Ultrabroadband Spacetime Nanoscopy of Terahertz Polaritons in a van der Waals Cavity

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.