Exceptional points in microdisk cavities with bound states in the continuum

In the past few decades, bound states in the continuum (BICs) have been intensively studied due to their superiority in suppressing radiation losses, highlighting potential applications in lasers, imaging, and sensors. Up to now, BICs have been theoretically and experimentally verified in microdisks, waveguides, and metasurfaces, which offer new perspectives to explore novel physics in such configurations. Among these preliminary research studies, microdisks based on Friedrich–Wintgen BICs (FW-BICs) show unprecedented properties, which are not accessible in the conventional system, allowing the existence of bound states in configurations possessing structural symmetry and mode interference or structures with a lower refractive index than that of the surrounding. However, compared to thoroughly explored cavity physics for conventional microdisks, only the merits of high quality and etchless fabrication procedures in BIC microdisks are confirmed, and further explorations are still absent, significantly limiting the application potential of these structures. In this work, we investigate a new kind of singularity by merging two FW-BICs into one. The resultant state not only exhibits enhanced sensitivity to perturbations, but also shows totally suppressed radiations. We believe that our findings illustrate an effective way to design devices with novel functions and expand the application potential of FW-BIC microdisks.