We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy-loss spectra. We show that in the case of a uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction.

中文翻译：

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使用快速电子探测和操纵单轴材料中的体积和表面声子极化子：六方氮化硼

通过分析电子能量损失谱，我们从理论上描述了快速电子如何耦合到单轴材料中的极化模式。我们表明，在具有双曲分散的单轴介质的情况下，可以通过快速电子穿过体积或沿材料与真空之间的无限界面行进来激发体模和表面模。有趣的是，与各向同性材料中的激发相反，体模可以由在单轴介质外传播的快速电子激发。我们用代表性的单轴材料六角形氮化硼证明了我们的发现。我们表明，体声子极化和表面声子极化模式的激发与电子速度密切相关，并且高度取决于电子束轨迹与材料光轴之间的夹角。