Dual-polarization interferometric sensing for independent characterization of surface and bulk layers

Photonic integrated biosensors are a promising solution for biomarker detection in applications ranging from clinical diagnostics to food quality monitoring. However, their response is not only affected by molecular binding at the sensor surface, but also by bulk refractive index variations, background composition changes and temperature fluctuations. Most reported implementations cannot separate these effects, leading to inaccurate measurements. In this work, we present a fully integrated dual-polarization Mach-Zehnder interferometer with coherent detection, capable of distinguishing refractive index changes occurring at different distances above the waveguide surface, thereby enhancing sensor robustness. This is achieved through two separate measurements, one using the Transverse Electric (TE) mode and the other using the Transverse Magnetic (TM) mode. By exploiting their different evanescent field penetration depths and postprocessing the respective signals, we solve a system of equations to decouple surface and bulk contributions. Beyond refractive index sensing, this method could be extended to estimate additional parameters such as molecular layer thickness or temperature variations. The good agreement between simulation and experimental results confirms that the proposed sensor can effectively differentiate between contributions due to protein adsorption or biorecognition events within the 10 nm layer closest to the surface (surface effects) from bulk refractive index variations (background effects). To the best of our knowledge, this is the first demonstration of spatially resolved refractive index discrimination by an integrated photonic biosensor with coherent interrogation, highlighting its competitiveness against current state-of-the-art solutions.