Theory and sensitivity optimization of plasmo-photonic Mach-Zehnder interferometric sensors

We demonstrate a novel theoretical framework for refractive index Mach-Zehnder interferometric (MZI) sensors that can accurately calculate the sensor FSR and sensitivity while taking into account waveguide effective index dispersion and dip splitting effects. In contrast to the state-of-the-art mathematical equation that relates sensitivity with FSR, our analysis concludes to a mathematical expression that retains its validity and accuracy both in low and large FSR sensor layouts, suggesting its suitability for use in optimizing sensor performance even in the high-sensitivity, high-FSR configurations. This is validated by applying our theory to integrated plasmo-photonic MZI sensors with FSR values up to hundreds of nm, confirming our theoretical results through accurate numerical and circuit-level simulations and demonstrating how sensitivity can be boosted to >10⁵ nm/RIU values exploiting dispersion engineering of the waveguides. To this end, our analytical formula that relates sensitivity with FSR and waveguide effective index dispersion can lead to reliable designs and well-matched fabricated modules when targeting high FSRs and high sensitivity MZI photonic integrated sensors.