Surface-Based Sensor Detects Single Molecules

Optical sensing is one of the most important appli­cations of light science. It plays crucial roles in astronomy, environ­mental science, industry and medical diagnoses. Despite 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. The finger­print can be its trans­mission, reflection or absorption. The stronger these effects are, the stronger the response of the system.

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. (Source: R. El-Ganainy & Q. Zhong)

While this works well at the macro­scopic 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. Some of these techniques, which rely on complex quantum optics concepts and imple­mentations, have indeed proved useful, such as in sensing gravi­tational waves in the LIGO project. Others, which are based on trapping light in optical resonators, have succeeded in detecting micro-particles and relatively large biological components.

Nonetheless, the ability to detect small nano-particles and eventually single molecules remains a challenge. Current attempts focus on a special type of light trapping devices – microring or micro­toroid resonators – these enhance the inter­action between light and the molecule to be detected. The sensi­tivity of these devices, however, is limited by their fundamental physics. Now, physicists and engineers from Michigan Techno­logical Uni­versity, Pennsyl­vania State University and the Univer­sity of Central Florida 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 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,” said Ramy El-Ganainy, associate professor of physics, adding that the sensi­tivity neces­sitated clever tuning tricks in previous 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,” said graduate student Qi Zhong. 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. (Source: MTU)

Reference: Q. Zhong et al.: Sensing with Exceptional Surfaces in Order to Combine Sensitivity with Robustness, Phys. Rev. Lett. 122, 153902 (2019); DOI: 10.1103/PhysRevLett.122.153902

Link: Henes Center for Quantum Phenomena, Michigan Technological University, Houghton, USA

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