Recent advances in near-field technology with an ultrahigh spatial resolution breaking optical diffraction limit, make it possible to further identify surface-enhanced Raman scattering (SERS) enhancement theories, and to monitor the SERS substrates. Here we verify the electromagnetic enhancement mechanism for SERS with a close-up view, using scattering-type scanning near-field optical microscopy. The array of metal-insulator-metal (MIM) subwavelength structures is studied, in which the field enhancement comes from the strong coupling between gap plasmon polariton and surface plasmon polariton modes. The near-field optical measurements reveal that SERS enhancement factor (EF) varies from one MIM subwavelength unit to another in a finite array. Besides the enhancement of isolated unit, the loss exchange phenomenon in strong coupling with a large Rabi splitting can give rise to an additional enhancement of more than 2 orders of magnitude in periodic arrays and close to 3 orders of magnitude in finite arrays. The SERS EF of the array composed of only 5 units is demonstrated to yield the best SERS performance. Our near-field optical measurements show evidence that finite-size structures embodied with strong coupling effect are a key way to develop practical high-performance SERS substrates.

中文翻译：

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散射型扫描近场光学显微镜揭示的强耦合拉曼散射增强

具有突破光学衍射极限的超高空间分辨率的近场技术的最新进展，使得进一步识别表面增强拉曼散射 (SERS) 增强理论和监测 SERS 基板成为可能。在这里，我们使用散射型扫描近场光学显微镜通过近距离观察来验证 SERS 的电磁增强机制。研究了金属-绝缘体-金属 (MIM) 亚波长结构阵列，其中场增强来自间隙等离子体激元和表面等离子体激元模式之间的强耦合。近场光学测量表明，SERS 增强因子 (EF) 在有限阵列中从一个 MIM 亚波长单元到另一个不同。除了隔离单元的增强，强耦合中的损失交换现象与大 Rabi 分裂可以在周期阵列中产生超过 2 个数量级的额外增强，在有限阵列中产生接近 3 个数量级的额外增强。仅由 5 个单元组成的阵列的 SERS EF 被证明可以产生最佳的 SERS 性能。我们的近场光学测量表明，具有强耦合效应的有限尺寸结构是开发实用的高性能 SERS 基板的关键途径。