A refractive index sensor with a free spectral range that is unlimited by neighboring mode spacing (10 fold increase with respect to 20 nm of an unmodified ring), based on an optical silicon-on-insulator microring resonator patterned with periodically arranged set of gold nanodisks, is presented and numerically verified. It is shown that the particular periodic arrangement of nanodisks selects a single resonance from a wide set of ring resonator modes and removes mode splitting. Extraction of the waveguided electromagnetic energy into evanescent plasmonic modes enhances light-analyte interaction and increases device sensitivity to variation of refractive index up to 176  nm/RIU (about 2-fold increase compared to the unmodified ring), which is useful for sensor applications. Proof of the concept is presented by finite-difference time-domain simulations of a design readily practicable by means of modern nanotechnology.

We present the experimental and numerical analysis of a microring resonator with an integrated one-dimensional photonic crystal fabricated on a silicon-on-insulator platform and show its applicability in bulk refractive index sensing. The photonic crystal is formed by periodically patterned, partially etched cylindrical perforations, whose induced photonic bandgap is narrower than the range of measurable wavelengths (1520-1620 nm). Of particular interest is that the microring operates in both air and dielectric bands, and the sensitivities of the resonances on both edges of the bandgap were investigated. We showed that a higher field localization inside the volume of the perforations for the air band mode leads to an increase in sensitivity.

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We suggest that implementation of a circular element second order Bragg grating 1D photonic crystals into the inner-wall of silicon-on-insulator microring resonators increases the light-matter interaction strength and device free spectral range. Introduction of a specifically tailored grating changes the quality factor of a selected resonance and modulates the losses of the system, leading to the presence of longitudinal resonant air and dielectric Bloch modes. This phenomenon can be harnessed for the development of a self-referenced sensor that is immune to changes in ambient temperature and is well suited for both biomolecule and hydrogen gas exposure sensing. The recent results on numerical modeling, lithographic fabrication, and characterization of microring resonators with circular elements on inner-wall are presented.

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