Quasi-BIC realized in a subwavelength volumetric split ring-based resonator

In recent years, resonant structures with quasi-bound states in the continuum (quasi-BICs) have significantly expanded the practical possibilities in optics and nanophotonics. A similar mode, the so-called supercavity mode, observed in single dielectric cylinders with high permittivity and low material losses, allows achieving extremely high quality (Q) factors. Resonators supporting quasi-BICs are also promising for applications in the radio frequency range. However, creating compact structures using high-permittivity materials at frequencies below 300 MHz is challenging. This study introduces a subwavelength (∼λ/13) volumetric structure composed of two arrays of coupled split ring resonators, with one array located inside the other, which provides a supercavity mode. The numerical Q factor of this mode is increased by approximately 100 times under lossless conditions and by about 1.5 times when accounting for material losses compared to that of non-interacting modes of the two arrays. The Q factor enhancement is confirmed experimentally by near-field measurements. The advantages of the proposed resonator include its hollow cavity, ease of fabrication, and frequency tunability within the radio frequency range.