Quasi-bound states in the continuum empowering transmissive terahertz narrowband filter with high Q and broad sideband free of interferential distortion

• Pan Zhou, Kaiyu Wu, and Qingkang WANG
• received 03/25/2024; accepted 04/25/2024; posted 04/26/2024; Doc. ID 524394
• Abstract: Most currently available terahertz (THz) narrowband filters contain metal and substrate, which introduces absorption loss and spectral fluctuations caused by Fabry-Pérot interference in substrates. To address these issues, we employ quasi-bound states in the continuum (BIC) for design and realization of a substrate-free all-dielectric THz transmissive narrowband filter. Under oblique incidence, the symmetry-protected BICs break and collapse into high-Q transmissive quasi-BIC modes, thereby achieving narrowband filtering. The filter not only minimizes energy loss, but also demonstrates a smooth filtering response without interferential spectral fluctuation associated with the substrate. An experimental high Q value of ~127 at 4.1 THz with a broad sideband of ~2.6 THz with transmittance below 10% is achieved.