Highly Confined Surface and Epsilon-Near-Zero Phonon Polaritons in SrTiO₃ Nanomembrane

Free-standing, wafer-scale, single-crystalline complex oxide membranes have enabled a new set of optical, electrical, and magnetic functionalities and structural and quantum phase transitions. Recent theoretical studies suggest that oxide perovskites nanomembranes support surface phonon polaritons with low-loss and strong confinement. In this work, we explore the SrTiO₃ nanomembrane as a photonic platform for phonon polaritons in the far-infrared regime. Our combined far-field and near-field infrared spectroscopic and nanoscopic studies reveal two types of phonon polaritons on a SrTiO₃ membranes transferred on gold and thermally oxidized silicon substrate, namely surface phonon polaritons and epsilon-near-zero(ENZ) polaritons. Radiative Berreman modes and bound ENZ polaritons confined within the nanomembrane are detected in the frequency window with epsilon zero, resulting from the deep subwavelength thickness of the membrane . Moreover, we reveal highly confined, propagating surface phonon polaritons through real-space hyperspectral nanoimaging. Surface polaritons with up to 7 times increased momenta as compared to surface polaritons of the same energy in bulk SrTiO₃ are detected. Our results suggest that perovskite oxide nanomembranes are promising two-dimensional polaritonic platforms within the terahertz regime.