Chiral BIC can reach ultrahigh quality factors (Q-factor) based on its asymmetry, with broken mirror symmetries and in-plane inversion. Only by in-plane structural perturbation can chiral quasi-BIC (q-BIC) appear, so it is much more realizable and reasonable for the manufacturers in practical productions and fabrications considering the technology and means that are available. In this paper, we design a new dielectric metasurface employing H-shaped silica meta-atoms in the lattice, which is symmetrical in structure, obtaining chiral BIC with ultrahigh Q-factor (exceeding 10⁵). In this process, we change the length of the limbs of the structure to observe the specific BICs. Previous scholars have focused on near-infrared-wavelength bands, while we concentrate on the terahertz wavelength band (0.8–1 THz). We found that there is more than one BIC, thus realizing multiple BICs in the same structure; all of them exhibit excellent circular dichroism (CD) (the maximum value of CD is up to 0.8127) for reflectance and transmittance, which provides significant and unique guidance for the design of multi-sensors. Meanwhile, we performed temperature sensing with chiral BIC; the sensitivity for temperature sensing can reach 13.5 nm/°C, which exhibits high accuracy in measuring temperature. As a consequence, the result proposed in this study will make some contributions to advanced optical imaging, chiral sensors with high frequency and spectral resolution, optical monitoring of environmental water quality, multiple sensors, temperature sensing, biosensing, substance inspection and ambient monitoring and other relevant optical applications.

Bound states in the continuum (BIC) with high quality factor (Q-factor) and narrow bandwidth can manipulate electromagnetic waves and enhance light-matter interactions. This phenomenon has garnered significant attention in the field of photonics, particularly in the context of all-dielectric metamaterials and metasurfaces. Recently, the introduction of BIC into chiral structures has shown that upon circularly polarized light irradiation of metasurfaces with chiral asymmetry, BIC leaks into chiral quasi-BIC (Q-BIC) and induces near-unity circular dichroism (CD). This has broad applications in fields including efficient nonlinear spin optics and chiral biosensing. Here, we numerically demonstrate in the terahertz band that simultaneous breaking of in-plane inversion symmetry and mirror symmetry in monolayer and bilayer cross-shaped structures can lead to Q-BICs with strong chirality, and achieve near-unity CD with high Q-factor. Especially, the bilayer metasurface can realize two Q-BICs in the considered frequency band without introducing complexity of the pattern in each layer. It has been demonstrated that there is a positive correlation between the number of chiral Q-BICs and the layers, which contributes to enhancing the modulation depth of the chiral optical response. Besides, the CD of the two Q-BIC modes in the bilayer metasurface can be flexibly regulated with more freedoms by varying the chiral asymmetry parameters in each layer and the distance between the layers. Additionally, the chiral response induced by Q-BIC in the bilayer structure exhibits a more significant CD and higher Q-factor in both operational modes, indicating higher sensitivity. For example, the figure of merit (FOM) reaches of 357 RIU-1 and 914 RIU-1 for Mode-I and Mode-II in bilayer metasurface, compared to only 85 RIU-1 for the Q-BIC mode in the monolayer metasurface. This work demonstrates a viable technique for controlling terahertz waves on chiral bilayer metasurface, which is expected to enable the practical application of terahertz chiral sensors with high spectral resolution.

Bound state in the continuum (BIC) as a novel non-radiating state of light in the continuum of propagating modes has received great attention in photonics. Recently, chiral BICs have been introduced in the terahertz regime. However, strong chiroptical effects of transmitted waves remain challenging to achieve in metallic terahertz metasurfaces, especially for intrinsic chirality at normal incidences. Here, we propose a chiral quasi-BIC by simultaneously breaking the out-of-plane mirror and in-plane C₂ rotation symmetries in a bilayer metallic metasurface, in which spin-selective terahertz transmittance is successfully realized. Benefiting from the symmetry-protected nature of our proposed BIC, precise tuning of structural parameters can lead to anticipated chiroptical performance. As a degree of freedom, the rotation angle of the split ring gaps can fully determine the handedness, linewidth, and working frequency with strong circular dichroism. Besides, the sensing performance shows a surrounding refractive index sensitivity of 200 GHz/RIU, which is similar to those of previous works based on terahertz metasurfaces. Taking advantage exclusively of symmetry-protected BICs to realize transmitted terahertz chiroptical response provides fresh insights into the creation of novel BICs, which enables profound advancements in the surging field of novel terahertz devices.

Bound states in the continuum (BICs) with high quality factors (Q-factors) by confining resonances embedded in a continuous spectrum by eliminating radiation loss can boost substantially light-matter interaction for various applications in ultrathin, active, and nonlinear metadevices. Regulating and improving the modulating freedom of the BICs are important in tailoring the spectra lineshape. Here, we suggest a way to manipulate the number of BICs in the spectra by controlling the layers and the shape or arrangement of the rectangular nanoholes in each layer of the metasurface, illuminated by a p-polarized plane wave. It is convenient to achieve symmetry protected multiple at-Γ BICs and multiple off-Γ BICs associated with the decoupling of the radiation waves and improve their regulation depth only by adjusting the layers and one single nanohole arrangement and shape in one lattice in each layer, without introducing extra geometric complexity of the metasurface. Additionally, we demonstrate the multiple BICs are robust and have a good tolerance to fabrication imperfections. Beyond the multiple BICs of both types, we also explore the huge sensitivity around the BICs of the metasurface on the surrounding medium refractive index. Multiple BICs with high Q-factor realized in this multi-layer metasurface promise various practical applications including ultra-high sensitivity sensors and filters.

All-dielectric metasurface with ultra-high quality resonances underpinned by bound states in the continuum (BICs) have attracted lots of attention in recent years for they enable new methods of wavefront control and light focusing. We study a metasurface composed of one transverse nanohole (TNs) and two identical vertical nanoholes (VNs) in one lattice, which supports both symmetry-protected and accidental BICs (at-Γ and off-Γ BICs). Based on the destructive interference between the surface states from the TN element and the identical VNs element, two at-Γ BICs emerge, and they turn into quasi-BICs by rotating the electric field polarization direction of the incident plane wave from x to y. The off-Γ BICs come from destructive interference from different radiation channels, which are influenced by the in-plane structural parameters symmetry insignificantly. Two at-Γ BICs and one off-Γ BIC of the metasurface all have ultra-high Q-factors (exceeding 10⁶, 10⁴, and 10⁶, respectively), which means much in the application of biosensors. Especially, this nanostructure has outstanding ultra-slow light properties at BICs, with a group index about 10⁶, which underpin a new generation of flat-optics slow light devices.