CNR-SPIN U.O.S. Napoli, Complesso Universitario di Monte Sant'Angelo , Via Cintia, 80126 Napoli, Italy.
Department of Physics, University of California, San Diego , La Jolla, California 92093, United States.
ACS Nano. 2017 Sep 26;11(9):8741-8746. doi: 10.1021/acsnano.7b02323. Epub 2017 Sep 5.
Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging resolution of Δ/20 is shown (Δ being the polaritonic fringes' separation distance), comparable to the best resolution in s-SNOM. Importantly, this detection mechanism is free from light background, and it is in fact the first photonless detection of phonon polaritons.
到目前为止，二维（2D）材料的中红外纳米成象和光谱学一直局限于散射型扫描近场光学显微镜（s-SNOM）实验，其中来自样品的光被与材料相互作用的金属涂层原子力显微镜（AFM）尖端散射。这些实验最近允许对石墨烯中的等离子体极化激元以及六方氮化硼（hBN）中的双曲声子极化激元进行成像。在这里，我们展示了 AFM 悬臂的高机械灵敏度可用于成像 hBN 中的双曲声子极化激元。在我们的成像过程中，由激发出的声子极化激元局部增强了 hBN 微米大小的薄片的晶格振动。这些增强的振动与接触样品表面的 AFM 尖端耦合，并在扫描过程中记录下来。显示了 Δ/20 的成像分辨率（Δ 是极化激元条纹的分离距离），与 s-SNOM 的最佳分辨率相当。重要的是，这种检测机制不受光背景的影响，实际上是首次对声子极化激元进行无光子检测。
