efficient modulation of multipolar fano resonances in asymmetric ring-disk/split-ring-disk nanostructure

Generation of multiple fano resonances are theoretically investigated in asymmetry ring-disk and asymmetry split-ring-disk. the effects of structural parameter on multiple fano resonances are analyzed in detail, and it is found that the wavelength of multipole fano resonances can be extensively and accurately controlled by changing the gap size and the relative offset between the ring/split-ring and disk in asymmetric ring-disk/split-ring-disk nanostructure. simulation on scattering spectra of the asymmetric structure show that fano dips generally exhibit redshift in resonant wavelength and simultaneously with a varied modulation depth as symmetry of the structure is further broken. the results of the near-field distribution and phase simulation disclose that multiple fano resonances are caused by interference of dipolar bright mode of whole asymmetric structure with the combined high-order dark-dark modes, and the dip on the shorter resonant wavelength side corresponds to higher-order dark mode. furthermore, it is found that the multiple fano resonances of asymmetry ring-disk are polarization-independent. however, for the asymmetric split-ring-disk, resonances are sensitive to polarization angle and number of the dips can be switched on and off by tuning the polarization angle. the proposed asymmetric nanostructures could find wide applications in plasmon line shaping, multiband sensing, electromagnetic-induced transparency and many other fields.