Experimental study of Fabry–Pérot BICs in a microwave waveguide

We study Fabry–Pérot bound states in the continuum (FP-BIC) in the GHz frequency range in a metal-walled rectangular waveguide with two ceramic disks placed inside it. The disks act as perfect reflectors at the resonance frequency, and the energy becomes perfectly trapped between the disks, forming an FP-BIC when the distance between them satisfies the Fabry–Pérot quantization condition. We investigate the system both theoretically and experimentally, revealing how the total and radiative quality (Q) factor depends on the inter-disk distance. We gain valuable insights into the Fano features observed in the transmission spectra using the quasi-normal mode technique and temporal coupled-mode theory. Notably, we find that as the system approaches the BICs, the Fano asymmetry parameters diverge, resulting in a Lorentzian peak in the transmission spectrum. The experimentally measured radiative Q factor is on the order of 105, while the total Q factor, limited by material losses, remains around 103. These results open up new possibilities for utilizing BICs in microwave technology, with the potential to significantly enhance the performance of microwave devices.