Role of Resonances in the Transmission of Surface Plasmon Polaritons between Nanostructures

Understanding how surface plasmon polaritons (SPPs) propagate in metal nanostructures is important for the development of plasmonic devices. In this paper, we study the transmission of SPPs between single-crystal gold nanobars on a glass substrate using transient absorption microscopy. The coupled structures were produced by creating gaps in single nanobars by focused ion beam milling. SPPs were launched by focusing the pump laser at the end of the nanobar, and the transmission across the gaps was imaged by scanning the probe laser over the nanostructure. The results show larger losses at small gap sizes. Finite element method calculations were used to investigate this effect. The calculations show two main modes for nanobars on a glass surface: a leaky mode localized at the air gold interface, and a bound mode localized at the glass gold interface. At specific gap sizes (approximately 50 nm for our system), these SPP modes can excite localized surface plasmon modes associated with the gap, which dissipate energy. This increases the energy losses at small gap sizes. Experiments and simulations were also performed for the nanobars in microscope immersion oil, which creates a more homogeneous optical environment, and consistent results were observed.

理解表面等离激元极化激元（SPPs）在金属纳米结构中如何传播对于等离激元器件的发展至关重要。在本文中，我们利用瞬态吸收显微镜研究了玻璃基底上单晶金纳米棒之间SPPs的传输。通过聚焦离子束铣削在单个纳米棒中制造间隙来制备耦合结构。通过将泵浦激光聚焦在纳米棒的端部来激发SPPs，并通过在纳米结构上扫描探测激光对跨越间隙的传输进行成像。结果表明，在小间隙尺寸下损耗更大。采用有限元方法计算来研究这种效应。计算结果表明，玻璃表面上的纳米棒存在两种主要模式：一种是局域在空气 - 金界面的泄漏模式，另一种是局域在玻璃 - 金界面的束缚模式。在特定的间隙尺寸下（对于我们的系统约为50纳米），这些SPP模式能够激发与间隙相关的局域表面等离激元模式，从而耗散能量。这增加了小间隙尺寸下的能量损耗。还对处于显微镜浸油中的纳米棒进行了实验和模拟，这创造了一个更均匀的光学环境，并且观察到了一致的结果。