Scaling the Sensitivity of Integrated Plasmo-Photonic Interferometric Sensors

We present a new optical biosensing integration approach with multifunctional capabilities using plasmonic and photonic components on the same chip and a new methodology to design interferometric biosensors exhibiting record high sensitivity and enhanced resolution relying on a planar surface plasmon polariton (SPP) waveguide. First, we use this approach to demonstrate a proof of concept integrated plasmo-photonic liquid refractive index sensor based on a silicon nitride (Si3N4) Mach–Zehnder Interferometer (MZI). A 70 μm long, gold metal stripe is incorporated in the sensing arm serving as the transducer element. A variable optical attenuator and a thermo-optic phase shifter are deployed in the Si3N4 reference arm for performance optimization. The variable optical attenuator stage targets high extinction ratio of the resonance at the interferometer output by balancing the power between the two arms whereas the phase shifter is used to tune the MZI at the desired spectral window. Experimental results matched well with numerical simulations showing bulk sensitivity up to 1930 nm/RIU and a resonance extinction ratio of 37 dB. We also provide a theoretical analysis for correlating the sensitivity performance of the sensor with its free spectral range (FSR). Based on this analysis, we propose optimized sensor designs and show that, by engineering the free spectral range of the sensor in the range of 600 nm, sensitivity may be boosted up to 60000 nm/RIU.