标题： 极化子傅里叶晶体中具有异常色散的布洛赫声子极化激元 显示英文标题

标题： Bloch phonon-polaritons with anomalous dispersion in polaritonic Fourier crystals

摘要： 最近提出的极化子傅里叶晶体（PFC）概念是基于一个谐波起伏的镜面基底，用于一层薄而纯净的极化子晶体层。 PFC中的传播极化子经历谐波和模式选择性动量调制，导致出现几乎无模式间散射的布洛赫模式。 PFC首次在六方氮化硼（hBN）中的双曲声子极化激元中得到演示，其位于II型Reststrahlen带（RB-II）内，在此区域内，介电常数张量的面内分量是各向同性和负的，而面外分量是正的。 相比之下，I型Reststrahlen带（RB-I）的特点是面外分量为负，面内分量为正，因此与RB-II相比，声子极化激元的场对称性发生了反转。 此类RB-I模式在极化子晶体中的行为仍有待探索。 在这里，我们采用双轴晶体α相三氧化钼（{\alpha }-MoO3）和近场成像技术，研究在一维PFC中RB-I内的极化子布洛赫模式，其中{\alpha }-MoO3中的中红外声子极化激元具有异常色散和负相速度。 令人惊讶的是，我们在第一布里渊区中观察到布洛赫波作为无色散的近场图案出现，这与使用面内各向同性hBN展示的RB-II情况形成对比。 我们将这种差异归因于两种RB中最低阶声子极化激元模式的相反场对称性，从而导致极化子傅里叶晶体中的不同动量调制机制。 我们的结果揭示了模式对称性对于极化子晶体的一般重要性，特别是对于傅里叶晶体这一新兴领域，该领域有望为操控纳米光提供新的方法。 显示英文摘要

摘要： The recently suggested concept of a polaritonic Fourier crystal (PFC) is based on a harmonically-corrugated mirror substrate for a thin pristine polaritonic crystal layer. The propagating polaritons in PFC experience a harmonic and mode-selective momentum modulation leading to a manifestation of Bloch modes with practically zero inter-mode scattering. PFC was first demonstrated for the hyperbolic phonon-polaritons in hexagonal boron nitride (hBN) within its Type II Reststrahlen band (RB-II) where the in-plane components of the dielectric permittivity tensor are isotropic and negative, while the out-of-plane component is positive. By contrast, a Type I Reststrahlen band (RB-I) is characterized by negative out-of-plane and positive in-plane permittivity components, and consequently, the inversion of field symmetry of phonon-polaritons compared to RB-II. Behavior of such RB-I modes in a polaritonic crystal is yet to be explored. Here, we employ a biaxial crystal alpha-phase molybdenum trioxide ({\alpha}-MoO3) and near-field imaging to study polaritonic Bloch modes in a one-dimensional PFC within the RB-I where the mid-infrared phonon-polaritons in {\alpha}-MoO3 have anomalous dispersion and negative phase velocity. Surprisingly, we observe a manifestation of Bloch waves as a dispersionless near-field pattern across the first Brillouin zone, in contrast to RB-II case demonstrated with in-plane isotropic hBN. We attribute this difference to the opposite field symmetry of the lowest-order phonon-polariton mode in the two RBs, leading to a different momentum modulation regime in the polaritonic Fourier crystal. Our results reveal the importance of mode symmetry for polaritonic crystals in general and for the emerging field of Fourier crystals in particular, which promise new ways to manipulate the nanolight.