Bound States in the Continuum in Cylindrical Impedance Surface Cavities

Bound states in the continuum are highly localized modes, which are typically found in periodic photonic structures as well as multilayer structures utilizing exotic materials. However, recent efforts have demonstrated versions of such states in single dielectric and metallic resonators, as well as in cavity slabs formed by surface impedance sheets. Although the latter represents a simple configuration with many interesting properties, it is not suitable for free-space excitations due to large sizes of the underlying impedance surfaces. Presently, we investigate open cavities made of single cylindrical impedance surfaces for extreme field localizations. When the resonances in the cavity and the impedance surface overlap, a bound state in the continuum emerges. The resonant properties are investigated analytically and numerically showing large quality factor and great field localization. The proposed resonator may find applications within filters, antennas, sensing and nonlinear devices.