Surface phonon polaritons (SPhPs) are hybrid light-matter excitations arising from the coupling of photons with optical phonons in polar materials. They exist within Reststrahlen bands, where the real part of the permittivity is negative. In bulk crystals, SPhP confinement is weak, with wavelengths near those in free space. However, reducing the crystal thickness to the deep-subwavelength scale leads to the hybridization of SPhP modes at each interface, creating symmetric and antisymmetric branches. The antisymmetric mode propagates with higher momentum than free-space electromagnetic waves, while the symmetric mode, at sufficiently small thicknesses, becomes an ENZ mode with nearly flat momentum dispersion. These ENZ modes, along with Berreman modes (where the electric field is largely confined within the sample), offer strong subwavelength field enhancement, promising applications in infrared nanophotonics, including sensing, absorption, superlensing, and nonlinear optics. While two-dimensional van der Waals materials exhibit some of these properties, their limited size and anisotropic properties restrict scalability. Perovskite oxides, offering intense optical phonon modes and tunable properties, present a promising alternative. This study focuses on SrTiO3, a well-established material with tunable phononic and photonic properties, to experimentally demonstrate the existence of highly confined SPhPs in ultrathin membranes, a phenomenon previously only predicted theoretically.

Previous research extensively explored surface phonon polaritons (SPhPs) in various materials. Studies on materials like hexagonal boron nitride (hBN), germanium sulfide (GeS), gallium selenide (GaSe), and α-MoO3 have demonstrated low-loss and deeply subwavelength confined phonon polaritons, advancing infrared nanophotonics. However, these van der Waals materials often come as micrometer-scale flakes, limiting scalability. Their inherent optical anisotropy can also complicate applications. Conversely, perovskite oxides, particularly SrTiO3, are known for their intense phonon modes and low-loss polaritons, alongside technological maturity and tunability through various means (electrical/optical excitation, strain, dopants). Theoretical work suggested the existence of highly confined SPhPs in ultrathin SrTiO3 membranes but lacked experimental validation, motivating this study.

This research employed a combination of experimental techniques and theoretical modeling to investigate phonon polaritons in a 100 nm thick SrTiO3 membrane. First, a high-quality single-crystal SrTiO3 membrane with a sacrificial layer was synthesized via pulsed-laser deposition. The sacrificial layer allowed for the transfer of the freestanding membrane onto different substrates (SiO2/Si and Au-coated SiO2/Si). The membrane's quality was verified using scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM). Far-field infrared reflectivity measurements were conducted using a Fourier-transform infrared (FTIR) spectrometer equipped with an infrared microscope and different objectives to control the angle of incidence. These measurements provided information on the Berreman modes. Near-field measurements were performed using synchrotron infrared nanospectroscopy (SINS) with an atomic force microscope (AFM) operating in tapping mode, using a metal-coated tip. SINS provided information on the spatial distribution and propagation of the phonon polaritons. The experimental results were analyzed using a factorized-formula fitting of the reflectivity to obtain the dielectric function of the bulk SrTiO3, and simulations employing the extended-dipole model and finite-difference time-domain (FDTD) methods. The extended-dipole model, which treats the AFM tip as an extended dipole, was used to simulate the near-field scattering signals from the SINS experiments. FDTD simulations were employed to model the electromagnetic field distribution in the SrTiO3 membrane on both substrates at various frequencies.

The study successfully demonstrated the existence of both symmetric and antisymmetric SPhP modes in a 100 nm SrTiO3 membrane. Far-field FTIR reflectivity revealed Berreman modes at frequencies near the longitudinal optical (LO) phonon frequencies. Near-field SINS revealed both ENZ modes (symmetric, observed on both substrates) with strong field enhancement inside the membrane and highly confined antisymmetric modes (observed only on the SiO2/Si substrate). The antisymmetric modes exhibited a momentum 10 times larger than in bulk SrTiO3, demonstrating significant confinement. The theoretical modeling, using the extended dipole model and FDTD simulations, closely matched the experimental SINS and FTIR results, validating the interpretations. The FDTD simulations showed that the antisymmetric modes were suppressed on the gold-coated substrate due to the metal's high conductivity, preventing the field from extending outside the SrTiO3 membrane. Measurements of propagation length and quality factors indicated a propagation length ranging from 6 to 2 µm and quality factors comparable to those observed in graphene and other materials. A redshift in phonon mode frequencies was observed in the membrane compared to bulk SrTiO3, possibly due to strain or surface effects.

The observed symmetric-antisymmetric mode splitting and the highly confined propagating antisymmetric mode in the SrTiO3 membrane directly confirm theoretical predictions. The strong light confinement and field enhancement demonstrated are highly significant for various nanophotonic applications. The ability to easily transfer the SrTiO3 membranes onto different substrates offers flexibility in device design and integration. The suppression of the antisymmetric mode on the metallic substrate highlights the importance of substrate selection for controlling polariton properties. The relatively lower quality factor compared to some other materials suggests potential improvement through using less lossy substrates or employing suspended membranes.

This study provides the first experimental verification of highly confined SPhPs in ultrathin SrTiO3 membranes. The observed ENZ modes and highly confined antisymmetric modes open new possibilities for advanced infrared nanophotonics. Future work could explore further optimization of the membrane properties, investigating different substrates or suspension techniques to improve propagation length and quality factor. The integration of SrTiO3 membranes with other photonic materials to create hybrid polariton structures is also a promising direction for future research. These findings significantly contribute to advancing the field of infrared polaritonics and nanophotonics, paving the way for applications in sensing, light manipulation, and novel devices.

The study focused on a single membrane thickness (100 nm). Further investigation across various thicknesses would provide a more comprehensive understanding of the thickness dependence of the observed phenomena. While the theoretical models well-replicated experimental findings, the extended-dipole model relies on approximations. The extrinsic optical loss introduced by the SiO2/Si substrate could be reduced using alternative substrates.