Organic polaritons, strongly hybridized light-matter excitations, have arisen as a platform to device optical interfaces at room temperature. Despite their inherent complexity, polaritons are commonly regarded as coherent excitations described by Landau's quasiparticle theory. Here, we theoretically study and experimentally demonstrate the role of incoherent matter excitations in terms of generalized Hopfield coefficients, which unveil the fading of the upper polariton at its entrance to a continuum of molecular excitations. This marks the breakdown of the simplistic quasiparticle picture and the formation of a more complex quantum state beyond Landau's formalism.

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• Exciton polariton
• Feynman diagrams
• Quasiparticles & collective excitations
• Organic semiconductors
• Optical microscopy