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.

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.

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.

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