Li Ning, Guo Xiangdong, Yang Xiaoxia, Qi Ruishi, Qiao Tianyu, Li Yifei, Shi Ruochen, Li Yuehui, Liu Kaihui, Xu Zhi, Liu Lei, García de Abajo F Javier, Dai Qing, Wang En-Ge, Gao Peng
Division of Nanophotonics, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, China.
International Center for Quantum Materials, Peking University, Beijing, China.
Nat Mater. 2021 Jan;20(1):43-48. doi: 10.1038/s41563-020-0763-z. Epub 2020 Aug 17.
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 10c. 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.
声子极化激元能够在深亚波长尺度上实现光的限制，具有潜在的技术应用，如亚衍射成像、传感和自发辐射工程。然而，声子极化激元的限制程度与激发效率之间的权衡阻碍了在单层六方氮化硼（h-BN）中直接观测这些模式，而在单层h-BN中预计它们能达到超高限制。在此，我们使用扫描透射电子显微镜中的单色电子能量损失谱（能量分辨率约为7.5 meV）来测量单层h-BN中的声子极化激元，随着声子Reststrahlen带（RS）消失，直接证明了这些模式的存在。我们发现单层h-BN中的声子极化激元表现出高限制（波长比自由空间中的光小>487倍）和低至约10c的超慢群速度。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带范围内激发声子极化激元。这些结果为超表面工程和强烈增强的光与物质相互作用开辟了广泛的机会。
