Plasmon Hybridization in Individual Gold Nanocrystal Dimers: Direct Observation of Bright and Dark Modes

• Published in: Nano letters Vol. 10; no. 2; pp. 632 - 637
• Main Authors: Yang, Shu-Chun, Kobori, Hiromu, He, Chieh-Lun, Lin, Meng-Hsien, Chen, Hung-Ying, Li, Cuncheng, Kanehara, Masayuki, Teranishi, Toshiharu, Gwo, Shangjr
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 10.02.2010
• Subjects: Collective excitations (including excitons, polarons, plasmons and other charge-density excitations); Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Dimerization; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Exact sciences and technology; Gold - chemistry; Light; Magnetics; Materials science; Metal Nanoparticles - chemistry; Microscopy, Electron, Scanning - methods; Nanocrystalline materials; Nanoparticles - chemistry; Nanoscale materials and structures: fabrication and characterization; Nanostructures - chemistry; Nanotechnology - methods; Optics and Photonics; Physics; Spectrophotometry - methods; Surface and interface electron states; Surface Plasmon Resonance; gold nanocrystal; plasmonic dimer; dark plasmon mode; Surface plasmon; plasmon hybridization; Interfaces; Gold; Red shift; Tin oxide; Plasmons; Dimers; Dipole moments; Hybridization; Spectral line shift; Nanostructured materials; Indium oxide; Nanocrystal
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl903693v

We apply a nanomanipulation technique to assemble pairs of monodispersed octahedral gold nanocrystals (side length, 150 nm) along their major axes with a varying tip-to-tip separation (25−125 nm). These pairs are immobilized onto indium tin oxide coated silica substrates and studied as plasmonic dimers by polarization-selective total internal reflection (TIR) microscopy and spectroscopy. We confirm that the plasmon coupling modes with the scattering polarization along the incident light direction result from the transverse-magnetic-polarized incident light, which induces two near-field-coupled dipole moments oriented normal to the air−substrate interface. In such cases, both in-phase (antibonding) and antiphase (bonding) plasmon coupling modes can be directly observed with the incident light wave vector perpendicular and parallel to the dimer axis, respectively. The observation of antiphase plasmon coupling modes (“dark” plasmons) is made possible by the unique polarization nature of the TIR-generated evanescent field. Furthermore, with decreasing nanocrystal separation, the plasmon coupling modes shift to shorter wavelengths for the incident light perpendicular to the dimer axis, whereas relatively large red shifts of the plasmonic coupling modes are found for the parallel incident light.