## Abstract

A refractive index (RI) sensor using a silicon-on-insulator hybrid plasmonic waveguide (HPWG) difference interferometer is proposed. We first provide a rigorous analysis that enables the construction of the modal characteristic equations of the TM- and TE-polarized modes in a HPWG. These equations are solved to get the modes’ complex effective indices in addition to their electromagnetic power distributions. The feature of polarization spatial diversity in HPWGs is then exploited to design a difference interferometer with a superior sensing capability. Our results reveal that the proposed device has a lower limit of detection of $1.32 \times {10^{- 6}}$ refractive index unit (RIU) and a sensitivity of $756.2\pi \;{\rm{rad}}/{\rm{RIU}}$ at a wavelength of 1550 nm, accounting for a greater than 50% improvement to the sensitivities of recently published all-dielectric interferometers. Moreover, the sensing interactive length in this work is shown to be 1396 µm, acquiring more than a threefold reduction compared to those reported in the literature. In addition, the propagation loss is reduced by more than $5 \times {10^2}$ times compared to its value in the case of a surface plasmon interferometer, which is found to enhance the detectable power level by 13.6 dB. Furthermore, the proposed HPWG difference interferometer is investigated with a broadband interrogation. Our findings come out with a high density of local extrema of the light interference intensity, for a short sensing interactive length, in the spectral range of interest (from 1490 to 1610 nm), which is favorable for accurate statistical analysis of the interference intensity signal.

© 2021 Optical Society of America

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