Polarization-normalized differential Fano response in phosphorene metasurfaces for ultra-highly sensitive sensing

We propose a phosphorene-integrated metasurface enabling an ultra-highly sensitive refractive-index sensing scheme based on a polarization-normalized differential Fano response. Unlike conventional Fano-resonant sensors relying on single-polarization excitation of an isolated resonance, the strong in-plane anisotropy of phosphorene provides two intrinsically correlated polarization channels that selectively excite a pair of distinct Fano modes. By introducing a controlled aspect-ratio offset in a phosphorene tetramer, the two resonances are driven toward near spectral degeneracy and converted into a single ultra-narrow differential spectral feature through polarization normalization. This correlated-polarization readout effectively suppresses common-mode intensity drifts, yielding a <i>Q</i>-factor that exceeds 1.97×10⁵ and a figure of merit of 1.76×10⁴, offering great potential of anisotropic 2D materials for correlated-polarization photonic readout.