Vibrational Polaritons in Disordered Molecular Ensembles

Abstract

Disorder is an intrinsic attribute of any realistic molecular system. It is known to lead to localization, which hampers efficient transport. It was recently proposed that in molecular ensembles strongly coupled to photonic cavities, moderate disorder leads to delocalization and increases of the transport and chemical reaction rates. Vibrational polaritons involve molecular vibrations hybridized with an infrared cavity. When the coupling strength largely exceeds the molecular inhomogeneity, polaritons are unaffected by disorder. However, in many experiments, such a homogeneous limit does not apply. We investigated vibrational polaritons involving molecular ensembles with systematically modified disorder. Counterintuitively, moderate disorder leads to an increase in Rabi splitting and the modification of the polariton bandwidths. Experimental spectroscopic data agree with a Tavis–Cummings-like model that suggests enhanced delocalization of the reservoir states occurs via the admixture of the cavity mode. Our results provide new insights into the paradigm of disorder-induced cavity-assisted delocalization in molecular polaritons.

Embrace the darkness: An experimental perspective on organic exciton–polaritons

“…Further computational and experimental work has revealed that this description holds true only when the coupling strength exceeds the excitonic inhomogeneous linewidth; for such large Rabi splittings the polariton states exhibit a homogeneous linewidth unaffected by the excitonic disorder. 81,130,131 However, this regime is rare, particularly in molecular systems, and in the more common situation where the Rabi splitting is commensurate with molecular disorder the polaritonic linewidths are directly modified. Indeed, experimental results suggest the LP and UP linewidths can differ strongly, dependent on their overlap with molecular absorption bands.…”

“…Indeed, experimental results suggest the LP and UP linewidths can differ strongly, dependent on their overlap with molecular absorption bands. 131 In the presence of moderate disorder, they can even drop below the expected homogeneous linewidth, an effect described elsewhere as motional narrowing. 130,132,133 More recently, there has been renewed focus on the influence of disorder on the eigenstates and the polariton linewidths and splittings.…”

Controlling the Manifold of Polariton States Through Molecular Disorder

“…[29][30][31][32] Not only is mixing predicted between different material states, the presence of disorder additionally causes these dark states to mix with the photon mode. [29,33,34] The resulting eigenstates have been variously described as subradiant or gray states. This term refers to eigenstates of the cavity system that in the absence of disorder, i.e., in the idealized TC model, would be unable to interact with the photon mode and remain optically forbidden, but in the presence of static disorder acquire varying degrees of photonic character.…”

Non-Hermitian Hamiltonians for linear and nonlinear optical response: A model for plexcitons

“…48 Models of disordered polaritons have suggested the existence of exceptional points in these non-Hermitian Hamiltonians, which vary according to the degree of disorder, 49 and these have been measured experimentally in infrared cavities. 50 Non-Hemiticity of the Hamiltonian can arise from a number of different scenarios not only due to energy relaxation but also from particle decay, for example, in photoionization. Moiseyev et al proposed a more realistic model of molecules, including the ionized continuum states, and studied their modification inside a cavity.…”