Experimental Study of Fabry-Perot BICs in a Microwave Waveguide

TL;DR

This study investigates Fabry-Perot bound states in the continuum within a microwave waveguide, combining theoretical and experimental approaches to understand their properties and potential applications.

Contribution

It provides the first combined theoretical and experimental analysis of FP-BICs in microwave waveguides, revealing how Q factors depend on inter-disk distance and Fano resonance features.

Findings

Experimental radiative Q factor around 10^5

Total Q factor limited to about 10^3 by material losses

Fano asymmetry parameters diverge near BICs, leading to Lorentzian profiles

Abstract

We study Fabry-Perot bound states in the continuum (FP-BIC) in the GHz frequency range, formed by two ceramic discs placed inside a metallic-walled rectangular waveguide, that act as perfect reflectors at the resonant frequency. The energy becomes perfectly trapped between the discs, forming a FP-BIC, when the distance between them matches the Fabry-Perot quantization condition. We present both theoretical and experimental analyses to investigate how the total and radiative quality factors (Q factors) depend 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 transmission profile. Experimentally, we measure a radiative Q factor on the order of $10^5$, while the total Q factor, limited by material losses, remains around $10^3$. These results offer new opportunities for the application of BICs in microwave technology, significantly advancing the potential for high-performance devices.