Objective With the characteristics of low power consumption, high sensitivity, and easy integration, nanophotonic sensors have become important research targets in the fields of biological detection, food safety, and internet of things. Surface plasmon resonance (SPR) provides an effective way to control photons at the sub-wavelength scale and has been extensively studied in the field of nano-sensors. In the past decade, many excellent surface plasmon resonance-based sensing structures have been used to construct photoelectric sensors such as nanoscale arrays of holes, disks, rings, and so on. As a type of SPR, localized SPR (LSPR) shows the advantages of low cost and high sensitivity. However, the potential application of LSPR is hindered by radiation damping. Moreover, many metal micro- and nano-structured SPR devices are limited by the current laboratory applications such as electron beam printing, electron beam lithography, and other processing methods, which are difficult to commercialize. How to realize low-cost, large-scale preparation of high-performance nano sensors is a difficult point in current research. This paper proposes a surface lattice resonant refractive index sensor with an excellent performance and large-scale production. This resonance mode has the characteristic of narrowing the resonance wavelength linewidth to achieve an excellent and stable structural performance. In the aspect of simulation, the finite difference time domain method (FDTD) is used to study the optical performance of this structure. The experiment uses the nanosphere lithography and nanoimprint technologies to prepare large-area, high-quality silver nanoring arrays. The experimental results show that the sensitivity of the structure is 663 nm/RIU and the figure of merit (FoM) is 9. 2. The change of structural geometric parameters can not only realize the tuning of resonance wavelengths, but also improve the refractive index sensitivity. Methods In this paper, the plane and 3D diagrams of the periodic silver nanoring array structure used for sensing are presented in Fig. 1, and the main parameters of the structure are given. In order to explore the dependence of resonance modes on structural parameters, a set of fixed structures are selected, and the reflection, transmission and absorption spectra of the structure are obtained by using the FDTD method. The physical mechanism of two reflectivity dips is analyzed and the more obvious one is the focus of our next study, manifested as the reflection decline of the SLR mode. Then we change the parameters of the structure, get the relationship between the parameters and the SLR mode, and summarize the optimized parameter range. Finally, we give the whole experimental preparation process. Firstly, 1 mL polystyrene (PS) microsphere solution (mass fraction of 10%) with a diameter of 690 nm is mixed with 1 mL ethanol solution to obtain self-assembled polystyrene microspheres. Then through reactive ion etching, magnetron sputtering and imprinting processes, the optimized silver nanoring arrays are obtained. Finally, we change the environmental refractive index around the sample to obtain the experimental refractive index sensitivity. Results and Discussions In the reflection spectrum of the optimized structure, the wavelength corresponding to the decrease of reflection is consistent with the simulation results, which verifies the feasibility of the experiment. In order to study the refractive index sensing characteristics of the silver nanoring structure after parameter optimization, five different refractive index solutions are selected as the measurement environment in the experiment. By recording and comparing the shifts of experimental and simulated resonance wavelengths (Fig. 10 (c)), the simulated and experimental refractive index sensitivities are both 663 nm/RIU. The FoM obtained by simulation is 18. 9, which is significantly higher than the FoM of 9. 2 in the experiment, indicating that the morphology and order of the samples still need to optimize in the preparation process. In order to explore the influence of structural parameters on refractive index sensitivity, we simulate the reflection spectra under different structural parameters when the background refractive index changes from 1 (air) to 1.4722 (glycerol). The thickness of silver film and the radii of inner and outer rings have little influence on the resonance wavelength shift. The position of the resonance wave shifts to the same degree, and the sensitivity is basically stable at 660-700 nm/RIU. We keep other parameters unchanged and change the height of the ring cavity template within the optimized range to obtain template height H. In this case, the refractive index sensitivity corresponding to the resonance wavelength is shown in Fig. 11(a). The simulation results show that the refractive index sensitivity increases with the increase of template height H, and reaches the maximum sensitivity of 800 nm /RIU when H = 100 nm. Finally, when only the structural period changes, the refractive index sensitivity corresponding to the resonance wavelength is obtained as shown in Fig. 11(b). The simulation results show that with the increase of structural period P, the refractive index sensitivity shows an approximately linear growth trend. The increase of template height H and structural period P can improve the refractive index sensitivity, but the increase of H reduces the FoM and the stability of the structure. Unlike H, the increase of P can at least increase the refractive index sensitivity and the FoM. Therefore, it is better to increase the P to improve the refractive index sensitivity of the structure. Conclusions In this paper, an SLR refractive index sensor based on a silver nanoring array is designed, and the optical and sensing performances of this structure are studied from two aspects of simulation and experiment. The optical simulation software FDTD is used to calculate the reflection spectra of the structure under different parameters, explore the optical performance of the structure, and optimize the structural parameters. The experimental part includes the initial preparation and the final test using methanol, ethanol, isopropanol, ethylene glycol, and glycerol. These five solutions with different refractive indexes are used as the test environment to explore the refractive index transmission of the structure. The simulation results show that the structure can be adjusted to the resonance wavelength and refractive index sensitivity. The experimental results of the optimized structure are in good agreement with the simulation data. The final experimental refractive index sensitivity is 663 nm/RIU and the quality factor is 9.2. The proposed structure exhibits potential application value in biosensing detection in terms of preparation and performance.
