A spherical metallic nanoparticle is the simplest and most frequently used example of plasmonic nanostructures. In such a highly symmetric structure the plasmon modes consist of degenerate multipoles, which cannot be separately observed by only utilizing energy resolved means. We here demonstrate nanoscale optical field mappings of degenerate multipole modes in spherical silver nanoparticles using an angle- and polarization-resolved cathodoluminescence technique combined with scanning transmission electron microscopy. By properly selecting the detection angle and polarization, the observed optical field maps of spherical silver nanoparticles exhibit dipole or quadrupole features, depending on the energy. The angle-dependent multipole patterns visualize their three-dimensional rotation, which can be explained as superposition of the degenerate modes due to the spherical symmetry.

中文翻译：

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阴极发光观察的球形银纳米粒子的三维多极旋转

球形金属纳米颗粒是等离子体纳米结构的最简单和最常用的例子。在这种高度对称的结构中，等离子体激元模由简并的多极子组成，仅通过利用能量分解手段无法单独观察到。我们在这里展示了使用角度和偏振分辨阴极发光技术与扫描透射电子显微镜相结合的球形银纳米粒子中简并多极模的纳米级光学场映射。通过适当地选择检测角和极化，球形的银纳米颗粒观察到的光学场图表现出偶极或四极的功能，这取决于能量。角度相关的多极模式可视化了它们的三维旋转，