High‑Quality Fano Resonances in a Stacked Silver Nanoring/Half‑Nanoring Resonator

Background

Surface plasmon polaritons (SPPs) enable unprecedented control of light–matter interactions at the nanoscale, driving advances in nanofabrication, optical characterization, and full‑field computational electromagnetics.1–6 These developments have broadened our understanding of localized plasmon resonances in metallic nanostructures such as disks, triangles, rods, and rings.7–13 Fano resonances, arising from the interference between broad and narrow excitation channels, have been demonstrated in ring‑rod assemblies, plasmonic oligomers, nonspherical clusters, graphene‑based devices, and quantum dots.14–18 However, achieving Fano resonances at specific wavelengths remains challenging due to the complex hybridization of available modes and retardation effects that depend on the angle of incidence.19–32 These factors limit the activation of higher‑order or spatially extended modes, which are critical for high‑performance nanoplasmonic devices. The present study introduces a stacked nanoring/half‑nanoring system that circumvents these limitations by enabling efficient coupling of sub‑radiant modes under normal incidence, thereby offering robust tunability and high quality factors for practical applications.

Results and Discussion

Figure 2a and 2c display the transmission spectra of the isolated nanoring and half‑nanoring, respectively. The nanoring supports a third‑order superradiant mode at 1027 nm (A), while the half‑nanoring exhibits a first‑order dipolar resonance at 1297 nm (B). The Fano line shape in the nanoring arises from the hybridization of a disk‑like dipole (D_D) with an anti‑dot (D_H), forming bonding (D_B) and antibonding (D_AB) states (Fig. 2b). The electric‑field maps confirm the distinct charge distributions for these modes.

a Transmission of the complete nanoring. b Hybridization diagram of D_D and D_H, illustrating bonding and antibonding modes. c Transmission of the half‑nanoring.

Stacking the half‑nanoring onto the nanoring (Fig. 1) yields a coupled system where the second‑order Fano resonance emerges at 1160 nm, while the third‑order resonance at 1027 nm remains largely unaffected (Fig. 3a–c). The strong blueshift of the fundamental mode in the stacked configuration enables the activation of the even‑order resonance. The quality factors (Q) are exceptionally high: Q = 82.8 for the second‑order (FWHM = 14 nm) and Q = 114 for the third‑order (FWHM = 9 nm). These values surpass previously reported Qs of 20–62 in similar plasmonic systems.14,19,37–39 The dephasing times, calculated from T_r = 2ħ/Γ_L, are 0.10 ps (second‑order) and 0.12 ps (third‑order), well above the typical 10 fs for conventional Fano resonances, confirming the robustness of the stacked design.

a Transmission of the coupled system. b–c Electric‑field distributions at 1027 nm (third‑order) and 1160 nm (second‑order).

Rotating the half‑nanoring by a small angle θ introduces a rotation mode (m_r) that further splits the Fano resonances. At θ = 5°, three asymmetric dips appear (Fig. 4a). Charge‑distribution maps (Fig. 4b–d) confirm that the rotation mode originates from the hybridization of the half‑nanoring’s first‑order mode with the nanoring’s second‑order mode. As θ increases, the rotation resonance shifts to longer wavelengths, while the second‑order resonance weakens due to reduced dipole overlap along the y‑axis. Figure 5 demonstrates that at θ = 30°, the rotation mode becomes prominent, generating a distinct Fano profile alongside the remaining resonances.

a Transmission spectra for θ = 5°. b–d Electric‑field maps at 1027 nm, 1160 nm, and 1346 nm.

Transmission spectra for θ = 0°, 10°, 20°, 30° (blue, green, red, black). Other parameters unchanged.

These results demonstrate that the stacked nanoring/half‑nanoring platform offers unprecedented control over multiple Fano resonances, including the ability to switch on/off specific modes via a simple rotation of the half‑nanoring. The high Q factors and long dephasing times make this structure attractive for ultra‑compact, high‑sensitivity sensing and on‑chip photonic applications.

Conclusions

We have introduced a silver nanostructure comprising a complete nanoring stacked with a half‑nanoring that supports multiple, high‑quality Fano resonances in the near‑infrared. The resonances are highly tunable through the rotation angle of the half‑nanoring, providing a straightforward route to modulate spectral features on demand. The achieved Q factors (up to 114) and dephasing times (≈ 0.1 ps) underscore the suitability of this architecture for nanoscale sensors and integrated photonic devices.