Friedrich-Wintgen bound states in the continuum in a photonic and plasmonic T-shaped cavity: Application to filtering and sensing

Bound states in the continuum (BICs) in open cavities have attracted considerable attention in wave physics due to their ability to confine light and produce high-quality-factor resonances with promising applications for filtering and sensing. One of the most interesting types of BICs is Friedrich-Wintgen (FW) BICs, which result from destructive interference of two interacting modes belonging to the same radiation channel. Here, we investigate theoretically and experimentally FW BICs in a photonic and plasmonic T-shaped cavity made of two horizontal guides of lengths ${d}_{2}$ and ${d}_{3}$ coupled to a vertical stub of length ${d}_{1}$. We demonstrate that the necessary condition for obtaining BICs consists in taking the lengths of the two horizontal guides ${d}_{2}$ and ${d}_{3}$ commensurate. This BIC is a common mode of the guides of lengths ${d}_{2}$ and ${d}_{3}$, such as the electric field vanishes at their connection point with the stub of length ${d}_{1}$; this BIC is independent of ${d}_{1}$ and the infinite waveguide to which the whole cavity will be attached. We show that, depending on ${d}_{1}$, the FW BIC appears as the consequence of the interaction between two eigenmodes of the originally isolated cavity where the width of one mode vanishes giving rise to FW BIC, while the width of the second mode becomes broad. In addition, we show that by slightly deviating from the BIC condition, the latter transforms to either electromagnetically induced transparency (EIT) or reflection or Autler-Townes splitting (ATS) resonances. Both EIT and ATS effects are qualified as a transparency window between two transmission zeros, but with different physical origins. We exploit the Akaike's information criterion test to discern EIT from ATS and distinguish the regime where the EIT or ATS effect dominates. The theoretical results, obtained by means of the Green's function method, are validated both by experimental measurements using coaxial cables in the radiofrequency domain and numerical simulations using metal-insulator-metal plasmonic waveguides operating in the infrared domain. The sensitivity of the PIT (the plasmonic analogue of EIT) resonances to the dielectric inside the waveguides can be used to design a highly sensitive sensor, which makes it suitable for an on-chip optical sensing platform.