Nanostructured plasmonic surfaces allow for precise tailoring of electromagnetic modes within sub-diffraction mode volumes, boosting light–matter interactions. This study explores vibrational strong coupling (VSC) between molecular ensembles and subradiant “dark” cavities that support infrared quadrupolar plasmonic resonances (QPLs). The QPL mode exhibits a dispersion characteristic of bound states in the continuum (BIC). That is, the mode is subradiant or evanescent at normal incidence and acquires increasing “bright” dipole character with larger in-plane wavevectors. We deposited polymethyl methacrylate (PMMA) thin films on QPL substrates to induce VSC with the carbonyl stretch in PMMA and measured the resulting infrared (IR) spectra. Our computational analysis predicts the presence of “dark” subradiant polariton states within the near-field of the QPL mode, and “bright” collective molecular states. This finding is consistent with classical and quantum mechanical descriptions of VSC that predict hybrid polariton states with cavity-like modal character and N−1 collective molecular states with minimal cavity character. However, the behaviour is opposite of what is standardly observed in VSC experiments that use “bright” cavities, which results in “bright” polariton states that can be spectrally resolved as well as N−1 collective molecular states that are spectrally absent. Our experiments confirm a reduction of molecular absorption and other spectral signatures of VSC with the QPL mode. In comparison, our experiments promoting VSC with dipolar plasmonic resonances (DPLs) reproduce the conventional behavior. Our results highlight the significance of cavity mode symmetry in modifying the properties of the resultant states from VSC, while offering prospects for direct experimental probing of the N−1 molecule-like states that are usually spectrally “dark”.

中文翻译：

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次辐射等离子体空腔使明亮的极化子状态变暗

纳米结构等离子体表面允许在亚衍射模式体积内精确定制电磁模式，从而增强光与物质的相互作用。这项研究探索了分子系团和支持红外四极等离子体共振（QPL）的次辐射“暗”腔之间的振动强耦合（VSC）。 QPL 模式表现出连续体 (BIC) 中束缚态的色散特性。也就是说，该模式在法向入射时是次辐射或渐逝的，并且随着较大的面内波矢而获得增加的“明亮”偶极子特征。我们在 QPL 基底上沉积聚甲基丙烯酸甲酯 (PMMA) 薄膜，通过 PMMA 中的羰基拉伸诱导 VSC，并测量所得红外 (IR) 光谱。我们的计算分析预测 QPL 模式近场内存在“暗”次辐射极化子态和“亮”集体分子态。这一发现与 VSC 的经典和量子力学描述一致，这些描述预测具有类腔模态特征的混合极化子态，并且N−1具有最小空腔特征的集体分子状态。然而，这种行为与使用“明亮”腔的 VSC 实验中标准观察到的行为相反，这会产生“明亮”的极化子态，可以进行光谱解析以及N−1光谱上不存在的集体分子态。我们的实验证实 QPL 模式可减少 VSC 的分子吸收和其他光谱特征。相比之下，我们的实验通过偶极等离子体共振（DPL）促进 VSC 再现了传统行为。我们的结果强调了腔模对称性在改变 VSC 所得态的性质方面的重要性，同时为直接实验探测N−1通常在光谱上是“暗”的分子状状态。