for controlling the divergence and radiation direction of a light beam [186,187] and plasmonic structures for steering the radiation direction of light depending on frequency [167] or controlling the propagation direction of plasmons based on phase [188]. The antenna excitation patterns [189,190] can be altered by changing the phase and polarization of the incident light, which affects the plasmon modes that are excited [191].…”

Near Field Properties and Plasmonic Effects of a Periodic Bowtie Antenna Array Embedded in a Substrate with Different Depths

“…An important field pattern of the excitation of plasmonic waves is significant enhancement of the incident electric fields near the surface of the metallic regions by several orders of magnitude [4]. In recent years, plasmonic antennas [5]- [8] have attracted of great interest by researches owing to their ability to support the localized surface plasmon resonance (SPR) and provide the enhanced and confined electromagnetic fields. Various applications of plasmonics antennas have been proposed and demonstrated [9]- [12].…”

“…1: the gap width, d; the antenna length along x-axis, a; the antenna width along y-axis, b; and the antenna thickness, c. It should also be noted that the area within the gap has an eventful impact on the optical performance of antenna. Limited by the uncertainty of conventional nanofabrication, it is often difficult to fabricate antenna arrays with a reproducible gap size below 20 nm [6], therefore a gap width of d=30 nm is chosen throughout this paper. The area of gap region d × e=30 nm × 30 nm is chosen for this study.…”