Exceptional Points in a Spiral Ring Cavity for Enhanced Biosensing

The unique point of non-Hermitian degeneracy in an open system, known as exceptional point (EP), holds great potential in cavity quantum electrodynamics, optical signal processing, and sensing applications. For on-chip miniaturized optical cavities, auxiliary means have been employed for steering a non-Hermitian system to an EP, which typically requires either precision post-fabrication tuning or external off-chip components. Here we report a route of engineering non-Hermitian degeneracies in spiral ring microcavities without any external or temporary assistance. In such a deformed whispering gallery mode cavity which can be readily obtained through the self-rolling of pre-strained nanomembranes, the coupling strength of original non-degenerate modes can be fine tailored by spiral edge-induced backscatterings. Our theoretical results suggest that an EP can be reached by adjusting the physical scales and relative positions of inner and outer spiral edges. For EP-enhanced sensing, a functional regime upon a tiny perturbation where splitting scales as the square root of the perturbation and a saturated regime upon a sufficiently large perturbation are distinguished. For EPs in such a single-resonator system, the enhanced sensitivity towards localized perturbations and robustness towards homogeneous perturbations are desirable for single-molecule detections. We expect that this work will serve as a generic guideline for realizing compact EP-empowered sensing systems and provide new insights into ultra-sensitive bioanalyses.