The Fate of the Upper Polariton: Breakdown of the Quasiparticle Picture in the Continuum

TL;DR

This paper investigates how incoherent matter excitations affect organic polaritons, revealing the breakdown of the quasiparticle picture and the emergence of complex quantum states at the upper polariton branch.

Contribution

It experimentally demonstrates the failure of the quasiparticle model for upper polaritons and introduces a theoretical framework to understand this complex hybridisation.

Findings

Well-defined lower polariton branch observed

Fading of the upper polariton at continuum entrance

Breakdown of the quasiparticle picture for the upper branch

Abstract

Organic polaritons resulting from the strong hybridisation between photons and matter excitations have arisen as a suitable platform to device light-matter technological interfaces at room temperature. Despite their inherent complexity, organic polaritons are commonly regarded as coherent light-matter excitations that can be described in terms of Landau's quasiparticle approach. Here, we experimentally unveil the role of incoherent matter excitations on the polaritons by exploring the relevant energy-momentum parameter space. We demonstrate the emergence of a well-defined lower polariton branch and an intriguing fading of the upper polariton at its entrance to a continuum of matter excitations. This marks the breakdown of the simplistic quasiparticle picture for this branch and the formation of a more complex quantum state. Our experimental results are sustained by a general theoretical framework that allows understanding the light-matter hybridisation beyond the quasiparticle approach. Our work expands the understanding of organic polaritons in all their complexity and increases their technological significance.