Dark plasmon in asymmetric nanoring arrays on conducting substrates and related applications

We put forward a theoretical analysis of the optical properties of an asymmetric nanoring array on a gold metal layer. We show that for normal incidence, the spectrum of the symmetric system is characterized by a general dipolar ring resonance. For the asymmetric nanoring with an offset inner surface, obviously both dark monopolar and quadrupolar ring resonances can be generated under a normal incidence, which depends sensitively on the asymmetry of the structure. Meanwhile, the symmetry breaking of the structure provides the desirable near-field characteristics for biosensing. As a result, monopolar and multipolar modes reveal ultrahigh refractive sensitivities as large as 792 nm/RIU and 742 nm/RIU, respectively. Furthermore, due to their subradiant features, the resulting FOM is 72 RIU−1 for the monopolar mode and the record high value of 137.4 RIU−1 for the quadrupolar mode in a wide refractive index range of 1.33-1.38. These results indicate that an asymmetric ring system could be promising for biosensing applications with high performance.