A surface plasmon polariton refractive index sensor based on Fano resonances in metal–insulator–metal (MIM) waveguides coupled with rectangular and ring resonators is proposed and numerically investigated using a finite element method. Fano resonances are observed in the transmission spectra, which result from the coupling between the narrow-band spectral response in the ring resonator and the broadband spectral response in the rectangular resonator. Results are analyzed using coupled-mode theory based on transmission line theory. The coupled mode theory is employed to explain the Fano resonance effect, and the analytical result is in good agreement with the simulation result. The results show that with an increase in the refractive index of the fill dielectric material in the slot of the system, the Fano resonance peak exhibits a remarkable red shift, and the highest value of sensitivity (S) is 1125 nm/RIU, RIU means refractive index unit. Furthermore, the coupled MIM waveguide structure can be integrated with other photonic devices at the chip scale. The results can provide a guide for future applications of this structure.

The core‐shell structured Co₃O₄‐PANI nanocomposites have been successfully prepared using an in situ polymerization method, while the core Co₃O₄ nanoparticles were synthesized by carbon‐assisted method using degreasing cotton as a template. The obtained samples were characterized by XRD, TEM, FTIR, and XPS. The results indicated that the amorphous PANI was well covered on the surface of the spinel Co₃O₄ and the Co₃O₄‐PANI with core‐shell structure was formed with particle size of about 100 nm. The interfacial interaction of the core‐shell nanocomposite greatly enhances the microwave absorption properties. The maximum reflection loss of Co₃O₄‐PANI is up to −45.8 dB at 11.7 GHz with a thickness of 2.5 mm and the adsorption bandwidth with the reflection loss below −10 dB reaches 14.1 GHz ranging from 3.9 to 18 GHz when the thickness is between 2 and 5.5 mm. Therefore, the facilely synthesized and low‐cost Co₃O₄‐PANI nanocomposite with superior microwave absorption properties can be a promising nanomaterial for high efficient microwave absorption.