In this paper, we propose a refractive index sensor based on surface plasmon Fano resonance in eccentric circular ring arrays. By altering the distance between the center of the outer ring and the inner cavity, asymmetry is introduced, enabling coupling between the dark mode of the outer ring and the bright mode of the inner cavity to form a unique Fano resonance mode. We aim to explore the formation mechanism of Fano resonance and its sensing performance, providing theoretical references for designing high-performance plasmonic sensors.

In this paper, we propose a heterocentric ring array refractive index sensor based on surface plasmon Fano resonance. By changing the distance between the outer ring and the inner cavity of the unit structure, asymmetry is introduced, causing coupling between the dark mode of the outer ring and the bright mode of the inner cavity, resulting in a unique Fano resonance mode. In addition, the surface plasmon polariton (SPP) excited by the metal/dielectric interface can further reduce the linewidth of the Fano resonance. Using the finite element method, we study the formation mechanism of the Fano resonance by changing the centroid distance d and perform fitting analysis using Fano resonance. In this paper, we also analyze and discuss the evolution process and reflectance spectral characteristics of Fano resonance modes with different geometric parameters (outer ring radius R, inner cavity radius r, ring thickness h, array period P, and silicon dioxide layer thickness t). Finally, by changing the external refractive index, the sensitivity and quality factors of the refractive index sensor based on wavelength sensing and intensity sensing are calculated.

The eccentric torus array exhibits a Fano resonance pattern. Fano resonance is formed by coupling the dark mode of the outer ring and the bright mode of the inner cavity, while the SPP at the metal-dielectric interface further reduce the resonant linewidth. As the centroid distance d increases from 0 to 40 nm, the FWHM of the reflection dip at the longer wavelength (dip 2) reaches a minimum of 4.5 nm at d=10 nm (Fig. 2). The influence of different geometric parameters on the Fano resonance is investigated. Increasing the outer ring radius R from 80 nm to 120 nm broadens the resonance linewidth and deepens the reflection dip (Fig. 4). The resonance wavelength redshifts and the reflectance increases as the inner cavity radius r increases from 30 nm to 70 nm (Fig. 5). The ring thickness h affects the resonance strength and linewidth, with an optimal thickness of 30 nm yielding the deepest reflection dip (Fig. 6). The array period P and silica layer thickness t also significantly influence the resonance wavelength and reflectance, with redshifts observed as P and t increase (Fig. 7). By changing the geometric parameters of the structure in multiple dimensions, coupling modes of dipole, quadrupole, and hexapole can be achieved in different combinations. The optimized structure (d=10 nm, R=100 nm, r=50 nm, h=30 nm, P=550 nm, t=30 nm) demonstrates excellent sensing performance. The resonance wavelength redshifts systematically with the external refractive index n, achieving a wavelength sensitivity S of 516 nm/RIU and a wavelength figure of merit of 114.7. For intensity-based sensing, the light intensity at the resonance wavelength (λ=804 nm) shows an intensity sensitivity S^* of 84.2 RIU^-1 and an intensity figure of merit of 1754.2 (Fig. 7). Therefore, it holds potential application value in the field of label-free biosensing.

In this paper, we propose a heterocentric toroidal array refractive index sensor based on surface plasmon Fano resonance and study the Fano resonance effect of the structure and its application in refractive index sensing. The Fano resonance is caused by changing the distance between the outer ring and the inner cavity of the structural unit, breaking the symmetry, and enabling coupling between the dark mode of the outer ring and the bright mode of the inner cavity. In addition, the SPP excited by the metal/dielectric interface can further reduce the linewidth of the Fano resonance. After parameter optimization, we obtain a Fano resonance valley with a FWHM of only 4.5 nm and a reflectance of almost 0 at this wavelength. By multi-dimensionally changing the geometric parameters of the structure, coupling modes of dipole, quadrupole, and hexapole can be achieved in different combinations. Thanks to the narrow linewidth of Fano resonance and the substantial enhancement of the electromagnetic field, the sensor exhibits excellent refractive index sensing performance, with a wavelength sensitivity S of 516 nm/RIU, an intensity sensitivity S^* of 84.2 RIU^-1, an FOM of 114.7, and an FOM^* of 1754.2. This provides a theoretical reference for the design of high-performance plasmonic refractive index sensors and holds potential application value in high-sensitivity refractive index sensing.