  Tailoring light-matter interaction is essential to realize nanophotonic
components. It can be achieved with surface phonon polaritons (SPhPs), an
excitation of photons coupled with phonons of polar crystals, which also occur
in 2d materials such as hexagonal boron nitride or anisotropic crystals.
Ultra-confined resonances are observed by restricting the SPhPs to cavities.
Phase-change materials (PCMs) enable non-volatile programming of these cavities
based on a change in the refractive index. Recently, the new plasmonic PCM
In$_3$SbTe$_2$ (IST) was introduced which can be reversibly switched from an
amorphous dielectric state to a crystalline metallic one in the entire infrared
to realize numerous nanoantenna geometries. However, reconfiguring SPhP
resonators to modify the confined polaritons modes remains elusive. Here, we
demonstrate direct programming of confined SPhP resonators by phase-switching
IST on top of a polar silicon carbide crystal and investigate the strongly
confined resonance modes with scanning near-field optical microscopy.
Reconfiguring the size of the resonators themselves result in enhanced mode
confinements up to a value of $\lambda/35$. Finally, unconventional cavity
shapes with complex field patterns are explored as well. This study is a first
step towards rapid prototyping of reconfigurable SPhP resonators that can be
easily transferred to hyperbolic and anisotropic 2d materials.
