Sensitive label-free sensor with high figure of merit based on plasmonic metasurface with unit cell of double two-split nanorings

We theoretically propose a sensitive label-free sensor with high figure of merit (FoM) based on Fano resonances on a plasmonic metasurface whose unit cell consists of double two-split nanorings by finite-element method. The unit cell of the proposed Fano resonant structure comprises of two thin gold nanorings each with two splits. Two Fano modes are generated in near-infrared regime through symmetry breaking by rotating the splits in nanorings in opposite directions. The fundamental feature of the proposed sensor is its relatively simple structure, double-mode usable, a large single-side quality factor of 566, and high FoM value of 378. According to our knowledge, these values are higher than those previously reported. The sensor has high substance resolving capability that its minimum detectable refractive index change can be as small as 0.00236, making it possible to distinguish different bio-tissues.