Quantum theory of longitudinal-transverse polaritons in nonlocal thin films

TL;DR

This paper develops a quantum theory for longitudinal-transverse polaritons in nonlocal thin films, revealing how nonlocal effects enable hybrid modes that can channel energy from electrical currents to radiation.

Contribution

It introduces an analytical polaritonic theory that quantifies nonlocal coupling in dielectric nanolayers, advancing understanding of hybrid longitudinal-transverse polaritons.

Findings

Calculated the spectrum of longitudinal-transverse polaritons in nanolayers.

Developed an analytical model for nonlocal coupling effects.

Identified conditions for the emergence of hybrid modes.

Abstract

When mid-infrared light interacts with nanoscale polar dielectric structures optical phonon propagation cannot be ignored, leading to a rich nonlocal phenomenology which we have only recently started to uncover. In properly crafted nanodevices this includes the creation of polaritonic excitations with hybrid longitudinal-transverse nature, which are predicted to allow energy funnelling from longitudinal electrical currents to far-field transverse radiation. In this work we study the physics of these longitudinal-transverse polaritons in a dielectric nanolayer in which the nonlocality strongly couples epsilon-near-zero modes to longitudinal phonons. After having calculated the system's spectrum solving Maxwell's equations, we develop an analytical polaritonic theory able to transparently quantify the nonlocality-mediated coupling as a function of the system parameters. Such a theory provides a powerful tool for the study of longitudinal-transverse polariton interactions and we use it to determine the conditions required for the hybrid modes to appear.