title: Evolution of Plasmonic Metamolecule Modes in the Quantum Tunneling Regime.
abstract: Plasmonic multinanoparticle systems exhibit collective electric and magnetic resonances that are fundamental for the development of state-of-the-art optical nanoantennas, metamaterials, and surface-enhanced spectroscopy substrates. While electric dipolar modes have been investigated in both the classical and quantum realm, little attention has been given to magnetic and other "dark" modes at the smallest dimensions. Here, we study the collective electric, magnetic, and dark modes of colloidally synthesized silver nanosphere trimers with varying interparticle separation using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). This technique enables direct visualization and spatially selective excitation of individual trimers, as well as manipulation of the interparticle distance into the subnanometer regime with the electron beam. Our experiments reveal that bonding electric and magnetic modes are significantly impacted by quantum effects, exhibiting a relative blueshift and reduced EELS amplitude compared to classical predictions. In contrast, the trimer's electric dark mode is not affected by quantum tunneling for even Ångström-scale interparticle separations. We employ a quantum-corrected model to simulate the effect of electron tunneling in the trimer which shows excellent agreement with experimental results. This understanding of classical and quantum-influenced hybridized modes may impact the development of future quantum plasmonic materials and devices, including Fano-like molecular sensors and quantum metamaterials.
authors.affiliation: Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Scholl
authors.affiliation: Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.|Center for Material Physics, CSIC - UPV/EHU and DIPC , Donostia, San Sebastian 20018, Spain.
authors.forename: Aitzol
authors.initials: A
authors.lastname: Garcia-Etxarri
authors.affiliation: Center for Material Physics, CSIC - UPV/EHU and DIPC , Donostia, San Sebastian 20018, Spain.
authors.forename: Garikoitz
authors.initials: G
authors.lastname: Aguirregabiria
authors.affiliation: Center for Material Physics, CSIC - UPV/EHU and DIPC , Donostia, San Sebastian 20018, Spain.
authors.forename: Ruben
authors.initials: R
authors.lastname: Esteban
authors.affiliation: Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.
authors.forename: Tarun C
authors.initials: TC
authors.lastname: Narayan
authors.affiliation: Stanford Nanocharacterization Laboratory, Stanford University , Stanford, California 94305, United States.
authors.forename: Ai Leen
authors.initials: AL
authors.lastname: Koh
authors.affiliation: Center for Material Physics, CSIC - UPV/EHU and DIPC , Donostia, San Sebastian 20018, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Aizpurua
authors.affiliation: Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.|Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory , 2575 Sand Hill Road, Menlo Park, California 94025, United States.
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Dionne
keywords: electron energy-loss spectroscopy; nanoparticle; plasmon; quantum tunneling; quantum-corrected model