Description of Surface Polaritons Within a Classical Coupled Harmonic Oscillator Model

Coupled harmonic oscillator models are widely used to describe strong and ultrastrong light–matter coupling, providing a simple yet powerful framework for the quantitative analysis of hybrid modes across a broad range of physical systems. Surprisingly, this approach has not been applied to describe surface phonon polaritons and surface plasmon polaritons, which are among the most fundamental examples of strongly coupled light–matter modes. Here, we analyze their dispersion relations using a harmonic oscillator model based on momentum coupling. We show that the surface polaritons—hybrid modes derived from classical electromagnetic theory—are exactly reproduced when the uncoupled oscillators are chosen as the surface phonon (or surface plasmon) mode and the dielectric Brewster mode. This identifies the dielectric Brewster mode as the photonic component of surface polaritons—a role that is typically left unspecified in the literature—and demonstrates the relevance of the coupled harmonic oscillator model for describing surface polariton dispersions.