Dual-band polarization-insensitive toroidal dipole quasi-bound states in the continuum in a permittivity-asymmetric all-dielectric meta-surface

Multi-mode resonance of bound states in the continuum in dielectric metasurfaces

Bound states in the continuum (BIC) represent distinct non-radiative states endowed with infinite lifetime and vanishing resonance linewidth. Introducing asymmetric perturbation to the system can convert true BICs into high quality leaky modes which is useful in many photonic applications. Previously, such perturbation and resonance of interest is only limited to a single factor. However, different perturbations by unit cell gap, geometry and rotation angle result distinctive resonance modes. The combination of two perturbation factors can excite multi-mode resonance contributed from each asymmetric factor which coexist simultaneously; thus, the number of reflectance peaks can be controlled. In addition, we have carefully analyzed the electric field variations under different perturbation factors, followed by a multipolar decomposition of resonances to reveal underlying mechanisms of distinct resonance modes. Through simulations, we find that the introduction of multiple asymmetric perturbations also influences the metasurface sensitivity in refractive index sensing and compare the performance of different resonance modes. These observations provide structural design insights for achieving high quality resonance with multiple modes and ultra-sensitive sensing.

Efficient polarization-insensitive quasi-BIC modulation by VO2 thin films

Bound states in the continuum (BIC) offer great design freedom for realizing high-quality factor metasurfaces. By deliberately disrupting the inherent symmetries, BIC can degenerate into quasi-BIC exhibiting sharp spectra with strong light confinement. This transformation has been exploited to develop cutting-edge sensors and modulators. However, most proposed quasi-BICs in metasurfaces are composed of unit cells with Cs symmetry that may experience performance degradation due to polarization deviation, posing challenges in practical applications. Addressing this critical issue, our research introduces an innovative approach by incorporating metasurfaces with C4v unit cell symmetry to eliminate polarization response sensitivity. Vanadium Dioxide (VO2) is a phase-change material with a relatively low transition temperature and reversibility. Here, we theoretically investigate the polarization-insensitive quasi-BIC modulation in Si-VO2 hybrid metasurfaces. By introducing defects into metasurfaces with Cs, C4, and C4v symmetries, we enable the emergence of quasi-BICs characterized by strong Fano resonance in their transmission spectra. Via numerically calculating the multipole decomposition, distinct dominant multipoles for different quasi-BICs are identified. A comprehensive investigation into the polarization responses of these structures under varying directions of linearly polarized light reveals the superior polarization-independent characteristics of metasurfaces with C4 and C4v symmetries, a feature that ensures the maintenance of maximum resonance peaks irrespective of polarization direction. Utilizing the polarization-insensitive quasi-BIC, we thus designed two different Si-VO2 hybrid metasurfaces with C4v symmetry. Each configuration presents complementary benefits, leveraging the VO2 phase transition's loss change to facilitate efficient modulation. Our quantitative calculation indicates notable achievements in modulation depth, with a maximum relative modulation depth reaching up to 342%. For the first time, our research demonstrates efficient modulation using polarization-insensitive quasi-BICs in designed Si-VO2 hybrid metasurfaces, achieving identical polarization responses for quasi-BIC-based applications. Our work paves the way for designing polarization-independent quasi-BICs in metasurfaces and marks a notable advancement in the field of tunable integrated devices.

High-Q Fano resonances in all-dielectric metastructures for enhanced optical biosensing applications

Fano resonance with high Q-factor is considered to play an important role in the field of refractive index sensing. In this paper, we theoretically and experimentally investigate a refractive index sensor with high performance, realizing a new approach to excite multiple Fano resonances of high Q-factor by introducing an asymmetric parameter to generate a quasi-bound state in the continuum (BIC). Combined with the electromagnetic properties, the formation mechanism of Fano resonances in multiple different excitation modes is analyzed and the resonant modes of the three resonant peaks are analyzed as toroidal dipole (TD), magnetic quadrupole (MQ), and magnetic dipole (MD), respectively. The simulation results show that the proposed metastructure has excellent sensing properties with a Q-factor of 3668, sensitivity of 350 nm/RIU, and figure of merit (FOM) of 1000. Furthermore, the metastructure has been fabricated and investigated experimentally, and the result shows that its maximum Q-factor, sensitivity and FOM can reach 634, 233 nm/RIU and 115, respectively. The proposed metastructure is believed to further contribute to the development of biosensors, nonlinear optics, and lasers.

