30Fig. 1. (a) Photograph of the fabricated device. (b) SEM image of the fabricated metasurface. Inset shows the simulated electric field |E| profile (right). (c) Normalized reflectance spectra measured in different concentrations of IPA with (left) and without (right) nanogaps, respectively. Insets show the SEM images with the scale bars of 1 μm. This study proposed silicon block metasurfaces with nanogaps to overcome the key challenges.1) Fig. 1(a)(b) shows a fabricated metasurface with the measured gap size of approximately 33 nm. Fabrication was performed using electron-beam lithography on silicon-on-quartz wafers and dry-etching process of silicon. The metasurface has quasi-BIC modes whose field distributions correspond to magnetic dipole modes, and their electric fields Research Digest experience the nanogap regions. Consequently, the metasurface increased the environmental refractive index sensitivity by up to 2.7 times while keeping a high Q factor (Fig. 1(c)) when the structural asymmetry parameter was 2%. We achieved a figure-of-merit (FOM) of 239 for refractometric sensors, which was among the highest in the similar all-dielectric metasurfaces reported so far.We then determined the optimum structural condition that yielded the lowest limit of detection (LOD) under the trade-off between the FOM and spectral signal-to-noise ratio when the asymmetries of the unit cell were changed. Here, silicon metasurfaces with asymmetric pair-rod arrays were fabricated experimentally, and the minimum LOD was obtained under a critical coupling condition with equal radiative and nonradiative Q factors. Fig. 2 shows the measured real-time tracking of the resonance peaks at the critical coupling condition, yielding a minimum LOD of 2.8 × 10−5 RIU. These results agreed well with the theoretical model derived from the temporal coupled-mode theory. We revealed that the LOD and optimum asymmetry are largely affected by the nonradiative losses in the nanostructure, indicating the importance of loss reduction in refractometric sensors using dielectric metasurfaces. These findings provide design guidelines for highly sensitive biochemical sensors based on all-dielectric BIC metasurfaces.Fig. 2. Real-time tracking of the resonance peaks when asymmetry parameter α = 5%. Gray-shaded regions indicate the injection of different concentrations of index solutions, while the other white regions indicate the injection of water. Inset shows a top-view SEM image of the fabricated silicon metasurface and the simulated electric field intensity.References1) K. Watanabe and M. Iwanaga, Nanophotonics 12, 99 (2023).large enhancement at 1. Outline of ResearchEnhancing light–matter interactions at the nanoscale is important for highly sensitive sensing applications. Recently, high refractive index dielectric metasurfaces have attracted significant attention as new strategies because of the diverse tunability of their optical properties based on their Mie resonances, small material absorption losses, and low-cost fabrication. In particular, all-dielectric metasurfaces at a singularity called the bound state in the continuum (BIC) offer design flexibility of controlling the resonance wavelength, linewidth, and amplitude. Experimentally, the sharp peaks are accessible from the free space by breaking the symmetry of the unit structure (quasi-BIC). This feature possesses great potential as highly sensitive refractometric sensors relying on their spectral shifts. However, simultaneously realizing both high quality (Q) factors and the large interplay of light with external medium remains one of the key challenges for their better performance. In this study, we demonstrate the strong light-matter interactions capable of demonstrating high-performance sensing and spectroscopy. Specifically, we investigate the unknown sensing characteristics of BIC metasurfaces and clarify the conditions under which the optimal resolution can be obtained. 2. Research ActivitiesStrong Light-Matter Interactions in All-Dielectric MetasurfacesKeisuke WATANABE
元のページ ../index.html#32