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Atomic-scale lightning rod effect in plasmonic nanoparticles: a classical view to a quantum effect

AuthorsUrbieta, Mattin; Barbry, Marc; Koval, P. ; Sánchez-Portal, Daniel ; Aizpurua, Javier ; Zabala, Nerea
Issue Date2018
CitationCEN 2018
AbstractPlasmonic nanoparticles (NPs) are known to produce localization and enhancement of electromagnetic fields, providing effective mode volumes of nanometer scale. Atomistic quantum calculations based on Time-Dependent Density Functional Theory (TDDFT) reveal the effect of subnanometric localization of optical fields due to the presence of atomic-scale features at the surface of metallic NPs and interparticle gaps. Using classical electrodynamics (Boundary Element Method, BEM), we explain this effect as a non-resonant lightning rod effect at the atomic scale that produces an extra enhancement over that of the plasmonic background. The near-field distribution of atomic-scale hot-spots around atomic features is robust against dynamical charge screening and spill-out effects, and follows the potential landscape determined by the electron density around the atomic sites. A detailed comparison of the field distribution around atomic hot-spots from full quantum atomistic calculations and from the local classical approach considering the geometrical profile of the atoms’ electronic density, validates the use of a classical framework to determine the field distribution and effective mode volume in these extreme subnanometric optical cavities. This finding is of great practical importance for surface-enhanced molecular spectroscopy and quantum nanophotonics, as it provides an adequate description of atomic-scale hot-spots with use of simplified classical methods. We have further studied this effect for plasmons excited with fast electrons and calculated the electron energy loss spectra (EELS) of electron beams passing nearby and through the same atomistic structures. Both classical and quantum atomistic calculations have been performed to study the impact of the geometry at the atomicscale on EELS, which nowadays is a technique with atomic resolution. The comparison between both models shows direct identification of the localized surface plasmons, while differences arise for the excitation of bulk plasmons in penetrating trajectories. Our results bear out the reliability of classical models at such dimensions, to both allow the identification of the excited modes in TDDFT calculations and unveil the limits of the classical approach with nanometer sized NPs.
DescriptionResumen del trabajo presentado a la Spanish Conference on Nanophotonics (Conferencia Española de Nanofotónica-CEN), celebrada en Donostia-San Sebastián (España) del 3 al 5 de octubre de 2018.
Appears in Collections:(CFM) Comunicaciones congresos
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