The ability of surface polaritons (SPs) to enhance and manipulate light fields down to deep-subwavelength length scales enables applications in optical sensing and nonlinear optics at the nanoscale. However, the wavelength mismatch between light and SPs prevents direct optical excitation of surface-bound modes, thereby limiting the widespread development of SP-based photonics. Free electrons are a natural choice to directly excite strongly confined SPs because they can supply field components of high momentum at designated positions with subnanometer precision. Here, we theoretically explore free-electron–SP coupling mediated by small scatterers and show that low-energy electrons can efficiently excite surface modes with a maximum probability reached at an optimum surface–scatterer distance. By aligning the electron beam with a periodic array of scatterers placed near a polariton-supporting interface, in-plane Smith–Purcell emission results in the excitation of surface modes along well-defined directions. Our results support using scattering elements to excite SPs with low-energy electrons.

中文翻译：

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由小散射体介导的与表面极化激元的自由电子耦合


表面极化激元 （SP） 能够增强和操纵低至深亚波长长度尺度的光场，从而在纳米级的光学传感和非线性光学中的应用。然而，光和 SP 之间的波长不匹配阻止了表面结合模式的直接光学激发，从而限制了基于 SP 的光子学的广泛发展。自由电子是直接激发强约束 SP 的自然选择，因为它们可以在指定位置以亚纳米级精度提供高动量的场分量。在这里，我们从理论上探讨了由小散射体介导的自由电子-SP 耦合，并表明低能电子可以有效地激发表面模式，在最佳表面-散射体距离处达到最大概率。通过将电子束与放置在极化激元支撑界面附近的周期性散射体阵列对齐，面内史密斯-珀塞尔发射导致沿明确定义的方向激发表面模式。我们的结果支持使用散射元件用低能电子激发 SP。