Active and Passive Tuning of Mid-Far Infrared Surface Phonon Polariton Resonances*

Resonant nanocavities designed for the manipulation of localized and propagating surface phonon polaritons on polar dielectrics hold great promise as an essential platform for mid-far infrared metasurfaces. Passive nanocavity arrays are constructed with deep sub-wavelength grooves of a metal-insulator-polar dielectric layered structure, which acts as a surface waveguide that supports coupled plasmon-phonon polariton hybrid modes. For active nanocavity arrays, vanadium dioxide (VO₂) is a promising candidate as a spacer layer in the surface waveguide because VO₂ undergoes a reversible insulator-to-metal phase transition near room temperature. Here, we employ SiO₂ films as the passive and VO₂ films for the active platform. These materials are applied on two polar dielectric substrates: sapphire and gallium arsenide (GaAs), which Reststrahlen band corresponding to the deep mid-infrared (10 - 20 microns) and far-infrared (28 – 33 microns) regions, respectively. we numerically and experimentally demonstrate passive and active tunable surface phonon polaritonic devices working within the Reststrahlen band. The devices consist of 40 nm thick gold and 100 nm thick spacer on sapphire and GaAs substrates. The cavity resonance shows redshifts as temperature increases for active tuning.