Generation of multiple Fano resonances in plasmonic panda's eye structures

A kind of dimer nanostructure similar to panda's eye (P-E), consisting two center biased elliptical rings, is proposed in this paper. The finite element method is used to study the nanostructure with the vertical incident light. The structure exhibits high order magnetic modes and the magnetic modes intensity can be manipulated independently by changing the structure parameters. The intensities of the magnetic dipole and magnetic quadrupole modes can be controlled by changing the angle between two rings and the distance of two cavity centers, respectively. Moreover, when two circular cavities increase simultaneously, the intensity of the magnetic octupole mode increases accordingly. Fano resonance can be induced when the electric mode couples with the magnetic mode. When the radius of the left circular cavity decreases, triple Fano resonance is formed. And when the whole structure is rotated, quintuple Fano resonance can be formed. More interesting, when the radii of the circular cavities of the two elliptical rings increase to 50 and 55 nm, the nanostructure becomes crescent dimer, and the enhancements of magnetic field and electric field reach 38 and 390, respectively. The P-E structure has potential application value in multiwavelength surface enhanced spectroscopy and biochemical sensing.