| Rapid detection and sensing of biochemical molecule concentration and refractive index using optical sensors is a hot topic of current research.Surface Plasmon Polaritons(SPPs)are sensitive to refractive index,so the sensors based on the resonance effect of SPPs have high refractive index sensitivity,especially the Fano resonance generated by the dark mode and bright mode coupling,which has a sharp asymmetric line shape and can greatly improve the sensitivity and detection limit of sensing.In this paper,we design two types of Metal-Insulator-Metal(MIM)waveguides coupled with resonant cavities to generate Fano resonances,and use the Finite Difference Time Domain Method(FDTD)to analyze the mechanism of Fano resonances generated by these structures and study their sensing characteristics for alcohol concentration and temperature,using sensitivity and factor of merit(FOM)parameters to characterize the sensing characteristics,and the main research is as follows:A double-opening circular resonant resonator structure is proposed,where the split ring constitutes two resonant cavities that can generate light and dark modes simultaneously.The light field distribution inside the double-opening ring is simulated by numerical simulation,and the mechanism of Fano resonance generation is analyzed.To study the performance of the structure as a sensor,the sensitivity and FOM of the structure were calculated to be 840 nm/RIU and 15,respectively,with alcohol concentration and temperature sensitivity up to 0.77 nm/% and 0.446 nm/℃.Due to the limited spectral width of the bright mode in this structure,resulting in a wider Fano resonance peak,the structure was optimized on this basis by adding a double silver baffle in the waveguide to form an F-P resonator to generate the bright mode,which produced a sharper Fano resonance peak.The refractive index sensitivity and FOM of this structure are 1386 nm/RIU and 18,respectively,and the alcohol concentration and temperature sensitivity can reach 0.918 nm/% and 0.550nm/℃.In order to improve the coupling length,a bent waveguide was used to couple with the circular resonator.It was determined by calculation that the coupling length of 251 nm(corresponding to the circular angle of the circular resonator)was the critical coupling,and the intensity of the Fano resonance peak was greatly improved,and the sensitivity and FOM of the optimized structure were 1400 nm/RIU and 19,respectively,and the sensitivity of alcohol concentration and temperature could reach 0.918 nm/% and-0.572 nm/℃,0.572 nm/℃.To effectively utilize the spectral bandwidth and improve the device integration,a rectangular resonant resonator is used to generate the dark mode.First,the Fano resonance characteristics in a single rectangular resonator are analyzed.The dark mode is generated with three rectangular resonant cavities of different lengths to obtain the triple Fano resonance,while the resonant cavities with larger deviations from the silver baffle produce weaker Fano resonances.In order to improve the coupling between the light and dark modes,an "E" shaped resonator is constructed.Four dark modes are generated in the "E" resonator,three of which can produce Fano resonance,and the mechanism of Fano resonance is analyzed.After optimizing the structure,the sensitivity and FOM of the "E"-shaped resonator sensor are 1380nm/RIU and 18,respectively,and the sensitivity of alcohol concentration and temperature can reach 0.936 nm/% and 0.568 nm/℃.This study is important for the MIM structure to generate tunable Fano resonance peaks. |
