Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal
Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal
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DOI：
10.1038/s41565-022-01264-4
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发表时间：
2022-12-12
影响因子：
38.3
通讯作者：
Qiu, Cheng-Wei
中科院分区：
文献类型：
--
作者：
Hu, Guangwei;Ma, Weiliang;Qiu, Cheng-Wei
各种光学晶体在中红外区域沿不同主方向具有相反符号的介电常数分量，表现出奇异的各向异性声子共振。这种具有双曲极化激元（具有开放等频轮廓的混合光物质准粒子）的材料具有大动量光学模式和波限制，这使得它们在纳米光子片上技术方面很有前景。到目前为止，双曲极化子已经在具有高对称性的晶体中被观察到并进行了表征，包括六方晶（氮化硼）、三方晶（方解石）和斜方晶（α-MoO3 或 α-V2O5）晶体，它们遵循一定的传播模式。然而，最近证明单斜晶体等低对称性材料可以为极化子现象提供更丰富的机会。在这里，我们使用扫描近场光学显微镜，报告了单斜晶 CdWO4 晶体中对称破缺双曲声子极化子的直接实空间纳米级成像，并展示了与纳米级剪切现象相关的固有不对称极化子激发和传播。我们还引入了一个定量理论模型来描述这些极化子，从而得出通过声子模式的阻尼损失来增强晶体不对称性的方案。最终，我们的研究结果表明，使用低对称性的天然材料可以实现极化纳米光子学，有利于在纳米尺度上操纵光的通用且通用的方法。
Various optical crystals possess permittivity components of opposite signs along different principal directions in the mid-infrared regime, exhibiting exotic anisotropic phonon resonances. Such materials with hyperbolic polaritons-hybrid light-matter quasiparticles with open isofrequency contours-feature large-momenta optical modes and wave confinement that make them promising for nanophotonic on-chip technologies. So far, hyperbolic polaritons have been observed and characterized in crystals with high symmetry including hexagonal (boron nitride), trigonal (calcite) and orthorhombic (alpha-MoO3 or alpha-V2O5) crystals, where they obey certain propagation patterns. However, lower-symmetry materials such as monoclinic crystals were recently demonstrated to offer richer opportunities for polaritonic phenomena. Here, using scanning near-field optical microscopy, we report the direct real-space nanoscale imaging of symmetry-broken hyperbolic phonon polaritons in monoclinic CdWO4 crystals, and showcase inherently asymmetric polariton excitation and propagation associated with the nanoscale shear phenomena. We also introduce a quantitative theoretical model to describe these polaritons that leads to schemes to enhance crystal asymmetry via the damping loss of phonon modes. Ultimately, our findings show that polaritonic nanophotonics is attainable using natural materials with low symmetry, favouring a versatile and general way to manipulate light at the nanoscale.