Theoretical investigation of an ultra-sensitive angular refractive index sensor based on compound parabolic micro-funnel arrays.

A high-precision, angular-selective refractive index (RI) sensor based on an array of Ag-coated parabolic hexagonal micro-funnels is proposed and numerically demonstrated. Leveraging étendue conservation in a micro-scale compound parabolic concentrator (CPC) geometry, the device achieves a remarkably sharp angular transmission cutoff. Infiltrating the cavities with liquid analytes induces a highly linear angular shift of this cutoff. We demonstrate a high angular sensitivity of 235.26 deg/RIU (R2 > 0.999) and a detection limit of 4.25 × 10-6 RIU. Unlike resonance-based sensors, this geometric-confinement-induced mechanism fundamentally bypasses the constraints of narrow spectral bandwidths and polarization dependence, offering a robust, broadband platform for trace fluidic sensing. The proposed design is compatible with established micro-fabrication techniques, providing a concrete route toward future experimental validation.