Extreme Localization of Fields in Open Cylindrical Impedance Surface Cavities

Bound states in the continuum (BIC) as well as their approximations are highly localized modes, enabling extreme wave–matter interactions. While typically found in array type of structures, recent efforts have demonstrated versions of such states in cavity slabs formed by surface impedance sheets. However, such slab configurations are not suitable for free-space excitations due to large sizes of the underlying impedance surfaces (ISs). To alleviate these issues, the present work introduces an open cavity made of a single cylindrical IS for extreme field localizations. The overall structure behaves as a quasi-open cavity, enabled by overlapping resonances in the cavity and the ISs, effectively forming a quasi bound state in the continuum. A cavity formed of conductive strips of Jerusalem crosses is designed in the microwave part of the spectrum and analyzed by analytical and numerical means. The analytical solution, which assumes a homogeneous IS, predicts a quality factor ( $Q$ -factors) of $\sim 7\times 10^{10}$ , whereas the numerical $Q$ -factor is 3000 due to material losses and inhomogeneity of the modeled IS fully accounted for in the numerical model. The proposed resonator may find applications within filters, antennas, sensing, and nonlinear devices.