Probing Hyperbolic Shear Polaritons in β-Ga₂O₃ Nanostructures Using STEM-EELS

Phonon polaritons, quasiparticles arising from strong coupling between electromagnetic waves and optical phonons, have potential for applications in subdiffraction 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 beta-Ga2O3 by free electron laser (FEL) measurements. However, detailed behaviors of HShPs in beta-Ga2O3 nanostructures still remain unknown. Here, by using monochromatic electron energy loss spectroscopy in conjunction with scanning transmission electron microscopy, the experimental observation of multiple HShPs in beta-Ga2O3 in the mid-infrared (MIR) and far-infrared (FIR) ranges is reported. HShPs in various beta-Ga2O3 nanorods and a beta-Ga2O3 nanodisk are excited. 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, beta-Ga2O3 is an excellent candidate for nanophotonic applications.