  Surface phonon polaritons (SPhP) and surface plasmon polaritons (SPP),
evanescent modes supported by media with negative permittivity, are a
fundamental building block of nanophotonics. These modes are unmatched in terms
of field enhancement and spatial confinement, and dynamical all-optical control
can be achieved e.g. by employing phase-change materials (PCMs). However, the
excitation of surface polaritons in planar structures is intrinsically limited
to p-polarization. On the contrary, waveguide modes in high-permittivity films
can couple to both p- and s-polarized light, and in thin films, their
confinement can become comparable to surface polaritons. Here we demonstrate
that the s-polarized waveguide mode in a thin Ge$_3$Sb$_2$Te$_6$ (GST) film
features a similar dispersion, confinement, and electric field enhancement as
the SPhP mode of the silicon carbide (SiC) substrate, while even expanding the
allowed frequency range. Moreover, we experimentally show that switching the
GST film grants non-volatile control over the SPhP and the waveguide mode
dispersions. We provide an analytical model for the description of the GST/SiC
waveguide mode and show that our concept is applicable to the broad variety of
polar crystals throughout the infrared spectral range. As such, complementarily
to the polarization-limited surface polaritons, the s-polarized PCM waveguide
mode constitutes a promising additional building block for nanophotonic
applications.
