A differential phase sensor based on a narrowband perfect absorber uses the phase singularity at the point of darkness (100% absorption). Herein, a structure with a top SiO2 layer is proposed that is index matched with a glass flow cell that shows the point of darkness for both s‐ and p‐polarizations. Detailed numerical results to optimize the structure, study the role of fluctuation in thickness and roughness of analyte layer show that the proposed structure has refractive index sensitivity of 147.3° RIU−1. Experimental results of launch angle independent ≈99.9% absorption in the 45°–73° range are reported, with the highest reported till date quality factor in planar narrowband absorbers of ≈42, and high figure of merit of 556 for p‐polarization. The structure also exhibits the generalized Brewster effect with a perfect absorption (≈99.99%) for s‐polarization at ≈74° angle. Experimental results are presented to demonstrate the sensor operation for two different biomolecules at the p‐polarization resonance. The configuration presented is ideal for reusable and cost‐effective biosensors.