title: Raspberry-like metamolecules exhibiting strong magnetic resonances.
abstract: We report a synthetic approach to produce raspberry-like plasmonic nanostructures with unusually strong magnetic resonances, termed raspberry-like metamolecules (raspberry-MMs). The synthesis based on the surfactant-assisted templated seed-growth method allows for the simultaneous one-step synthesis and assembly of well-insulated gold nanoparticles. The aromatic surfactant used for the syntheses forms a thin protective layer around the nanoparticles, preventing them from touching each other and making it possible to pack discrete nanoparticles at close distances in a single cluster. The resulting isotropic gold nanoparticle clusters (i.e., raspberry-MMs) exhibit unusually broad extinction spectra in the visible and near-IR region. Finite-difference time-domain (FDTD) modeling showed that the raspberry-MMs support strong magnetic resonances that contribute significantly to the broadband spectra. The strong magnetic scattering was also verified by far-field scattering measurements, which show that in the near-IR region the magnetic dipole resonance can be even stronger than the electric dipole resonance in these raspberry-MMs. Structural parameters such as the size and the number of gold nanoparticles composing raspberry-MMs can be readily tuned in our synthetic method. A series of syntheses with varying structure parameters, along with FDTD modeling and mode analyses of corresponding model structures, showed that the close packing of a large number of metal nanoparticles in raspberry-MMs is responsible for the unusually strong magnetic resonances observed here.
authors.affiliation: Department of Chemistry, ‡Department of Physics, and §Department of Electrical and Systems Engineering, University of Pennsylvania , Philadelphia, Pennsylvania 19104, United States.
authors.forename: Zhaoxia
authors.initials: Z
authors.lastname: Qian
authors.forename: Simon P
authors.initials: SP
authors.lastname: Hastings
authors.forename: Chen
authors.initials: C
authors.lastname: Li
authors.forename: Brian
authors.initials: B
authors.lastname: Edward
authors.forename: Christine K
authors.initials: CK
authors.lastname: McGinn
authors.forename: Nader
authors.initials: N
authors.lastname: Engheta
authors.forename: Zahra
authors.initials: Z
authors.lastname: Fakhraai
authors.forename: So-Jung
authors.initials: SJ
authors.lastname: Park
keywords: gold nanoparticle cluster; magnetic dipole; magnetic quadrupole; magnetic resonance; metamaterial; metamolecule; surface plasmon resonance