Fano-Assisted Tunable X-Band Microwave Ring Resonator

Control of the resonance frequency of microwave cavities is of great interest in many applications such as sensing and wavelength filtering. However, this task normally relies on active control via functional materials (i.e., polymers and ferrites, or a combination of both) excited by an external electric or magnetic field. In this article, instead, we propose an innovative whispering gallery mode cavity resonator with tunable bandwidth for sensing and wavelength filtering applications. The proposed approach exploits Fano-type resonances (FTRs) to improve the cavity performance in the microwave regime. We have measured a ~74% increase in the loaded quality factor compared to regular ring cavities, with only 1.9-dB additional loss. In addition, by carefully choosing the size of the transmitting and receiving monopole antennas placed inside the cavity and their relative angular spacing, we can easily tune the cavity response. The extra degree-of-freedom introduced by such variable angular spacing, responsible for the onset of FTRs, allows the cavity quality factor Q to be continuously tuned from 385 to 670.