Self-Assembled Plasmonic Metamolecules Exhibiting Tunable Magnetic Response at Optical Frequencies

A bottom-up self-assembly approach is used to synthesize pseudospherical clusters of Au nanoparticles with low size and shape polydispersity. Such plasmonic metamolecules (PMMs) with various diameters are prepared from two different AuNP building block sizes, enabling the investigation of the size dependent optical properties of these structures. Trends in the UV-vis spectra are reproduced theoretically using generalized multi particle Mie theory as either the AuNP or PMM diameter are changed. The calculated results indicate that features in the spectra of large PMMs can be associated with magnetic dipole resonances, which strengthen and red-shift with increasing PMM size. Finally, effective medium theory is used to demonstrate that materials composed of these PMMs could exhibit a bulk isotropic magnetic response at optical frequencies.