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 state in the continuum (BIC), as a novel eigenmode with infinitely high-quality factor, has received great attention in modern optical science. Mode coupling in dielectric metasurfaces opens possibilities in searching for robust BICs. Here, we discover multiple BICs in periodic dielectric resonators composed of a silicon rectangular bar and a silicon ring in one lattice. For the symmetry-protected BIC at-Γ point, a sharp electromagnetically induced transparency window can be formed by either tilting incident angle to induce the ‘bright-bright’ mode coupling, or by displacing the ring to generate the ‘bright-dark’ mode coupling. Besides, the coupling between two resonators leads to a new energy band in the dielectric metasurface. As a result, two off-Γ BICs are formed owing to avoided crossings with two energy bands, and another one belongs to the single-resonance parametric BIC. Thus, our coupled resonators possess superior abilities to judiciously engineer BICs via versatile physical mechanisms. Taking advantage exclusively of coupled resonators in dielectric metasurfaces provides fresh insights into the creation of both symmetry-protected and accidental BICs, which enables profound advancements in designing novel photonic devices.

Based on the ideas of modulation of multiple BICs, ultrahigh Q-factor resonance has been realized by array of planar metasurfaces with H-shaped nanoholes, perforated on a Si₃N₄ photonic crystal slab surrounded by a silica medium. Multiple BICs of both at Γ BICs and off Γ BICs are obtained. Two BICs at the Γ point show up in the lower band of the visible light, and they can be tuned by the center position and hole size of each part of the H-shaped, the refractive index of the surrounding medium, and the lattice constant. In detail, they move apart from each other or shift together with a fixed distance, with the structure parameters changing. On the other hand, the off Γ BICs behave only in the form of monotonously shifting. The at Γ BICs transform from BICs to quasi-BICs obviously with the broken symmetry of the H-shaped nanohole; however, its influence on off Γ BICs is insignificant. BICs in both types are ultra-sensitive to the surrounding medium, and the Q-factor of the BICs reaches a value more than 10⁶. The results here are helpful in the design of photonic filters and sensors.