Microscopic quantum description of surface plasmon polaritons: revealing intrinsic ultrastrong light-matter coupling

TL;DR

This paper develops a comprehensive quantum theory of surface plasmon polaritons, revealing intrinsic ultrastrong light-matter coupling and unconventional ground-state fluctuations, with implications for quantum plasmonics.

Contribution

It introduces a microscopic quantum framework based on Power-Zienau-Woolley electrodynamics for arbitrary geometries, highlighting ultrastrong coupling effects at metal-dielectric interfaces.

Findings

Reveals geometry-dependent renormalization of plasma frequency

Derives exact dispersion relations for surface plasmon polaritons

Shows inherent ultrastrong coupling regime at interfaces

Abstract

We develop a microscopic quantum theory of surface plasmon polaritons valid for arbitrary metal-dielectric geometries. Our framework is based on the Power-Zienau-Woolley representation of quantum electrodynamics, which provides an optimal separation between electronic and photonic degrees of freedom and is therefore particularly well suited for constructing quantum descriptions of polaritonic excitations in strongly dispersive media. Within this formulation, the fundamental electronic oscillator is identified as the bulk plasmon mode, which is nonperturbatively coupled to the radiative continuum of free photon modes. This coupling induces a geometry-dependent renormalization of the bulk plasma frequency, giving rise to confined plasmonic resonances. As specific applications, we recover the localized surface plasmon modes of metallic nanoparticles, including radiative frequency shifts and decay, as well as the exact dispersion relation of propagating surface plasmon polaritons at planar interfaces. Our quantum treatment further reveals that light-matter interactions at metal-dielectric interfaces are inherently in the ultrastrong coupling regime. As a result, in the quasistatic limit, the system exhibits unconventional ground-state quantum fluctuations that can be controlled through the refractive index. These results open new intriguing perspectives in the field of quantum plasmonics.