Phase-Change materials such as the Ge₃Sb₂Te₆ compound (GST) can be reversibly switched between the amorphous and crystalline crystal phases by ultrashort laser pulses. At infrared wavelengths, the two phases of GST exhibit a large permittivity contrast, which can be used to control the dielectric background sensed by guided optical modes that propagate in close proximity.

In their work, the authors demonstrate experimentally and theoretically that a thin GST film on a polaritonic substrate (SiC) serves as an actively tunable material system for the infrared spectral range. The system not only supports index-shifted p-polarized surface phonon polaritons (SPhPs), but additionally also s-polarized waveguide modes. Intriguingly, the waveguide modes replicate the properties of the SPhP that define its suitability for nanophotonic applications, such as the enhancement of local electric fields and subwavelength confinement.

The authors show experimentally that both modes can be actively tuned by switching the GST phase, with an exceptionally high tuning figure of merit of up to 7.7. With this unique tunability of its omnipolarized guided modes, the material system provides a promising platform for nanophotonic applications such as in-plane metasurfaces or polariton lenses.