One of the essential applications of light science is optical sensing. It plays crucial roles in astronomy, environmental science, industry, and medical diagnoses.
In spite of the variety of schemes used for optical sensing, they all share the same principle: the quantity to be measured must leave a "fingerprint" on the optical response of the system. It is likely the fingerprint can be its transmission, reflection or absorption. The stronger these impacts are, the stronger the response of the system.
Though this dynamic works well at the microscopic level, measuring tiny, microscopic quantities that induce weak response is a challenging task. Researchers have developed techniques to overcome this difficulty and improve the sensitivity of their devices.
One of these techniques, which rely on complex quantum optics concepts and implementations, have indeed proved useful including in sensing gravitational waves in the LIGO project. Other methods that involve trapping light in tiny boxes called optical resonators have succeeded in detecting micro-particles and relatively large biological components.
However, the ability to detect small nano-particles and eventually single molecules remains a challenge. Recent attempts focus on a particular type of light trapping devices called microring or microtoroid resonators; these enhance the interaction between light and the molecule to be detected. However, the sensitivity of these devices is limited by their fundamental physics.
In the new study published in Physical Review Letters, engineers and physicists from Michigan Technology University, Pennsylvania State University and the University of Central Florida propose a new type of sensor. They are based on the original notion of exceptional surfaces that consist of exceptional points.
A physical system that exhibits an exceptional point is quite fragile. In other words, any small perturbation will dramatically alter its behavior. The feature makes the system highly sensitive to tiny signals.
Ramy El-Ganainy, associate professor of physics, said that despite this promise, the same enhanced sensitivity of exceptional point-based sensors is also their Achilles heel. These devices are quite sensitive to unavoidable fabrication errors and undesired environmental variations. El-Ganainy added that the sensitivity necessitated clever tuning tricks in previous experimental demonstrations.
The lead author of the paper and a graduate student who is currently working towards his doctorate degree at Michigan Tech, Qi Zhong, said that their current proposal alleviates most of these problems by introducing a new system that has the same enhanced sensitivity reported in previous work, while at the same time robust against the majority of the uncontrivable experimental uncertainty.
While researchers continue to refine the design of microring sensors, they are hopeful that through the improvement of the devices, seemingly tiny optical observations will have significant effects.
