Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes

• Published in: Nano letters Vol. 12; no. 4; pp. 1885 - 1890
• Main Authors: Schertz, Florian, Schmelzeisen, Marcus, Mohammadi, Reza, Kreiter, Maximilian, Elmers, Hans-Joachim, Schönhense, Gerd
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 11.04.2012
• Subjects: Collective excitations (including excitons, polarons, plasmons and other charge-density excitations); Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electron and ion emission by liquids and solids; impact phenomena; Electron emission; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Exact sciences and technology; Far fields; Gold; Interfaces, heterostructures, nanostructures; Joining; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Materials science; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Nanostructure; Nonlinearity; Photoemission and photoelectron spectra; Physics; Plasmons; Scattering; Spectroscopy; Surface and interface electron states; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); subnanometer gap; gap resonance; near field; dark mode; sphere-on-plane; Plasmon coupling; Thin films; Nanoparticles; Gold; Photoemission; Far field; Plasmons; Dipole moments; Electron emission; Nanostructures; Gallium phosphide; Electron microscopy; Nanostructured materials
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl204277y

Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-on-plane, NPOP) are investigated using two complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), we detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. To our knowledge, this is the first time that both far- and near-field spectroscopy are carried out for identical individual nanostructures interacting via a subnanometer gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.