  Recent theoretical studies have suggested that transition metal perovskite
oxide membranes can enable surface phonon polaritons in the infrared range with
low loss and much stronger subwavelength confinement than bulk crystals. Such
modes, however, have not been experimentally observed so far. Here, using a
combination of far-field Fourier-transform infrared (FTIR) spectroscopy and
near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the
phonon-polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO3
transferred on metallic and dielectric substrates. We observe a
symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and
Berreman modes as well as highly confined (by a factor of 10) propagating
phonon polaritons, both of which result from the deep-subwavelength thickness
of the membranes. Theoretical modeling based on the analytical finite-dipole
model and numerical finite-difference methods fully corroborate the
experimental results. Our work reveals the potential of oxide membranes as a
promising platform for infrared photonics and polaritonics.
