Nanoplasmonics: Classical down to the Nanometer Scale

• Published in: Nano letters 2012-03, Vol.12 (3), p.1683-1689
• Main Authors: Duan, Huigao, Fernández-Domínguez, Antonio I, Bosman, Michel, Maier, Stefan A, Yang, Joel K. W
• Format: Article
• Language: English
• Subjects: Brightening; Collective excitations (including excitons, polarons, plasmons and other charge-density excitations); Computer Simulation; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Contact; Electrodynamics; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Evolution; Exact sciences and technology; Gold; Gold - chemistry; Light; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Mathematical models; Models, Chemical; Models, Molecular; Nanostructure; Nanostructures - chemistry; Nanostructures - ultrastructure; Particle Size; Physics; Plasmonics; Plasmons; Scattering, Radiation; Surface and interface electron states; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl3001309

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light–matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms.