Enhancing the sensitivity in measuring nanoparticle sizes

Rapid detection of nanoscale particles on chips with ultrahigh sensitivity has great significance for applications such as atmospheric environmental monitoring and medical diagnosis. Li et al investigate an integrated nanoparticle sensor that implements a condition of anti-parity-time (anti-PT) symmetry via two whispering-galley mode (WGM) microcavities indirectly coupled through two waveguides.

The ability to detect individual nanoparticles with high precision is crucial for sensing applications. In current measurement techniques, a nanoparticle would cause a perturbation as it enters the mode volume of a cavity, which leads to a split in the eigenfrequency that is proportional to the strength of the perturbation. The authors explore a different way for detecting single nanoparticles based on anti-PT symmetry, implemented by utilizing two indirectly coupled WGM microcavities, whose coupling can be tailored to operate at exceptional points (EPs) where the two eigenvalues and eigenstates coalesce.

“The anti-PT symmetric solution is more suitable for single nanoparticle detection in experiments because it exhibits an entirely real frequency splitting, which is different from the parity-time symmetric work,” said author Hao Zhang.

Their method utilizes the modal coupling between the clockwise and anticlockwise modes in the microcavity induced by the scattering of the nanoparticle, which would lead to mode broadening or splitting in the transmission and reflection spectra. Based on their calculations, the authors concluded that the configuration operating at the EP would show a significant sensitivity enhancement, achieving an eigenfrequency splitting of three orders of magnitude larger than a single WGM sensor.

“With advanced nanofabrication technologies, on-chip microcavity sensors operating at EPs are feasible,” said Zhang. “Our work provides a new method for exploring EPs for nanoparticle sensing applications in photonic systems.”

Source: “Real frequency splitting indirectly coupled anti-parity-time symmetric nanoparticle sensor,” by Wenxiu Li, Hao Zhang, Peng Han, Xiaoyang Chang, Shuo Jiang, Yang Zhou, Anping Huang, and Zhisong Xiao, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0020944 .