Negative Polariton Dispersion in Freestanding Oxide Membranes Tuned by Interfacial Strong Coupling

Here we demonstrate that polaritons in free-standing strontium titanate (SrTiO3) membranes hold promise as a versatile platform for nanophotonics. In bulk form, SrTiO3 exhibits tunable insulating, ferroelectric, metallic, and superconducting states. Recently, advances in strain engineering have also realized ferroelectricity in thin-film SrTiO3. In this work, we investigate the photonic properties of mid-infrared surface phonon-polaritons (SPhPs) in SrTiO3 membranes of varied thickness from 10 to 120 nm using scanning near-field optical microscopy (SNOM). High resolution near-field imaging clearly resolves propagating substrate-tunable polariton modes near membrane edges and surface defects. Meanwhile, nano-infrared spectroscopy measurements acquired across these membranes allow us to determine the real-space dispersion of SPhPs in this novel photonic platform. As predicted by coupled mode theory, interfacial coupling of polarization across the membrane thickness gives rise to unconventional polariton dispersion and attendant negative group velocity. Additionally, we quantify the symmetric-asymmetric polariton mode splitting in varying dielectric environments and membrane thicknesses, and propose a novel design for in-plane Veselago lensing in oxide membranes. Thus, our work lays the foundation for tunable photonics in a wide range of complex-oxides, including negative-index photonics in SrTiO3 membranes.