Utilizing Membrane Thickness for Tunable Surface Phonon-Polaritons in Strontium Titanate Membranes

Nanophotonics with polaritons in 2D photonic membranes is emerging as a rapidly evolving field with numerous applications in sensing, nano-optics, and integrated circuits. Here we demonstrate that strontium titanate (SrTiO₃) holds promise as a versatile platform for nanophotonics in oxide membranes. In bulk form, SrTiO₃ exhibits tunable insulating, ferroelectric, metallic, and superconducting states. Recently, advances in strain engineering have also allowed ferroelectric transitions to be engineered 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 100 nm using scanning near-field optical microscopy (SNOM). High resolution near-field imaging clearly resolves propagating polariton modes near membrane edges. Meanwhile, nano-infrared spectroscopy measurements acquired across these membranes allow us to determine the real-space dispersion of SPhPs in this novel photonic platform. Owing to inter-interface coupling of polarization across the membrane thickness, we observe unconventional polariton dispersion and attendant negative group velocity for polariton wave packets, in accord with predictions based on coupled mode theory. Comparisons of our measured and simulated SPhP dispersion demonstrate the tunability of polaritons with the thickness of the SrTiO₃ membrane. Our work lays the foundation for tunable photonics in a wide range of complex-oxides, including negative-index photonics in SrTiO₃ membranes.