Physicists and engineers propose a new type of sensor. They are based on the new notion of exceptional surfaces: surfaces that consist of exceptional points.
In order to understand the meaning of exceptional points, consider an imaginary violin with only two strings. In general, such a violin can produce just two different tones — a situation that corresponds to a conventional optical resonator. If the vibration of one string can alter the vibration of the other string in a way that the sound and the elastic oscillations create only one tone and one collective string motion, the system has an exceptional point.
A physical system that exhibits an exceptional point is very fragile. In other words, any small perturbation will dramatically alter its behavior. The feature makes the system highly sensitive to tiny signals.
Despite the promise, the same enhanced sensitivity of exceptional point-based sensors is also their Achilles heel: These devices are very sensitive to unavoidable fabrication errors and undesired environmental variations. The sensitivity necessitated clever tuning tricks in previous experimental demonstrations. The new system is robust against the majority of the uncontrivable experimental uncertainty.
Though the design of microring sensors continues to be refined, researchers are hopeful that by improving the devices, seemingly tiny optical observations will have large effects.