Physicists Propose a New Type of Sensor for Exceptionally Sensitive Detection

The sensitivity of microring sensors can be improved by modifying their design and without introducing any additional implementation complications.

An exceptional surface-based sensor. The microring resonator is coupled to a waveguide with an end mirror that partially reflects light, which in turn enhances the sensitivity. (Image credit: Ramy El-Ganainy and Qi Zhong)

One of the most significant applications of light science is optical sensing, which plays vital roles in medical diagnoses, astronomy, industry, and environmental science.

In spite of the numerous strategies utilized for optical sensing, they all have one common principle—the amount to be determined should leave a “fingerprint” on the system’s optical response. The fingerprint can be its absorption, reflection, or transmission. Stronger effects translate to a stronger response of the system.

Although this works suitably at the macroscopic level, determining very small, microscopic amounts that promote weak response is a difficult operation. In this regard, scientists have created a number of methods to overcome this challenge and enhance the sensitivity of their devices. A few of these methods, which depend on intricate concepts and implementations of quantum optics, have been certainly shown to be useful, for example, in perceiving gravitational waves in the LIGO project. Others, which are predicated on capturing light in small boxes known as optical resonators, have succeeded in identifying tiny particles and comparatively bulk biological components.

Nevertheless, the potential to detect tiny nanoparticles and ultimately individual molecules continue to pose a problem. Present attempts concentrate on a unique kind of light trapping devices known as microtoroid resonators or microring resonators—devices that improve the interaction between the molecule to be detected and light. However, the sensitivity of these kinds of devices is somehow restricted by their fundamental physics.

Now, in their article titled, “Sensing with Exceptional Surfaces in Order to Combine Sensitivity with Robustness” that appeared in Physical Review Letters, engineers and physicists from Michigan Technological University, the University of Central Florida, and Pennsylvania State University suggested a unique kind of sensor. The sensors are predicated on the novel concept of excellent surfaces—surfaces that contain exceptional points.

Exceptional Points for Exceptionally Sensitive Detection

In an effort to figure out the connotation of exceptional points, let us consider an imaginary violin that has just a pair of strings. Generally, a violin like that will be able to create only two varied tones—a circumstance that corresponds to a traditional optical resonator. If the vibration of a single string is able to modify the vibration of the other string in a way that the elastic oscillations and sound produce only one collective string motion and one tone, the system is said to have an exceptional point.

A physical system exhibiting an exceptional point is extremely delicate. To put this in simple terms, any slight perturbation will considerably change its behavior. The aspect renders the system highly responsive to small signals.

Despite this 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.

Ramy El-Ganainy, Associate Professor, Physics, Michigan Technological University

El-Ganainy added that the sensitivity called for ngenious tuning tricks in earlier experimental demonstrations.

Our 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.

Qi Zhong, Study Lead Author and Graduate Student, Michigan Technological University

At present, Zhong is working towards his doctorate degree at Michigan Tech.

While the design of microring sensors continues to be improved further, scientists believe that device enhancement will allow apparently small optical observations to have huge effects.

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