Real Frequency Splitting Indirectly Coupled Anti-Parity-Time Symmetric Nanoparticle Sensor

Detecting the size of a single nanoparticle with high precision is crucial to understanding the characteristic of the nanoparticle. Traditionally, a nanoparticle as a perturbation enters the mode volume of a cavity, leading to eigenfrequency splitting that is proportional to the perturbation strength. Here, we demonstrate the single particle detection based on the anti-parity-time (anti-PT) symmetric indirectly coupled whispering-gallery mode (WGM) cavities, which can be tailored to operate at exceptional points (EPs) where the two eigenvalues and the corresponding eigenstates coalesce. Due to the complex-square-root topology characteristic at the EPs, the frequency splitting exhibits a square-root dependence on the small perturbation. When operating the system at such an EP, the anti-PT symmetric WGM nanoparticle sensor exhibits giant enhancement (approximate to 3 orders) in frequency splitting compared with a traditional single WGM sensor with the same perturbation. With respect to the PT symmetric nanoparticle sensor, our research exhibits a real eigenfrequency splitting, which can be directly detected.