Probing Phonon Polaritons via Electron Microscopy: A Review

Phonon polaritons (PhPs) are hybrid excitations arising from the coupling of optical phonons and electromagnetic waves, providing a versatile platform for exploring strong light-matter interaction studies and engineering the infrared response of polar materials. Their capability to confine and manipulate light well below the diffraction limit, combined with low losses, has enabled breakthroughs in subwavelength waveguiding, hyperlensing, infrared imaging, and molecular sensing. While far-field spectroscopy and scanning near-field optical microscopy have been widely used to characterize PhPs, these photon-based techniques often suffer from limited spatial resolution, momentum mismatch, and a scarcity of suitable light sources and detectors in the infrared region. In contrast, scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) has recently emerged as a powerful method for probing PhPs, offering broadband excitation and detection, access to large momentum transfers, sub-nanometer spatial resolution, and substrate-free measurements. Looking ahead, STEM-EELS holds promise for exploring PhPs under in situ conditions-including electrostatic gating, variable temperatures, and mechanical strain-as well as for validating next-generation polaritonic device concepts. When combined with emerging ultrafast electron techniques, STEM-EELS further offers the potential to access polaritonic dynamics, enabling real-time tracking of PhP propagation and damping processes. Addressing challenges such as radiation damage, low signal-to-noise ratios at meV losses, and complex data interpretation will further establish STEM-EELS as an indispensable tool for guiding the design of infrared nanophotonic devices.