Atomistic Quantum Plasmonics of Gold Nanowire Arrays

The dielectric properties of a regular 2D array of Au nanowires are investigated using time-dependent density-functional theory employing a fully atomistic quantum description. Longitudinal modes produce a Drude-like peak in the infrared that is rather insensitive to geometrical parameters. Transverse modes, instead, give rise to a plasmonic peak in the optical region, which exhibits a nontrivial dependence on the spatial separation between the wires: a strong resonant enhancement and a shift from the optical to the far-infrared region is observed as the interwire distance is decreased, with the formation of "hot spots" in which induced field and charge distributions exhibit nondipolar shape and rapidly alternating quantum phase. The general character of this phenomenon is confirmed by its occurrence in Au nanoparticle arrays. Addition of ligand species in the hot spot region can lead to the appearance of new resonances due to strong coupling between plasmonic and molecular modes, as exemplified in a proof-of-concept case. This shows the possibilities of atomistic quantum plasmonics effects and subwavelength control of electromagnetic field intensity in properly engineered nanogaps.