Plasmonic Meta-Molecules in the Quantum Tunneling Regime

Aitzol Garcia-Etxarri DIPC, Donostia International Physics Center, San Sebastian, Gipuzkoa, Spain Department of materials science and engineering, Stanford University, Stanford, California, USA Jonathan Scholl Department of materials science and engineering, Stanford University, Stanford, California, USA Garikoitz Aguirregabiria DIPC, Donostia International Physics Center, San Sebastian, Gipuzkoa, Spain Centro Mixto CSIC-UPV/EHU, Centro de Fisica de Materiales, San Sebastian, Gipuzkoa, Spain Ruben Esteban DIPC, Donostia International Physics Center, San Sebastian, Gipuzkoa, Spain Tarun Narayan Department of materials science and engineering, Stanford University, Stanford, California, USA Hadiseh Alaeian Department of materials science and engineering, Stanford University, Stanford, California, USA Department of Electrical Engineering, Stanford University, Stanford, California, USA Javier Aizpurua DIPC, Donostia International Physics Center, San Sebastian, Gipuzkoa, Spain Centro Mixto CSIC-UPV/EHU, Centro de Fisica de Materiales, San Sebastian, Gipuzkoa, Spain Jennifer A. Dionne Department of materials science and engineering, Stanford University, Stanford, California, USA Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, USA

Closely spaced plasmonic nanoparticles exhibit highly tunable hybridized resonances based on nanoparticle geometry, spacing, and arrangement. Such resonances have allowed for applications ranging from nanoantennas and metamaterials to novel surface-enhanced spectroscopy substrates. While most studies have focused on classical resonances, recently it has become possible to probe plasmonic resonances in the quantum regime. Here, we explore the quantum-influenced modes of one of the most basic plasmonic meta-molecules - a trimer - using scanning transmission electron microscope (STEM) and electron energy-loss spectroscopy (EELS). This technique enables spatially-selective excitation of the trimer, allowing observation of dark and higher-order plasmonic modes not readily detectable with optical microscopy. Additionally, the electron beam can interact with the nanoparticles to modify the interparticle distance and induce particle convergence.

Silver nanoparticles 25 nm in diameter were colloidally synthesized and self-assembled into trimers with ~1 nm interparticle gaps. STEM-EELS data was then collected as particles approached and converged due to the electron beam influence. We monitored the electric dipolar mode, the magnetic mode, and dark modes by positioning the electron beam at the vertex, edge, and center of the trimer, respectively. For separations greater than ~0.5nm, modal evolution with distances agrees with classical calculations using the boundary element method. However, trimers with smaller separations support electric dipolar and magnetic modes that exhibit relative blue-shifts and intensity quenching that are not predictable with classical simulations. To account for this behavior, we developed a quantum corrected model that incorporates tunneling between the particle junctions. Our combined experimental and theoretical work provides, to our knowledge, the first complete evolution of magnetic and dark plasmonic modes in the classical and quantum tunneling regime, and may enable new quantum plasmonic metamolecules and metamaterials.

aitzolgarcia@gmail.com