The impact of contact and contactless interactions between the meta-atoms on terahertz bound states in the continuum

The excitation and manipulation of symmetry-protected bound states in the continuum (SP-BIC) is significantly valuable for metasurface-based biosensors. The interactions between adjacent meta-atoms determine the fundamental properties of SP-BIC; however, this topic has not been profoundly explored. In this work, we experimentally and numerically investigate the effects of contactless and contact interactions between adjacent dual-gap split-ring resonators (DSRRs) on the SP-BIC. We demonstrate that there is only one SP-BIC at 0.9 THz when the incident radiation polarization is parallel to the gap in both contactless and contact coupling conditions. When the polarization is vertical to the gap, the individual SP-BIC shifts the frequency to 0.8 THz under contactless coupling. Under contact coupling, the SP-BIC degrades to be an electromagnetically induced transparency (EIT) windows at 0.8 THz. We calculated a 3.6 ps group delay of slow light for EIT. Numerical simulations indicate that the combination of one magnetic dipole (MD) in the inner arm and another electric dipole in the outer arm of DSRR results in quasi-BIC at 0.9 THz and 0.8 THz under contactless coupling. Under contact coupling conditions, the formation of quasi-BIC at 0.9 THz is similar to contactless coupling. However, two MDs of opposite polarity results in the EIT windows at 0.8 THz. Our results reveal excitation and manipulation of terahertz SP-BIC via contactless and contact coupling, which is significant for the innovation of terahertz biosensors.