Abstract
Guiding, storing, and processing light at the nanoscale hinges on understanding how polaritons - hybrid quasiparticles of light and matter - propagate and interfere in both space and time. This work introduces a synchrotron-based technique, SYnchrotron SpaceTimE Mapping (SYSTEM), which captures real-time evolution of polariton wave packets with ∼10 nm spatial and sub-100 fs temporal resolution across an ultrabroadband 5-50 THz range. Here, SYSTEM directly visualizes the creation, interference, and decay of multiple high-quality Fabry-Pérot phonon polariton cavity modes in an α-MoO(3) microcavity. These real-space, real-time observations reveal wave-packet dynamics and cavity resonances with record-high quality factors (Q ≈ 100) in the single-digit terahertz regime near 9 THz. SYSTEM thus offers a powerful and broadly applicable platform for probing and engineering ultraslow, deeply subwavelength polaritons, opening new avenues for tailoring light-matter interactions and advancing next-generation THz nanophotonic technologies.