Measurement of far-infrared surface phonon polaritons in AlN nanowires via electron microscope

Surface phonon polaritons (SPhPs) exhibit promising advantages (e.g., low loss, long lifetimes) for mid/far-infrared (MIR/FIR) nanophotonics. However, FIR SPhPs experiments remain challenging for conventional optics and scattering-type scanning near-field optical microscopy (s-SNOM) due to the lack of compatible light sources/detectors. In this work, we characterized ∼ 75–110 meV SPhPs in AlN nanowires using a monochromated scanning transmission electron microscope (STEM) equipped with electron energy loss spectroscopy (EELS). This technique provided exceptional 4.3 meV energy resolution and sub-angstrom spatial resolution. We observed the evolution of SPhP interference fringes with propagation distance, derived the dispersion curve, and clarified size effects on SPhP propagation by tuning AlN structure dimensions. Experimental-numerical cross-validation confirmed that the local continuum model (LCM) accurately describes AlN’s SPhP behaviors. This work advances the understanding of FIR SPhPs in polar dielectrics and establishes a robust platform for studying FIR phonon polariton materials.