| Fano resonance is a special form of resonance.Due to its unique optical characteristics,it is widely used in metasurface sensing,Raman and absorption.Optical Fano resonance is generated by the coupling of continuous resonance state and discrete resonance state.The most common way to generate Fano resonance in metasurfaces is to break the symmetry of the structure.When the symmetry of the nanostructure is broken,a high-order resonance state will be generated,and then the higher-order resonance state will couple with the localized surface plasmon resonance of the nanostructure to generate Fano resonance.However,this kind of structure requires fine control of the nanostructure and has a limit on the polarization of incident light.The longitudinal coupling structure of the metal nanostructure array and the Fabry–Pérot(F-P)resonant cavity is another structure that can generate Fano resonance.Compared with the asymmetric structure,the longitudinal coupling structure is more flexible in terms of design and regulation of incident light.Based on this longitudinal coupling structure,two Fano resonancebased metasurfaces are designed for surface enhanced Raman scattering(SERS)and refractive index sensing,respectively,and the properties of the metasurface are analyzed through simulation and theoretical calculation.In terms of SERS,the appearance of the dual resonance substrate promotes the development of surface enhanced Raman scattering.By controlling the frequency of the dual resonance to match the excitation and Raman scattering frequencies,the Raman signal can be enhanced more effectively.For the double-resonance substrate,while the resonance frequencies can be highly controllable,the electric field enhancement is also one of the important factors affecting the application in SERS.In this paper,a metasurface composed of nanoring dimer array,silicon dioxide dielectric and gold film is designed.The localized surface plasmon resonance of the nanoring dimer array is coupled with the cavity mode of the resonant cavity,resulting in a double Fano resonance with strong electric field enhancement,and the electric field enhancement can reach 100.By changing the structural parameters,the double Fano resonance peak can be flexibly adjusted while maintaining a large electric field enhancement.Therefore,the proposed metasurface provides the required enhanced and adjustable double Fano resonance for high-sensitivity,high-selectivity and high-throughput detection.In addition,this article also proves that the length of the resonant cavity can be determined by theoretical calculations,which avoids a lot of simulation processes.In terms of sensing applications,the combination of surface plasmon resonance and Fano resonance can improve some key indicators,i.e.sensing sensitivity,figure of merit(FOM),band number,and polarization sensitivity,which are all related to the comprehensive performance for high-precision and multi-band sensing.This paper proposes a refractive index sensor composed of a nanoring array and a F-P resonator.The coupling of the localized surface plasmon resonance of the nanoring array and the cavity mode of the F-P resonant cavity produces a double Fano resonance.The corresponding sensing sensitivity can reach 621.5nm/RIU and 906.9 nm/RIU,and the corresponding FOMs can reach 119.7 and 119.0.Then,through simulation calculation and theoretical analysis,the influence of structural parameters on the sensitivity and FOM of the sensor is studied,and the insensitivity of the structure to the polarization of incident light is verified.The proposed structure has high comprehensive performance and is more suitable for practical applications.This work provides a new idea for Fano resonance generation structure through theoretical and simulation analysis.Based on this structure,flexible adjustment of the Fano resonance wavelength can be achieved,and the structure is insensitive to incident light polarization.Therefore,this structure promotes the application of Fano resonance-based metasurfaces for fast,high-sensitivity,and high-throughput Raman sensing. |
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