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 micro-resonator 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 micro-cavities, it is possible to adjust the high-sensitivity 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 6.95×10
⁴
nm/RIU, 6.61×10
⁴
nm/RIU, and 6.46×10
⁴
nm/RIU, respectively. Within an effective RI range of 1.3395 to 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 existing fiber-optic structures, and microfluidic operation that supports real-time monitoring.