The gap length between plasmonic nanoparticles determines the strength of the optical coupling that results in electromagnetic field enhancement for spectroscopic and other applications. Although gap plasmon resonances have been the focus of increasing research interest, experimental observations have primarily been limited to the coupling of spherical nanoparticles that may not provide clear spectral contrast of the optical response as the interaction evolves from capacitive to charge transfer with the gap size decreasing to sub-nanometer. Here, by taking advantage of the sharp plasmon resonances of colloidal gold nanorods coupled to gold film, we present the spectral evolution of gap plasmon resonance as the particle–film spacing varies from over 30 nm to the touching limit. We find that the capacitive gap plasmon resonance of the coupled system red-shifts and narrows continuously until it vanishes at the quantum tunneling limit, in contrast to the nonlocal and Landau damping effects that are expected to result in relative blue-shifting and spectral broadening. When the spacer thickness is further decreased, high order cavity modes appear, and eventually single peak broad resonances that are characteristic of tunneling and direct contact particle–film interaction emerge. The experimental observations show that nanorods are better suited for creating cavity plasmon resonances with high quality factor, and the spectral contrast at the transition provides clarity to develop improved theoretical modeling of optical coupling at sub-nanometer gap lengths.

中文翻译：

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通过介电间距的原子控制将等离子耦合从电容型调整为导电型

等离子体纳米颗粒之间的间隙长度决定了光耦合的强度，从而增强了光谱和其他应用的电磁场。尽管间隙等离子体激元共振已成为越来越多的研究兴趣，但实验观察主要限于球形纳米粒子的耦合，随着相互作用的发展，随着间隙尺寸的减小，相互作用从电容转移到电荷转移，球形纳米粒子可能无法提供清晰的光学响应光谱对比。到亚纳米。在这里，通过利用胶体金纳米棒与金膜耦合的尖锐的等离子体激元共振，我们介绍了随着粒子与膜之间的距离从30 nm到接触极限变化，间隙等离子体激元共振的光谱演化。我们发现耦合系统的电容间隙等离子体激元共振红移并连续变窄直到其在量子隧穿极限处消失，这与非局部和兰道阻尼效应相反，后者预计会导致相对蓝移和光谱展宽。当间隔物的厚度进一步减小时，出现了高阶腔模，最终出现了具有隧穿和直接接触粒子-膜相互作用特征的单峰宽共振。实验观察表明，纳米棒更适合于产生具有高品质因数的腔等离子体激元共振，并且跃迁处的光谱对比为开发亚纳米间隙长度处的光学耦合的改进理论模型提供了清晰度。