Nanoplasmonics: Classical
down to the Nanometer Scale
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Abstract
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