Toroidal-dipole induced plasmonic perfect absorber

We present a new kind of perfect absorber which roots in a toroidal dipole resonance. The toroidal metastructure consists of a metallic circular groove with a depth asymmetry, which couples to the toroidal dipole field in the near-infrared region and thus realizes nearly unit absorbance, acting as a perfect absorber. Moreover, this absorber owns a high sensitivity of 609.6 nm/RIU to the dielectric surroundings. Furthermore, by tuning the geometric parameters, both the toroidal dipole resonance and perfect absorbance characteristics are insensitive to the circular groove width, providing profound fabrication tolerance in future experiments.