Investigation of out-of-plane symmetry-protected bound states in the continuum in terahertz metamaterials.

Currently, significant progress has been made in the development of in-plane symmetry-protected BIC sensors, but research on out-of-plane symmetry-protected BIC sensors has received much less attention. In this work, we introduce an approach where the tilt angle of rectangular holes is adjusted to break vertical mirror symmetry, successfully exciting the out-of-plane BIC mode. Our results show that increasing the tilt angle enhances electric field confinement at the BIC resonance, while promoting radiation leakage and facilitating stronger coupling to free space. Further analysis reveals that the gold coating significantly enhances the quality factor for tilt angles smaller than 5 ° , without compromising the sensor's detection sensitivity. These findings highlight the potential of out-of-plane symmetry-broken BIC metamaterials as highly sensitive platforms for THz refractive index and thickness sensing, driving advancements in this emerging field. Investigation of out-of-plane symmetry-protected bound states in the continuum in terahertz metamaterials.