Interplay Between Optical Bianisotropy and Magnetism in Plasmonic Metamolecules

Abstract

The smallness of natural molecules and atoms with respect to the wavelength of light imposes severe limits on the nature of their optical response. For example, the well-known argument of Landau and Lifshitz and its recent extensions that include chiral molecules show that the electric dipole response dominates over the magneto-electric (bianisotropic) and an even smaller magnetic dipole optical response for all natural materials. Here, we experimentally demonstrate that both these responses can be greatly enhanced in plasmonic nanoclusters. Using atomic force microscopy nanomanipulation technique, we assemble a plasmonic metamolecule that is designed for strong and simultaneous optical magnetic and magneto-electric excitation. Angle-dependent scattering spectroscopy is used to disentangle the two responses and to demonstrate that their constructive/destructive interplay causes strong directional scattering asymmetry. This asymmetry is used to extract both magneto-electric and magnetic dipole responses and to demonstrate their enhancement in comparison to ordinary atomistic materials.

Assembly of “3D” plasmonic clusters by “2D” AFM nanomanipulation of highly uniform and smooth gold nanospheres

“…For example, the colloidal self-assembly is mainly governed by a packing consideration or thermodynamics; thereby, available cluster geometries are in principle limited to symmetric cluster or random aggregate 6, 15, 17, 30, 31 . However, in case, the symmetry of plasmonic metamolecules is needed to be broken especially for enhancing artificial magnetism or inducing Fano resonance 12, 13, 18 . For e-beam lithography, each AuNP can be arbitrarily positioned with high flexibility, so as to form metamolecules with controlled asymmetry 32 .…”

“…Plasmonic metamolecules consisting of clustered metallic nanoparticles (NPs) (i.e., meta-atoms) have been extensively investigated over the last decade, because allowing for various electromagnetisms of interest at visible frequencies such as artificial magnetism, Fano-like interference, negative refractive index, and ultrahigh refractive index 1–18 . As accessible artificial electromagnetisms of plasmonic metamolecules are defined by materially realizable clusters, there is a pressing need for innovation of NP assembly methods.…”

“…In this context, atomic force microscopy (AFM) nanomanipulation has been viewed as a unique and deterministic method for the assembly of clustered NPs, because its core, a blueprint containing information for programmed linear vector motion of the AFM tip can be directly translated into the directional dribbling of NPs with a few nanometer precisions. This AFM nanomanipulation results in the assembly of plasmonic clusters with exquisite control over NP size, cluster geometry, and NP spacing, which would be difficult to achieve with other methods 12, 18, 19 . Furthermore, this technique can be used to bring quantum emitters close to plasmonic nanocavities as well 20, 21 .…”

“…Thus, dribbling polygonal-shaped and polydispersed AuNSs via AFM nanomanipulation has been error prone and difficult to achieve a few nanometer precisions 23, 24 . Even a few nanometer-error within gaps between AuNSs is non-trivial, as it leads to drastic changes in artificial magnetism 9, 12, 18 . Therefore, AFM nanomanipulation-enabled assembly of plasmonic metamolecules needs to address the relatively low robustness of AuNS dribbling.…”

“…This lack of robustness has been effectively mitigated by recent advances in the synthesis of highly uniform and spherical AuNPs 25–27 . It has been shown that the use of highly spherical and smooth AuNSs can increase the accuracy of AFM nanomanipulation 18, 19 ; however, limited dimensionality to the planar 2D motifs (e.g., 2D tetramer) has remained an obstacle for expanding design space for accessible plasmonic metamolecules via AFM nanomanipulation.…”