| With the rapid development of information technology,photonic devices have obvious advantages in performance and integration compared with traditional electronic devices.As a way to realize optical field regulation in the subwavelength range,surface plasmon polaritons（SPPs）can overcome the classical diffraction limit of photonic devices at nanoscale,thereby enabling photonic devices to achieve better size and performance.Metal-insulator-metal（MIM）waveguides have become an important structure for the transmission and control of SPPs due to their simple structure,easy fabrication,and strong field enhancement properties.SPPs can be excited at the two metal-dielectric interfaces in the MIM waveguide and propagate along the waveguide.When SPPs is coupled into a suitable resonant cavity,the continuous broadband mode and discrete narrowband mode will be excited directly and indirectly,and the coupling between the two modes will generate the Fano resonance.The special optical effect has an obviously sharp and asymmetric spectral feature,which makes the Fano resonance extremely sensitive to the change in the refractive index of the medium,thus showing great value in the field of sensing applications.In this paper,three MIM waveguide-coupled resonator structures are designed based on SPPs,and the Fano resonances formed by them are used to realize the refractive index sensing function.The transmission characteristics of the structures are studied by Coupled Mode Theory and Finite element method,and the sensing performance is optimized.The specific research work is as follows:1.A miniature sensing structure composed of a semi-rectangular ring cavity and a MIM waveguide with a rectangular cavity is designed.The structure is numerically analyzed based on FEM,and the Fano resonances appearing in its transmission spectra is investigated by normalizing the magnetic field distribution.The influence of structural parameters on transmission characteristics is analyzed by adjusting the parameters.The results show that the characteristics of Fano resonance have different dependencies on different parameters,and the discrete narrowband mode has a significant impact on the sensitivity.The maximum sensitivity of the optimized structure reaches 3477 nm/RIU,and the corresponding figure of merit is 66.8.2.A novel nanosensing structure based on MIM waveguide coupled double ring resonator is designed.The energy field distribution in the resonator cavity was changed to optimize the sensing performance of the structure under the premise of maintaining compactness.Its sensitivity reaches 1885 nm/RIU with a corresponding quality factor of 77.Compared with the previously designed structure,the Fano resonance generated by this structure has a higher sensing resolution.Furthermore,the proposed derivative structure splits the resonance modes to form two higher-order narrowband modes,which bring additional tunable Fano resonances to the structure.The presence of multiple sensing channels increases the reliability of the measurements and expands the parallel processing capabilities of the structure.3.A coupling structure composed of a ring resonator with stubs and a MIM waveguide with stubs is designed.The formation mechanism of Fano resonance and the effect of stubs are studied by splitting the structure.The sensitivity is greatly improved by enhancing the interference between the stubs inside the structure,which reaches 2660 nm/RIU with a corresponding figure of merit of 66.5.The application of the structure in measuring physical quantities related to refractive index is discussed.The results show that when the structure is used to detect hemoglobin concentration,its sensitivity is the highest at 2.524 nm·L/g,and when it is used to detect temperature,its sensitivity is 0.831 nm/℃.The MIM waveguide coupled resonator structures proposed in this paper have the advantages of high sensitivity and small size,and provide a new design idea for the design of highly integrated nanosensor devices. |
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