# Surface Polariton-Like s-Polarized Waveguide Modes in Switchable Dielectric Thin-Films on Polar Crystals

## TL;DR

This paper demonstrates that switchable dielectric thin films can support s-polarized waveguide modes with properties similar to surface phonon polaritons, enabling non-volatile control and expanding the functional range of nanophotonic devices.

## Contribution

It introduces a novel s-polarized waveguide mode in switchable dielectric films that mimics surface polaritons and can be dynamically controlled, broadening nanophotonic capabilities.

## Key findings

- s-polarized waveguide mode exhibits dispersion and confinement similar to SPhP
- Switching GST film controls mode dispersion non-volatilely
- Applicable to various polar crystals in the infrared range

## Abstract

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.