  Exceptional points, where eigenvalues and eigenvectors coalesce, impact the
behavior of different photonics components that show, e.g., enhanced sensing,
coherent perfect absorption, unidirectional lasing, and chirality. However,
only a few passive geometries have been developed that work on these points.
Here, we introduce a novel non-Hermitian structure based on a microresonator
shaped as the infinity symbol twice coupled to a bus waveguide: the
infinity-loop microresonator. Unlike other structures working on an exceptional
surface, the infinity-loop microresonator can achieve either high- or
low-contrast unidirectional reflection with a negligible or identical
reflection for counterpropagating light. It allows an easy walking through the
Riemann sheet by simply controlling the phase of the light propagating in the
bus waveguide, which makes it a tunable component to build more complex
topological structures. Furthermore, the infinity-loop microresonator allows
sensors that show the features of both an exceptional point device and a
diabolic point device simultaneously.
