Probing Hyperbolic Shear Polaritons in Β-Ga 2 O 3 Nanostructures Using STEM-EELS.

Phonon polaritons, quasiparticles arising from strong coupling between electromagnetic waves and optical phonons, have potential for applications in sub-diffraction imaging, sensing, thermal conduction enhancement, and spectroscopy signal enhancement. A new class of phonon polaritons in low-symmetry monoclinic crystals, hyperbolic shear polaritons (HShPs), have been verified recently in β-Ga 2 O 3 by free electron laser (FEL) measurements. However, detailed behaviors of HShPs in β-Ga 2 O 3 nanostructures still remain unknown. Here, by using monochromatic electron energy loss spectroscopy in conjunction with scanning transmission electron microscopy, we report the experimental observation of multiple HShPs in β-Ga 2 O 3 in the mid-infrared (MIR) and far-infrared (FIR) ranges. We excite HShPs in various β-Ga 2 O 3 nanorods and a β-Ga 2 O 3 nanodisk. The frequency-dependent rotation and shear effect of HShPs reflect on the distribution of EELS signals. The propagation and reflection of HShPs in nanostructures are clarified by simulations of electric field distribution. These findings suggest that, with its tunable broad spectral HShPs, β-Ga 2 O 3 is an excellent candidate for nanophotonic applications. This article is protected by copyright. All rights reserved.