It has become apparent that the plasmons of metallic nanostructures, while described by classical electromagnetic theory, exhibit certain characteristics that are analogous to electrons in quantum systems. Plasmonic dimers and ring-like hexamers are well studied examples, which possess similar symmetry to H2 and benzene molecules, respectively. This similarity helps scientists to unravel the energy levels in the conjugated atoms of these molecules.

Newly designed and fabricated nano-lithographic metallic quadrumers, with well controlled inter-particle gaps at sub-20 nm, help the team to study the formation of collective modes and obtain more information on the complex electronic states in trigonal planar molecules.

The group analysed the interference between a sub-radiant mode and a super-radiant mode and found that the D3h configuration of the plasmonic quadrumers leads to the exhibition of a pronounced Fano Resonance (FR). This resulting FR is attributed to the destructive interference among anti-parallel dipole modes, which can be observed independently of the excitation polarization at normal incident light without a need to break the geometrical symmetry, changing the incident light angle or the excitation polarization.

Meanwhile, using this configuration, the near-field energy distribution can be flexibly tuned by changing the excitation polarization, while the collective optical responses are still kept polarization-independent. It also provides a novel approach to localize the energy distribution at sub-20 nm gaps among nano-discs by normal incident light of a single source rather than co-illumination by two light sources at different incident angles or phase shifts. Consequently, the set-up can overcome the spatial restrictions of conventional optics with unique potential applications in nano-lithography and nonlinear spectroscopy, as well as opening up promising perspectives in molecular optical switching.

Decoding graphene

The work provides an opportunity to investigate the potentials of atomic interactions in planar trigonal molecules, which belong to the D3h symmetry group. Further study could help to explore the coupling mechanism of individual carbons atoms in graphene, which may lead to more optical applications of this wonder material.

The researchers presented their results in the journal Nanotechnology.