Molecular emission near metal interfaces: the polaritonic regime

The strong coupling of a dense layer of molecular excitons with surface-plasmon modes in a metal gives rise to polaritons (hybrid light-matter states) called plexcitons. Surface plasmons cannot directly emit into (or be excited by) free-space photons due to the fact that energy and momentum conservation cannot be simultaneously satisfied in photoluminescence. Most plexcitons are also formally non-emissive, even though they can radiate via molecules upon localization due to disorder and decoherence. However, a fraction of them are bright even in the presence of such deleterious processes. In this letter, we theoretically discuss the superradiant emission properties of these bright plexcitons, which belong to the upper energy branch and reveal huge photoluminescence enhancements compared to bare excitons. Our study generalizes the well-known problem of molecular emission next to a metal interface to collective molecular states and provides new design principles for the control of photophysical properties of molecular aggregates using polaritonic strategies.Comment: Replaced previous version, noticing that van Hove anomalies are only observed in the direct and reflected contributions of photoluminescence, but they cancel out when added up in the total photoluminescence. The correct phenomenology is that enhancements of photoluminescence are still huge (not infinite) and are near (not exactly at) the critical poin

Linear response of molecular polaritons

In this article, we show that the collective light-matter strong coupling regime, where $N$ molecular emitters couple to the photon mode of an optical cavity, can be mapped to a quantum impurity model where the photon is the impurity that is coupled to a bath of anharmonic transitions. In the thermodynamic limit where $N\gg1$, we argue that the bath can be replaced with an effective harmonic bath, leading to a dramatic simplification of the problem into one of coupled harmonic oscillators. We derive simple analytical expressions for linear optical spectra (transmission, reflection, and absorption) where the only molecular input required is the molecular linear susceptibility. This formalism is applied to a series of illustrative examples showcasing the role of temperature, disorder, vibronic coupling, and optical saturation of the molecular ensemble, explaining that it is useful even when describing an important class of nonlinear optical experiments. For completeness, we provide a comprehensive Appendix that includes a self-contained derivation of the relevant spectroscopic observables for arbitrary anharmonic systems (for both large and small $N$) within the rotating-wave approximation. While some of the presented results herein have already been reported in the literature, we provide a unified presentation of the results as well as new interpretations that connect powerful concepts in open quantum systems and linear response theory with molecular polaritonics.Comment: 15 pages, 6 figure

Remarks on time-dependent [current]-density functional theory for open quantum systems

Time-dependent [current]-density functional theory for open quantum systems (OQS) has emerged as a formalism that can incorporate dissipative effects in the dynamics of many-body quantum systems. Here, we review and clarify some formal aspects of these theories that have been recently questioned in the literature. In particular, we provide theoretical support for the following conclusions: (1) contrary to what we and others had stated before, within the master equation framework, there is in fact a one-to-one mapping between vector potentials and current densities for fixed initial state, particle–particle interaction, and memory kernel; (2) regardless of the first conclusion, all of our recently suggested Kohn–Sham (KS) schemes to reproduce the current and particle densities of the original OQS, and in particular, the use of a KS closed driven system, remains formally valid; (3) the Lindblad master equation maintains the positivity of the density matrix regardless of the time-dependence of the Hamiltonian or the dissipation operators; (4) within the stochastic Schrodinger equation picture, a one-to-one mapping from stochastic vector potential to stochastic current density for individual trajectories has not been proven so far, except in the case where the vector potential is the same for every member of the ensemble, in which case, it reduces to the Lindblad master equation picture; (5) master equations may violate certain desired properties of the density matrix, such as positivity, but they remain as one of the most useful constructs to study OQS when the environment is not easily incorporated explicitly in the calculation. The conclusions support our previous work as formally rigorous, offer new insights into it, and provide a common ground to discuss related theories.Chemistry and Chemical Biolog