Journal
ADVANCED MATERIALS
Volume 32, Issue 46, Pages -Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adma.202002014
Keywords
electron energy loss spectroscopy; hyperbolic phonon polaritons; alpha-MoO3
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Funding
- National Key R&D Program of China [2016YFA0300804]
- National Natural Science Foundation of China [11974023, 51672007]
- Key R&D Program of Guangdong Province [2018B030327001, 2018B010109009, 2019B010931001]
- National Equipment Program of China [ZDYZ2015-1]
- Bureau of Industry and Information Technology of Shenzhen [201901161512]
- 2011 Program Peking-Tsinghua-IOP Collaborative Innovation Center of Quantum Matter
- Electron Microscopy Laboratory at Peking University
- Beijing Advanced Innovation Center for Intelligent Robots Systems
- Analysis & Testing Center at Beijing Institute of Technology
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Hyperbolic phonon polaritons (HPhPs) in orthorhombic-phase molybdenum trioxide (alpha-MoO3) show in-plane hyperbolicity, great wavelength compression, and ultralong lifetime, therefore holding great potential in nanophotonic applications. However, its polaritonic response in the far-infrared (FIR) range remains unexplored due to challenges in experimental characterization. Here, monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) is used to probe HPhPs in alpha-MoO(3)in both mid-infrared (MIR) and FIR frequencies and correlate their behaviors with microstructures and orientations. It is found 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 theb-axis, 68-106 meV along thec-axis, and 101-121 meV along thea-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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