Vernier Ellipsometry Sensing with Ultralow Limit-of-Detection and Large Dynamic Range by Tuning of Zero-Reflection Points

Optical sensors using zero-reflection points (ZRPs) enable excellent sensitivity due to the accompanying phase singularities and the steepest slope of the reflectivity curve. Here, the collaborative manipulation of three ZRPs in a simple platform formed by a lithography-free, metal-dielectric-metal structure with unsurpassed, experimentally demonstrated, limit of detection ≈2 × 10⁻⁸ refractive index unit is reported. The sensor relies on: i) strong coupling between p-polarized surface plasmon polariton and photonic waveguide, leading to reflection suppression, Rabi splitting and phase singularities; ii) simultaneous implementation of two orthogonally polarized ZRPs, enabling spectral overlap of s-polarized photonic modes (R_s) with the coupled p-polarized resonances (R_p); and iii) ellipsometry-based sensing where the s-polarized ZRPs provide a stable reference to boost the sensor performance in terms of the amplitude ratio and phase difference of R_p and R_s thereby naturally forming a refinement measuring scale akin to a Vernier scale. Remarkably, the precise manipulation of ZRPs enables resetting the sensor to its optimal sensing point. The capability has been demonstrated for a biosensor of SARS-CoV-2 spike (S2) protein that can track the full functionalization process then reset to perform dose-dependent detection of the S2 protein. This work provides a new strategy for the development of optical sensors and perfect light absorbers.