Fate of the upper polariton: Breakdown of the quasiparticle picture in the continuum

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.

Figure 1

(a) Simplified energy diagrams for the photon and exciton fields, the latter being embedded in a nondispersive continuum of incoherent excitations. (b) Measured absorption spectrum of the active layer expressed as $\text{Im}\left[\chi \right(\omega \left)\right]$ (red curve). Blue curve corresponds to the theoretical modeling of the absorption, while the green one to that of a single exciton peak with a standard broadening ${\gamma }_X.$ (c) Experimental reflectance spectra at normal incidence as a function of the cavity detuning. Dashed black lines correspond to the LP and UP energies while the white lines are guidelines for the continuum broadening of the Frenkel exciton.

Figure 2

Quasiparticle residue of the polariton branches calculated at normal incidence. Red curves correspond to ideal polaritons, while blue ones denote the generalized Hopfield coefficients. The yellow area depicts the deviation from the ideal UP and illustrates the breakdown of the quasiparticle picture for this branch.

Figure 3

Top: Experimental $s$-polarized reflectance as a function of the in-plane momentum, $k_{\left|\right|}$, for several values of the detuning, $\delta$. (a) Negative detuning $\delta =-0.18\text{eV},$ (b) close to maximal mixing $\delta =-0.07\mathrm{eV}$, and (c) positive detuning $\delta =0.15\text{eV}$. The black squares denote the polariton energies obtained from the field theory. Bottom: Spectral function of the cavity field for the same parameters as the experiment.