Strongly confined Mid-infrared to Terahertz Phonon Polaritons in Ultra-thin SrTiO3

Abstract: Surface phonon polaritons (SPhPs) have emerged as a promising platform for subwavelength optical manipulation, offering distinct advantages for applications in infrared sensing, imaging, and optoelectronic devices. However, the narrow Reststrahlen bands of conventional polar materials impose significant limitations on their applications across the mid-infrared (MIR) to terahertz (THz) range. Addressing this challenge requires the development of materials capable of supporting SPhPs with broad spectral range, strong field confinement, slow group velocity, and high quality factor. Here, using monochromatic electron energy-loss spectroscopy in a scanning transmission electron microscope, we demonstrate that ultra-thin SrTiO3 membranes encompass the exceptional properties mentioned above that have long been sought after. Systematic measurements across varying membrane thicknesses reveal two distinct SPhP branches characterized by wide spectral dispersion, high field confinement, and anomalously slow group velocities spanning from the MIR (68 ~ 99 meV) to THz (12 ~ 59 meV) range. Notably, in membranes approaching ~ 3 nm thickness (~ 8 unit cells), these polaritons exhibit unprecedented confinement factors exceeding 500 and group velocities as low as ~ 7 * 10-5 c, rivaling the best-performing van der Waals materials. These findings establish perovskite oxide such as SrTiO3 as a versatile platform 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.