Multipolar Origin and Active Control of High-Q Quasi-BIC Fano Resonances in Dielectric Metasurfaces for Sensing Applications

TL;DR

This work tackles achieving and controlling ultra-high-Q Fano resonances in all-dielectric metasurfaces through quasi-BICs in GaP bowtie-hole cuboids. The authors combine FDTD simulations, Fano-profile fitting, and LC-circuit modeling to reveal that MD and TD multipoles dominate the resonances, and that symmetry breaking enables multiple quasi-BICs with Q factors up to about $6.38\times10^4$. They demonstrate polarization-tunable switching and strong refractive-index sensing, achieving a sensitivity of about $342$ nm/RIU and a figure of merit around $217$ RIU^-1 for Vibrio cholerae detection, highlighting the practical potential for on-chip sensing and active photonic devices. The proposed, CMOS-friendly design offers a compact platform with high spectral selectivity, paving the way for integrated biosensing, dynamic filtering, and nonlinear light–matter interactions in the near-infrared.

Abstract

We designed an ingenious all-dielectric metasurface, employing cuboid structures patterned with bow-tie-shaped nanoholes, exhibiting multiple Fano resonances induced by quasi-bound states in the continuum (quasi-BICs) through structural asymmetry. Among them, several resonant modes demonstrated high quality factors in the range of 10^3-10^4, along with near-unity modulation depth and strong spectral contrast. The optical responses were analyzed utilizing the finite-difference time-domain (FDTD) method, with Fano profiles fitted to theoretical models and the BIC governed modes validated via the squared inverse ratio law. Furthermore, multipolar decomposition and electromagnetic spatial field profile revealed the origins of the resonance, while LC circuit modeling provided additional physical insight into the Fano profiles. The proposed metasurface also exhibited strong polarization dependence, indicating its potential for active optical switching. Finally, refractive index sensing performance, including the detection of Vibrio cholerae, reached a sensitivity of 342 nm/RIU and a figure of merit of 217.14 RIU^-1. Advancing the control of high-Q quasi-BIC Fano resonances, this study highlights Fano resonators' potential for refractive index sensing and active switching.

Figures (10)

• Figure 1: (a) 3D schematic illustration of the proposed all-dielectric bow-tie etched metasurface, (b) x-y, and (c) x-z plane view of the structure. The optimized structural parameters: P = $650~\mathrm{nm}$, w = $550~\mathrm{nm}$, tSiO2 = $200~\mathrm{nm}$, tGaP = $150~\mathrm{nm}$, and $\delta$ = s2 - s1. (d) Optical simulation configuration of the proposed structure.
• Figure 2: (a) Transmittance spectra of our proposed structure for $\delta = 0~\mathrm{nm}$, and $100~\mathrm{nm}$. Plot of FDTD calculated and fitted graph for (b) $\delta = 0~\mathrm{nm}$ near $\lambda = 1044~\mathrm{nm}$, and $1217~\mathrm{nm}$ (c) $\delta = 100~\mathrm{nm}$ near $\lambda = 911~\mathrm{nm}$, $1159~\mathrm{nm}$, and $1278~\mathrm{nm}$ under TM-polarized incident light. (d) Schematics of the LC circuit models for Mode I and II ($\delta = 0~\mathrm{nm}$). (e) FDTD calculated and fitted curve with LC circuit for Mode I and II for $\delta = 0~\mathrm{nm}$.
• Figure 3: Transmittance spectra for different asymmetry parameters, $\delta$ = $0~\mathrm{nm}$ to $125~\mathrm{nm}$ under TM-polarized incident light (b) and (c) relationship between the Q-factor, $Q_{rad}$, and the degree of asymmetry $\alpha$ for Mode III and IV, respectively. $\alpha$ was defined by the ratio of $\Delta A$ and $A$. The solid red line represents the fitted data, indicating an inverse squared relationship between the $Q_{rad}$ and $\alpha$. Additionally, the inset of (b) highlights the magnitude of $\Delta A$, denoted by the marked red region.
• Figure 4: (a) 3D schematic representation of ED, MD, and TD modes. (b) Scattering power contributions from individual multipole moments—including ED, TD, MD, EQ, and MQ—for the proposed structure at $\delta = 0~\text{nm}$, where the red-shaded region highlights resonant modes I and II. Spatial distribution of electric and magnetic fields in the x-y plane (c-d) at $\lambda = 1044~\text{nm}$ and (e-f) at $\lambda = 1217~\text{nm}$. White arrows indicate the orientation of the corresponding field vectors.
• Figure 5: (a) Transmittance spectra of the proposed structure, incorporating the rotational asymmetry (one triangular nanohole rotated from $0^\circ$ to $360^\circ$ as in the figure). (b) Scattering power contributions from individual multipole moments at the rotation of $180^\circ$. (c) Electric and magnetic field distribution profile at $\lambda = 1201~\mathrm{nm}$ at $180^\circ$.