High-sensitivity refractive index sensing with the singular phase in normal incidence of an asymmetric Fabry-Perot cavity modulated by grating

Considerable attention has been paid to the conception of point of darkness for perfect optical absorption and refractive index biochemical sensing based around phase-sensitive detection, in which the maximum change in the phase could occur at the point of darkness where the reflectivity is almost zero. To date, this has been achieved by using different material systems and complex nanopatterning under oblique-incidence conditions. In this study, a simple system consisting of an asymmetric Fabry-Perot cavity covered by a gold grating on the topside was developed to achieve perfect absorption and extreme singularity of the differential phase of reflective light under normal incidence conditions. The proposed structure was also demonstrated to function as a refractive index sensing platform based on a singular phase. It exhibits a maximum phase sensitivity of approximately 18,900 degree/RIU with a wide dynamic range during operation. This provides a more efficient alternative to the conventional oblique-incidence phase measurement system in an integrated biosensor chip.