Polarization-Robust and Electrically Tunable Quasi-Bound States in the Continuum in C4-Symmetric Terahertz Metasurfaces for Refractive-Index Sensing
- 1School of Physics, Nanjing University of Science and Technology, Nanjing 210094, China
Abstract
Polarization sensitivity and limited tunability remain important challenges for high-Q terahertz (THz) metasurfaces based on quasi-bound states in the continuum (q-BICs), particularly when geometric perturbations are introduced to enable radiative coupling. Here, we numerically demonstrate a graphene-integrated all-dielectric THz metasurface that combines high-Q q-BIC resonances, polarization robustness, electrical tunability, and refractive-index sensing through a C4-symmetric supercell design. Each metamolecule consists of two orthogonal amorphous-silicon strips, in which a width perturbation δ introduces local geometric asymmetry and enables the symmetry-protected states to couple to free space, while the four metamolecules related by successive 90° rotations preserve the global C4 rotational symmetry. Two q-BIC modes are supported, with their Q factors exhibiting the characteristic inverse-square dependence on δ. Multipolar analysis reveals that the lower-frequency q-BIC is dominated by magnetic-quadrupole radiation, whereas the higher-frequency mode is primarily governed by toroidal-dipole radiation and exhibits weaker overall radiative leakage. Owing to the C4-symmetric arrangement, the resonance spectra remain nearly invariant for normally incident linearly polarized THz waves over polarization angles from 0° to 90°. Integration of monolayer graphene further enables continuous electrical modulation of the resonance contrast and Q factor by varying the Fermi level from 0 to 0.7 eV, while inducing only minor resonance-frequency shifts. For bulk refractive-index sensing, the two q-BIC modes achieve sensitivities of 345 and 475 GHz/RIU, with maximum figures of merit exceeding 800 and approaching 104, respectively. These results provide a versatile strategy for integrating polarization-robust high-Q resonances, electrical reconfigurability, and high-performance refractive-index sensing in THz metasurfaces.
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BibTeX
@article{geng2026polarizationrobust,
title = {{Polarization-Robust and Electrically Tunable Quasi-Bound States in the Continuum in C4-Symmetric Terahertz Metasurfaces for Refractive-Index Sensing}},
author = {Mingyu Geng and Qinghua Wang and Yunjing Ji and Jiancheng Lai and Zhenhua Li},
journal = {Nanomaterials},
year = {2026},
month = oct,
doi = {10.3390/nano16211377},
url = {https://www.mdpi.com/2079-4991/16/21/1377},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Polarization-Robust and Electrically Tunable Quasi-Bound States in the Continuum in C4-Symmetric Terahertz Metasurfaces for Refractive-Index Sensing
AU - Mingyu Geng
AU - Qinghua Wang
AU - Yunjing Ji
AU - Jiancheng Lai
AU - Zhenhua Li
PY - 2026
DA - 2026/10/28
JO - Nanomaterials
DO - 10.3390/nano16211377
UR - https://www.mdpi.com/2079-4991/16/21/1377
AB - Polarization sensitivity and limited tunability remain important challenges for high-Q terahertz (THz) metasurfaces based on quasi-bound states in the continuum (q-BICs), particularly when geometric perturbations are introduced to enable radiative coupling. Here, we numerically demonstrate a graphene-integrated all-dielectric THz metasurface that combines high-Q q-BIC resonances, polarization robustness, electrical tunability, and refractive-index sensing through a C4-symmetric supercell design. Each metamolecule consists of two orthogonal amorphous-silicon strips, in which a width perturbation δ introduces local geometric asymmetry and enables the symmetry-protected states to couple to free space, while the four metamolecules related by successive 90° rotations preserve the global C4 rotational symmetry. Two q-BIC modes are supported, with their Q factors exhibiting the characteristic inverse-square dependence on δ. Multipolar analysis reveals that the lower-frequency q-BIC is dominated by magnetic-quadrupole radiation, whereas the higher-frequency mode is primarily governed by toroidal-dipole radiation and exhibits weaker overall radiative leakage. Owing to the C4-symmetric arrangement, the resonance spectra remain nearly invariant for normally incident linearly polarized THz waves over polarization angles from 0° to 90°. Integration of monolayer graphene further enables continuous electrical modulation of the resonance contrast and Q factor by varying the Fermi level from 0 to 0.7 eV, while inducing only minor resonance-frequency shifts. For bulk refractive-index sensing, the two q-BIC modes achieve sensitivities of 345 and 475 GHz/RIU, with maximum figures of merit exceeding 800 and approaching 104, respectively. These results provide a versatile strategy for integrating polarization-robust high-Q resonances, electrical reconfigurability, and high-performance refractive-index sensing in THz metasurfaces.
SN - 2079-4991
ER -