$α$ - MoO3中双曲声子极化激元的宽谱范围可持续性和可控激发
Broad-Spectral-Range Sustainability and Controllable Excitation of Hyperbolic Phonon Polaritons in $α$-MoO3
摘要：
正交相三氧化钼（$α$ -MoO 3）中的双曲声子极化激元（HPhPs）具有面内双曲性、波长压缩和超长寿命等特性，在纳米光子学领域具有巨大的应用潜力。然而，由于实验表征方面的挑战，其在远红外（FIR）范围内的极化激元响应长期以来一直未被探索。在这里，使用单色电子能量损失谱（EELS）在扫描透射电子显微镜（STEM），我们探测HPhPs在$α$ -MoO 3中的中红外（MIR）和FIR频率和相关的微观结构和取向的行为。我们发现低结构对称性导致了在宽的光谱范围（超过70 meV）和不同方向（沿b轴55-63 meV和119-125 meV，沿c轴68-106 meV，沿a轴101-121 meV）上的各种声子模和多个Reststrahlen带（RB）。这些HPhPs可以通过控制快速电子的方向来选择性地激发。这些发现为纳米光子和光电应用提供了新的机会，例如定向光传播，超透镜和传热。
英文摘要：
Hyperbolic phonon polaritons (HPhPs) in orthorhombic-phase molybdenum trioxide ($α$-MoO3) show in-plane hyperbolicity, great wavelength compression and ultra-long lifetime, therefore holding great potential in nanophotonic applications. However, its polaritonic response in the far-infrared (FIR) range has long remained unexplored due to challenges in experimental characterization. Here, using monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), we probe HPhPs in $α$-MoO3 in both mid-infrared (MIR) and FIR frequencies and correlate their behaviors with microstructures and orientations. We find that low-structural symmetry leads to various phonon modes and multiple Reststrahlen bands (RBs) over a broad spectral range (over 70 meV) and in different directions (55-63 meV and 119-125 meV along b axis, 68-106 meV along c axis, 101-121 meV along a axis). These HPhPs can be selectively excited by controlling the direction of swift electrons. These findings provide new opportunities in nanophotonic and optoelectronic applications such as directed light propagation, hyperlenses and heat transfer.
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