title: Plasmonic nanoclusters with rotational symmetry: polarization-invariant far-field response vs changing near-field distribution.
abstract: Flexible control over the near- and far-field properties of plasmonic nanostructures is important for many potential applications, such as surface-enhanced Raman scattering and biosensing. Generally, any change in the polarization of the incident light leads to a change in the nanoparticle's near-field distribution and, consequently, in its far-field properties as well. Therefore, producing polarization-invariant optical responses in the far field from a changing near field remains a challenging issue. In this paper, we probe experimentally the optical properties of cruciform pentamer structures--as an example of plasmonic oligomers--and demonstrate that they exhibit such behavior due to their symmetric geometrical arrangement. We demonstrate direct control over hot spot positions in sub-20 nm gaps, between disks of 145 nm diameter at a wavelength of 850 nm, by means of scattering scanning near-field optical microscopy. In addition, we employ the coupled dipole approximation method to define a qualitative model revealing the relationship between the near and far field in such structures. The near-field profiles depend on particular mode superpositions excited by the incident field and, thus, are expected to vary with the polarization. Consequently, we prove analytically that the far-field optical properties of pentamers have to be polarization-independent due to their rotational symmetry.
authors.affiliation: Department of Physics, Imperial College London , London SW7 2AZ, U.K.
authors.forename: Mohsen
authors.initials: M
authors.lastname: Rahmani
authors.forename: Edward
authors.initials: E
authors.lastname: Yoxall
authors.forename: Ben
authors.initials: B
authors.lastname: Hopkins
authors.forename: Yannick
authors.initials: Y
authors.lastname: Sonnefraud
authors.forename: Yuri
authors.initials: Y
authors.lastname: Kivshar
authors.forename: Minghui
authors.initials: M
authors.lastname: Hong
authors.forename: Chris
authors.initials: C
authors.lastname: Phillips
authors.forename: Stefan A
authors.initials: SA
authors.lastname: Maier
authors.forename: Andrey E
authors.initials: AE
authors.lastname: Miroshnichenko
mesh_terms: D000042:Absorption; D015374:Biosensing Techniques; D003198:Computer Simulation; D006046:Gold / Q000737:chemistry; D008027:Light; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D053758:Nanoparticles / Q000737:chemistry*; D036103:Nanotechnology / Q000379:methods*; D055095:Optics and Photonics; D012399:Rotation; D012542:Scattering, Radiation; D013053:Spectrophotometry; D017550:Spectroscopy, Fourier Transform Infrared; D020349:Surface Plasmon Resonance
chemical_list: D006046:Gold