Controlled comparison of phase-tuned exceptional and diabolic point sensing in integrated optical microresonators

Exceptional Points (EPs)—non-Hermitian degeneracies where both eigenvalues and eigenvectors coalesce—have attracted significant attention for their potential to enhance the sensitivity of optical microresonators. Unlike conventional diabolic points (DPs), EPs exhibit an nth-root response to perturbations, enabling superior detection capabilities. In this work, we experimentally demonstrate this advantage through a direct and controlled comparison between microring resonators (MRRs) engineered to operate at either an EP or a DP. The devices are carefully matched in loss rates to eliminate extrinsic variability. Localized perturbations of systematically varied strength are introduced via cylindrical holes in the waveguide, modulating backscattering-induced coupling of clockwise and counterclockwise MRR modes. A phase shifter is integrated to dynamically tune the phase relationship between intrinsic and perturbation-induced coupling coefficients, thereby optimizing sensitivity. This study presents a direct comparison between integrated MRRs operating at EPs and DPs, with uncertainties due to differing losses and phase relations explicitly eliminated. Our results establish a highly sensitive and reconfigurable photonic platform for advanced nanoscale sensing applications.