A dual-disk ring (DDR) structure with broken symmetry and weakly dissipating material, silver, is proposed to achieve Fano resonance in visible wavelength range. Symmetry breaking of a metallic ring is realized by placing two disks inside the ring. The excitation of the Fano resonance is interpreted in terms of coupling of the ring and the dual-disk plasmonic modes. The potential of using an array of such DDR nano structures as a biochemical sensor is evaluated with the figure of merit (FOM). Based on our design and simulation, arrays with DDR structures are fabricated and the Fano resonance peak is observed in visible wavelength range of extinction spectra of individual silver DDR nanostructures.
The nanostructure comprises a dual-disk ring consisting of two silver disks, measuring tens of nanometers wide, placed inside a silver ring. The researchers calculated the optical modes of the structures using the finite-difference time-domain (FDTD) method. They found that the coupling between one of the dual-disk eigenmodes and one of the ring eigenmodes produces a Fano resonance just below 700 nanometers in wavelength, well within the visible spectrum.
Scheme (top) of the dual-disk ring structure and scanning electron micrscosopy images of fabricated devices (bottom)