Leveraging surface phonon polaritons for enhanced Q-factor of mid-infrared BaTiO3 nanoresonators

Abstract

All-dielectric metasurfaces supporting surface phonon polaritons (SPhPs) have lower optical loss and stronger optical confinement compared to plasmonic systems. However, most polar materials with SPhPs have narrow Reststrahlen bands, typically within a few micrometers, thus limiting their application range. Barium titanate (BTO) is an anisotropic polar dielectric with two Reststrahlen bands supporting SPhPs in the mid-infrared to terahertz range and exhibits high-quality SPhPs characteristics due to its high dielectric function ratio (−ε′/ε′′). Herein, we construct BTO anisotropic metasurfaces and investigate the excitation, coupling, and application of SPhPs modes within them. Localized optical modes at subwavelength scales can be excited over a wide range but require large meta-atomic heights (>300 nm). The higher-order hybrid mode predominantly displays monopole characteristics, boasting a high Q-factor of 125, while the transverse dipole mode exhibits robust field enhancement capabilities (|E/E₀| = 173, a penetration depth of 404.7 nm). Together, they surpass conventional all-dielectric metasurfaces by nearly a factor of two in correlation performance. These properties are theorized to offer significant advantages in thermal emission, self-referenced sensing, and molecular detection. The findings of this work confirm the considerable contribution of SPhPs to promoting BTO metasurfaces for applications in mid-infrared nanophotonics.