Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)—which exist in much broader spectral ranges than phonon polaritons—has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag₂Te platelets. The hybridization of the PPs with their mirror image—by placing the platelets above a Au layer—increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

面内各向异性材料的薄层可以支持超限域极化子，其波长取决于传播方向。这种极化子具有探索基本材料特性和开发新型纳米光子器件的潜力。然而，超限域面内各向异性等离子体激元（PP）的真实空间观测——其光谱范围比声子极化激元宽得多——一直难以实现。_{在这里，我们应用太赫兹纳米镜对单斜晶 Ag 2}中的面内各向异性低能量 PP 进行成像血小板。PP 与其镜像的杂交（通过将片晶放置在 Au 层上方）增加了方向依赖性相对极化子传播长度和定向极化子限制。这样可以验证动量空间中的线性色散和椭圆等频轮廓，揭示面内各向异性声太赫兹 PP。我们的工作展示了低对称性（单斜晶系）晶体上的高对称性（椭圆形）极化激元，并演示了使用太赫兹 PP 进行各向异性载流子质量和阻尼的局部测量。