Permittivity-asymmetric qBIC metasurfaces for refractive index sensing

TL;DR

This paper introduces a permittivity-asymmetric qBIC metasurface that enhances refractive index sensing by encoding environmental changes directly into asymmetry, leading to improved linearity, sensitivity, and robustness over traditional geometric asymmetry designs.

Contribution

The study presents a novel {ta}-qBIC design that integrates environmental permittivity into the asymmetry factor, enabling more effective and linear RI sensing and environmental permittivity restoration of high-Q BIC states.

Findings

Achieved transmittance sensitivity of ~5300%/RIU under single-wavelength conditions.

Demonstrated a 104-fold increase in linear response window compared to geometric asymmetry qBIC.

Numerical results show environmental permittivity asymmetry can restore ultra-high-Q BIC states.

Abstract

Bound states in the continuum (BICs) provide exceptional light confinement due to their inherent decoupling from radiative channels. Small symmetry breaking transforms BIC into quasi-BIC (qBIC) that couples to free-space radiation enabling ultra-high-quality-factor (Q-factor) resonances desirable for refractive index (RI) sensing. In practical implementations, geometric asymmetry is typically employed. However, since the radiative loss remains fixed once fabricated, such metasurfaces exhibit only a horizontal translation of the resonance spectrum in RI sensing, without modification of its overall shape. Here, we demonstrate a permittivity-asymmetric qBIC ({\epsilon}-qBIC) metasurface, which encodes environmental refractive index variations directly into the asymmetry factor, resulting in indexes response involving both resonance wavelength shift and modulation variation. In addition to exhibiting a competitive transmittance sensitivity of ~5300%/RIU under single-wavelength conditions, the {\epsilon}-qBIC design provides a substantially improved linear response. Specifically, the linear window area of its sensing data distribution, calculated as the integrated wavelength region where the linearity parameter remains above the preset threshold, is 104 times larger than that of the geometry-asymmetric qBIC (g-qBIC), enabling more robust and reliable single-wavelength signal readout. Additionally, numerical results reveal that environmental permittivity asymmetry can optically restore the g-qBIC to a state with ultra-high-Q (over 10^7), approaching to BIC condition. Unlike traditional BICs, which are typically inaccessible once perturbed, the permittivity-restored BIC becomes accessible through environmental perturbations. These findings suggest an alternative design strategy for developing high-performance photonic devices for practical sensing applications.