Strongly confined mid-infrared to terahertz phonon polaritons in ultrathin SrTiO₃

Phonon polaritons (PhPs) enable subwavelength light control for infrared sensing, imaging, and optoelectronics, but conventional polar materials have narrow Reststrahlen bands, limiting applications. Materials that support PhPs with broad spectral range, strong field confinement, slow group velocity, and high-quality factor are therefore needed. Here, using monochromatic electron energy loss spectroscopy in a scanning transmission electron microscope, we demonstrate that ultrathin SrTiO₃ membranes have the desired properties. Systematic measurements across varying thicknesses reveal two PhP branches with wide spectral dispersion, strong confinement, and exceptionally slow group velocities spanning from the mid-infrared to terahertz range. Notably, in 3-nanometer-thick membranes, these polaritons exhibit unprecedented confinement factors exceeding 500 and group velocities as low as ~7 × 10^-5c, rivaling the best-performing van der Waals materials. These findings establish perovskite oxide such as SrTiO₃ as versatile platforms for tailoring light-matter interactions at the nanoscale, providing critical insights for the design of next-generation photonic devices requiring broadband operation and enhanced optical confinement.