Yim Ju Eun, Brawley Zachary T, Sheldon Matthew T
Department of Chemistry, Texas A&M University, College Station, USA.
Department of Materials Science and Engineering, Texas A&M University, College Station, USA.
Nanophotonics. 2024 Mar 22;13(11):2035-2045. doi: 10.1515/nanoph-2024-0058. eCollection 2024 May.
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 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 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 molecule-like states that are usually spectrally "dark".
纳米结构的等离子体表面能够在亚衍射模式体积内精确调整电磁模式，增强光与物质的相互作用。本研究探索了分子系综与支持红外四极等离子体共振（QPL）的亚辐射“暗”腔之间的振动强耦合（VSC）。QPL模式表现出连续统中的束缚态（BIC）的色散特性。也就是说，该模式在正入射时是亚辐射或倏逝的，并且随着面内波矢增大而获得越来越多的“亮”偶极特性。我们在QPL衬底上沉积聚甲基丙烯酸甲酯（PMMA）薄膜，以诱导其与PMMA中的羰基伸缩振动发生VSC，并测量所得的红外（IR）光谱。我们的计算分析预测在QPL模式的近场内存在“暗”亚辐射极化激元态和“亮”集体分子态。这一发现与VSC的经典和量子力学描述一致，后者预测具有腔状模态特性的混合极化激元态和具有最小腔特性的集体分子态。然而，这种行为与在使用“亮”腔的VSC实验中通常观察到的情况相反，在后者中会产生可光谱分辨的“亮”极化激元态以及光谱中不存在的集体分子态。我们的实验证实了分子吸收以及QPL模式的其他VSC光谱特征的降低。相比之下，我们用偶极等离子体共振（DPL）促进VSC的实验重现了传统行为。我们的结果突出了腔模对称性在改变VSC所得态性质方面的重要性，同时为直接实验探测通常在光谱上“暗”的类分子态提供了前景。
