Phonon polaritons enable light confinement at deep subwavelength scales, with potential technological applications, such as subdiffraction imaging, sensing and engineering of spontaneous emission. However, the trade-off between the degree of confinement and the excitation efficiency of phonon polaritons prevents direct observation of these modes in monolayer hexagonal boron nitride (h-BN), where they are expected to reach ultrahigh confinement. Here, we use monochromatic electron energy-loss spectroscopy (about 7.5 meV energy resolution) in a scanning transmission electron microscope to measure phonon polaritons in monolayer h-BN, directly demonstrating the existence of these modes as the phonon Reststrahlen band (RS) disappears. We find phonon polaritons in monolayer h-BN to exhibit high confinement (>487 times smaller wavelength than that of light in free space) and ultraslow group velocity down to about 10⁻⁵c. The large momentum compensation provided by electron beams additionally allows us to excite phonon polaritons over nearly the entire RS band of multilayer h-BN. These results open up a broad range of opportunities for the engineering of metasurfaces and strongly enhanced light–matter interactions.

声子极化子可以将光限制在较深的亚波长范围内，具有潜在的技术应用，例如亚衍射成像，自发发射的传感和工程设计。但是，限制程度和声子极化子的激发效率之间的权衡取舍阻止了在单层六角形氮化硼（h-BN）中直接观察到这些模式的预期，因为在这些模式中它们有望达到超高限制。在这里，我们在扫描透射电子显微镜中使用单色电子能量损失谱（约7.5 meV能量分辨率）测量单层h-BN中的声子极化子，直接证明了这些模式在声子Reststrahlen带（RS）消失时的存在。我们发现单层h-BN中的声子极化子表现出高约束（>⁻⁵ c。电子束提供的大动量补偿还使我们能够在多层h-BN的几乎整个RS波段上激发声子极化子。这些结果为超颖表面的工程化和大大增强的光-质相互作用提供了广泛的机会。