Quasi-BIC based all-dielectric metasurfaces for ultra-sensitive refractive index and temperature sensing

In this paper, an all-dielectric metasurface that measures refractive index and temperature using silicon disks is presented. It can simultaneously produce three resonances excited by a magnetic toroidal dipole, magnetic toroidal quadrupole, and electric toroidal dipole, in the THz region. Asymmetric structures are used to generate two quasi-bound states in the continuum (BIC) resonances with ultra-high-quality factors driven by magnetic and electric toroidal dipoles. We numerically study and show the dominant electromagnetic excitations in the three resonances through near-field analysis and cartesian multipole decomposition. The effects of geometric parameters, scaling properties, polarization angles, incident angles, and silicon losses are also investigated. The proposed metasurface is an excellent candidate for sensing due to the extremely high-quality factor of the quasi-BICs. The results demonstrate that the sensitivities for liquid and gas detection are Sl = 569.1 GHz/RIU and Sg = 529 GHz/RIU for magnetic toroidal dipole, and Sl = 532 GHz/RIU and Sg = 498.3 GHz/RIU for electric toroidal dipole, respectively. Furthermore, the sensitivity for temperature monitoring can reach up to 20.24 nm/°C. This work presents a valuable reference for developing applications in the THz region such as optical modulators, multi-channel biochemical sensing, and optical switches.

An all-dielectric metasurface based on Fano resonance with tunable dual-peak insensitive polarization for high-performance refractive index sensing

A symmetric all-dielectric metasurface based on silicon and GaAs is proposed and numerically studied. In the mid-infrared region, two Fano resonant peaks with a reflectance exceeding 90% are observed. By altering the geometric parameters of the metasurface, the wavelength location and quality factor (Q-factor) of the resonant peaks can be tuned. The highest Q-factors can be 9609.67 and 3476.33, respectively. The proposed metasurface structure for optical refractive index sensing shows high performance and is insensitive to the plane wave's polarization state. In the refractive index range of 1.00 to 1.10, the highest sensitivity and figure of merit (FoM) are 1901.34 nm RIU-1 and 2492.04 RIU-1, respectively. The highest sensitivity is 2248.57 nm RIU-1 and FoM is 977.64 RIU-1 in the refractive index range of 1.30 to 1.40. These research results will help improve and innovate related sensing technologies and devices.

Dual-band refractive index sensor with cascaded asymmetric resonant compound grating based on bound states in the continuum

We propose a cascaded asymmetric resonant compound grating (ARCG) for high-performance dual-band refractive index sensing. The physical mechanism of the sensor is investigated using a combination of temporal coupled-mode theory (TCMT) and ARCG eigenfrequency information, which is verified by rigorous coupled-wave analysis (RCWA). The reflection spectra can be tailored by changing the key structural parameters. And by altering the grating strip spacing, a dual-band quasi-bound state in the continuum can be achieved. The simulation results show that the highest sensitivity of the dual-band sensor is 480.1 nm/RIU, and its figure of merit is 4.01 × 10⁵. The proposed ARCG has potential application prospects for high-performance integrated sensors.

Asymmetric Cross Metasurfaces with Multiple Resonances Governed by Bound States in the Continuum

The bound state in the continuum (BIC) has paved a new way to achieve excellent localization of the resonant mode coexisting with a continuous spectrum in the metasurface. Here, we propose an all-dielectric metasurface consisting of periodic pairs of asymmetric crosses that supports multiple Fano resonances. Due to the sufficient degrees of freedom in the unit cell, we displaced the vertical bars horizontally to introduce in-plane perturbation, doubling the unit cell structure. Dimerization directly resulted in the folding of the Brillouin zone in k space and transformed the BIC modes into quasi-BIC resonances. Then, simultaneous in-plane symmetry breaking was introduced in both the x and y directions to excite two more resonances. The physical mechanisms of these BIC modes were investigated by multipole decomposition of the scattering cross section and electromagnetic near-field analysis, confirming that they are governed by toroidal dipole (TD) modes and magnetic dipole (MD) modes. We also investigated the flexible tunability and evaluated the sensing performance of our proposed metasurface. Our work is promising for different applications requiring stable and tunable resonances, such as optical switching and biomolecule sensing.

Multiple Fano resonances on the metastructure of all-dielectric nanopore arrays excited by breaking two-different-dimensional symmetries

We have designed and analyzed the high quality-factor (Q-factor), multiple Fano resonances device on the basis of the all-dielectric metastructure. The unit structure consists of two rectangular air holes etched within a silicon cube and periodically aligns on the substrate of silicon dioxide. The results demonstrate that four Fano resonances are achieved by integrating the theory of bound states in the continuum (BIC)and breaking the symmetry (width symmetry or depth symmetry) of two rectangle air holes, and the resonant wavelength can be modified by altering structural parameters. The sensing characteristics of the presented structure are studied. The sensitivity(S) of 304 nm/RIU, the maximal Q-factor of 2142 and the figure of merit (FOM) of 515.3 are achieved while width symmetry is broken. Meanwhile, the sensitivity of 280 nm/RIU, the maximal Q-factor of 2517 and the FOM of 560 are gotted through breaking depth symmetry. The proposed metastructures can be used for the lasers, biosensing and nonlinear optics.