ICYS Annual Report 2023

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ICYS Annual Report 2023 Nanophotonics materials have great potential for versatile applications including sensors and spectroscopy. Resonantly coupled systems between photonic resonators and molecules are characterized by spectral variations resulting from light–matter interactions. Photonic nanostructures are advantageous owing to their strong near-field confinement at the nanoscale, which enables strong interactions with nearby molecules. According to recent studies, radiative loss engineering of coupled systems is a promising approach for tailoring coupling conditions and enhancing the molecular signals. However, this strategy has only been realized using the localized surface plasmon resonances of metal nanostructures, which suffer from increased ohmic loss in the mid-infrared region and face serious limitations in achieving high quality (Q) factors. In this study, we adopt silicon-based metasurfaces formed on silicon-on-insulator (SOI) wafers to achieve high Q factors and tune the coupling conditions between the quasi-bound states in the continuum (qBICs) and molecular vibrations.Figure 1(a) shows the proposed silicon metasurface with asymmetric pair-rod arrays on an SOI wafer with 400-nm thick silicon and a 2000-nm thick buried oxide (BOX) layer. The period of array P is 3900 nm, the primitive silicon rod length L is 2625 nm, the rod height w is 985 nm, and the distance between the centers of the pair-rod is 1825 nm. The qBIC condition arises from symmetry breaking of the unit structure of the metasurface that satisfies the BIC condition with ideally infinite Q factors, even inside the light cone [1]. In our study, asymmetry is added Research Digest 1. Outline of Research2. Research ActivitiesKeisuke WATANABEFig. 1. (a) SEM image and the photograph (inset) of the fabricated silicon metasurface (α = 0.2). (b) Experimental transmission spectra. The dashed lines indicate the approximate peak positions of the qBIC modes. The spectra are vertically shifted for clarity. (c) Chemical structures of methyl methacrylate. Orange dashed line indicates the C=O bond.Fig. 2. (a) Experimental peak positions of the split modes (red dots) and the superimposed eigenvalues. Ω represents Rabi splitting. The dashed line represents the peak wavelength of the qBIC modes (α = 0.12), and the dotted line represents the absorption wavelength of PMMA molecules. (b) Experimental transmission spectra of the coupled qBIC-PMMA metasurfaces.References 1) K. Watanabe and M. Iwanaga, Nanophotonics 12, 99 (2023). 2) K. Watanabe, H. R. Devi, M. Iwanaga, and T. Nagao, Adv Opt Mater 12, 6, 2301912 (2024).by varying the lengths of the upper and lower rods in the unit cell, whose asymmetry parameter is expressed as α = 2ΔL/L. Figure 1(b) shows the measured transmission spectra of bare silicon metasurfaces with different α. Both the resonance peak amplitudes and linewidths increased with parameter α while maintaining the peak positions. Next, a 111-nm polymethyl methacrylate (PMMA) layer was spin-coated onto a bare metasurface to characterize the coupling between the qBIC mode and the C=O stretching mode of PMMA (Fig. 1(c)). Fig. 2 shows the measured spectra for different α. When α = 0.12, we observed that the two newly formed polariton modes never cross each other (i.e., anti-crossing behavior), as shown in Fig. 2(a). In the experiment, a coupling strength Ω = 5.30 meV was obtained, suggesting that the strong coupling condition Ω > 2γm, 2γc, whereγc and γm are the loss rates of the qBIC and vibrational modes, respectively, was satisfied. Considering almost constant Ω = 5.30 meV and 2γm = 2.24 meV for all α, the coupling condition was changed by changing the linewidths of qBIC modes 2γc. A rough estimate indicated that the strong coupling condition was satisfied when α < 0.27, and the weak coupling condition Ω < 2γm, 2γc was satisfied when α > 0.27. This result clearly shows that the coupling conditions can be tailored using the asymmetry parameter α of silicon metasurfaces. These findings indicate that silicon metasurfaces exhibit strong interactions with molecular vibrations in the mid infrared regime, which are useful for surface infrared spectroscopy with large enhancement factors.28Strong Light-Matter Interactions in All-Dielectric Metasurfaces