Plasmonic Mode Interference Effect Based Sensors

We propose and theoretically analyze a novel sensor based on plasmonic mode interference in a one-dimensional degenerate n-doped silicon core waveguide. The waveguide supports both, the symmetric- as well as anti-symmetric surface plasmon polaritons (SPPs), with a large propagation constant difference between them, drastically miniaturizing the probe size to $\sim$ 100 $\mu$ m. Our study reveals that the symmetric plasmonic mode has significant field localization in the sensing region as compared to the anti-symmetric plasmonic mode which has a large field localization in the substrate region. This makes the symmetric SPP considerably more suitable for bio/chemical sensing applications. The core mode projection technique with an optimized transverse offset between the lead-in waveguide and plasmonic waveguide has been used to couple appreciable power into the two SPP modes enhancing the extinction ratio of the transmission spectra. The estimated sensitivity of the sensor is $\sim$ 3400 nm/RIU over biologically relevant refractive indices. Our study demonstrates the effectiveness of plasmonic mode interference in designing highly sensitive bio/chemical sensors with miniaturized probe length through careful design considerations. We also discuss the effect of temperature cross-sensitivity on the performance of the sensor and have presented a sensitivity matrix-based approach for the simultaneous detection of two perturbations using a single sensor probe. We have shown that using this sensitivity-matrix approach, the error associated with the estimated variations in the perturbations is of the order of 10 $^{-4}$ to 10 $^{-3}$ , making it a powerful tool to estimate simultaneously varying perturbation parameters by tracking multiple resonances.