Tightly confined optical near fields in plasmonic nanostructures play a pivotal role in important applications ranging from optical sensing to light harvesting. Energetic electrons are ideally suited to probing optical near fields by collecting the resulting cathodoluminescence (CL) light emission. Intriguingly, the CL intensity is determined by the near-field profile along the electron propagation direction, but the retrieval of such field from measurements has remained elusive. Furthermore, the conditions for optimum electron near-field coupling in plasmonic systems are critically dependent on such field and remain experimentally unexplored. In this work, we use electron energy-dependent CL spectroscopy to study the tightly confined dipolar mode in plasmonic gold nanoparticles. By systematically studying gold nanoparticles with diameters in the range of 20–100 nm and electron energies from 4 to 30 keV, we determine how the coupling between swift electrons and the optical near fields depends on the energy of the incoming electron. The strongest coupling is achieved when the electron speed equals the mode phase velocity, meeting the so-called phase-matching condition. In aloof experiments, the measured data are well reproduced by electromagnetic simulations, which explain that larger particles and faster electrons favor a stronger electron near-field coupling. For penetrating electron trajectories, scattering at the particle produces severe corrections of the trajectory that defy existing theories based on the assumption of nonrecoil condition. Therefore, we develop a first-order recoil correction model that allows us to account for inelastic electron scattering, rendering better agreement with measured data. Finally, we consider the albedo of the particles and find that, to approach unity coupling, a highly confined electric field and very slow electrons are needed, both representing experimental challenges. Our findings explain how to reach unity-order coupling between free electrons and confined excitations, helping us understand fundamental aspects of light–matter interaction at the nanoscale.

中文翻译：

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通过电子能量相关的阴极发光光谱法进行自由电子-等离子体耦合强度和近场检索


等离子体纳米结构中严格限制的光学近场在从光学传感到光捕获的重要应用中发挥着关键作用。高能电子非常适合通过收集产生的阴极发光 (CL) 发射光来探测光学近场。有趣的是，化学发光强度是由沿电子传播方向的近场分布决定的，但从测量中检索这种场仍然难以捉摸。此外，等离激元系统中最佳电子近场耦合的条件很大程度上依赖于这种场，并且仍未通过实验探索。在这项工作中，我们使用电子能量依赖的化学发光光谱来研究等离激元金纳米颗粒中严格限制的偶极模式。通过系统地研究直径在 20–100 nm 范围内、电子能量在 4 到 30 keV 范围内的金纳米粒子，我们确定了快电子和光学近场之间的耦合如何取决于入射电子的能量。当电子速度等于模式相速度时，实现最强的耦合，满足所谓的相位匹配条件。在超然实验中，电磁模拟可以很好地再现测量数据，这解释了更大的粒子和更快的电子有利于更强的电子近场耦合。对于穿透电子轨迹，粒子处的散射会对轨迹产生严重的修正，这违背了基于非反冲条件假设的现有理论。因此，我们开发了一个一阶反冲校正模型，使我们能够考虑非弹性电子散射，从而与测量数据更好地吻合。 最后，我们考虑了粒子的反照率，发现为了接近统一耦合，需要高度受限的电场和非常慢的电子，这两者都代表了实验挑战。我们的研究结果解释了如何在自由电子和受限激发之间实现单位阶耦合，帮助我们了解纳米尺度上光与物质相互作用的基本方面。