Coupled-Capillary-Based Sensitivity-Enhanced Higher Order Exceptional Point Microfluidic Refractive Index Sensor for Biochemical Applications

High-sensitivity property near higher order exceptional point (EP) of non-Hermitian systems provides a new approach to improve their eigenspectral sensitivity to external perturbations. Compared with the conventional whispering gallery mode (WGM) sensor, we propose a third-order EP (EP(3)) WGM microresonator system based on coupled-capillary configuration, which significantly improves the measurement sensitivity for biochemical applications due to the nonlinear frequency responses of the transmission spectrum in the vicinity of EP(3). By simultaneously changing the refractive indices of the three microcavities, it is possible to adjust the high-sensitivity refractive index (RI) measurement range. For the RI measurement ranges centered at 1.34, 1.40, and 1.44, respectively, the RI sensitivities at EP(3) reach <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.95 \times 10^{{4}}$ </tex-math></inline-formula> nm/RIU, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.61 \times 10^{{4}}$ </tex-math></inline-formula> nm/RIU, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.46 \times 10^{{4}}$ </tex-math></inline-formula> nm/RIU, respectively. Within an effective RI range of 1.3395–1.3405, which is applicable for most biochemical samples, the calculation results indicate that the EP(3) system exhibits an enhancement in RI sensitivity by 6.8 and 59.9 times in comparison with its EP(2) and diabolic point (DP) counterparts, respectively. Our proposed microfluidic EP(3) sensor provides an ideal candidate for applications in label-free and trace biochemical analytes owing to its high sensitivity, controllable RI measurement range, good compatibility with the existing fiber-optic structures, and microfluidic operation that supports real-time monitoring.