Successful development of localized surface plasmon resonance-based technologies (e.g. biosensing, plasmonic lasing) relies on a detailed control over the basic resonance characteristics - the resonance frequency, line width and line shape. Dipole activation of dark higher order modes and hybridization into anti-parallel oscillating dipole moments has emerged as one of the most promising methods to achieve line width and line shape tuning in plasmonic nanocavities. In both cases, the resulting subradiant modes are characterized by low radiative losses and, hence, narrow line widths. Spectral overlap with broader dipolar modes can furthermore result in destructive and constructive interferences with an asymmetric Fano resonance line shape. Typically, these coherent coupling effects are achieved in cavity designs consisting of two or more capacitively coupled nanoparticles.

Here, we show that a plasmonic nanocross geometry consisting of two or more conductively coupled nanobars, thus forming a single building-block nanocavity, can likewise support spectrally sharp Fano resonances in the visible and near infrared. Finite difference time domain calculations of absorption and scattering cross-sections, as well as charge density profiles, are used to reveal the nature of the different modes. Moreover, experimental spectra support these calculations.

The ability to posses discrete degrees of rotational symmetry situated between rotational invariance (C_∞), like a disk, and no rotational symmetry at all (C₁), like a non-equilateral triangle, makes the nanocross a fundamentally interesting plasmonic structure. The nanocross‘ rotational symmetry is shown to have a crucial influence on the dipole activation of dark quadrupolar and octupolar modes. Particularly important about nanocross geometries with reduced symmetry, is the fact that they allow the excitation of Fano resonances without the need for nanometer sized interparticle gaps.

Our results for relatively simple nanostructures can form a basis for the design and understanding of new, more advanced plasmonic nanosystems.