Note: This paper is part of the JCP Special Topic on Polariton Chemistry: Molecules in Cavities and Plasmonic Media.

Optical constants of organic thin films can be evaluated using the Lorentz oscillator model (LOM) which fails to fit inhomogeneously broadened absorption of highly concentrated molecular films. In modified LOM (MLOM), the inhomogeneous broadening is implemented through a frequency-dependent adjustable broadening function. In this work, we evaluate the optical constants of rhodamine 6G doped poly-vinyl alcohol thin films with varying doping concentration (including also extensively high concentrations) using MLOM, which outperforms LOM by showing a better agreement with the experimental results. Our proposed method provides a way to accurately determine optical constants of isotropic organic thin films only from their absorption spectra without spectroscopic ellipsometry.

Strong coupling between molecules and confined light modes of optical cavities to form polaritons can alter photochemistry, but the origin of this effect remains largely unknown. While theoretical models suggest a suppression of photochemistry due to the formation of new polaritonic potential energy surfaces, many of these models do not account for the energetic disorder among the molecules, which is unavoidable at ambient conditions. Here, we combine experiments and simulations to show that for an ultra-fast photochemical reaction such thermal disorder prevents the modification of the potential energy surface and that suppression is due to radiative decay of the lossy cavity modes. We demonstrate that by increasing the coupling strength we can reduce such losses and enhance reactivity of the strongly coupled system, in contrast to the theoretical paradigm, which would predict stronger suppression. We also show that the excitation spectrum under strong coupling is a product of the excitation spectrum of the ”bare” molecules and the absorption spectrum of the molecule-cavity system, suggesting that polaritons can act as gateways for channeling an excitation into a molecule, which then reacts ”normally”. Our results therefore imply that strong coupling provides a means to tune the action spectrum of a molecule, rather than to change the reaction.

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Fluorescence spectroscopy is commonly employed to study the excited-state photophysics of organic molecules. Planar Fabry-Pérot microcavities play an essential role in such studies and a strategic cavity design is necessary to attain an enhanced light-matter interaction. In this work, we computationally study different geometries for a planar metallic Fabry-Pérot microcavity tuned for the absorption of Sulforhodamine 101, a typical dye for fluorescence spectroscopy. The cavity consists of a polymer layer enclosed between two silver mirrors, where the thicknesses of all the three layers are varied to optimize the cavity. Our transfer-matrix and finite-difference time-domain simulations suggest that a cavity with 30 nm thin top mirror and 200 nm fully reflective thick bottom mirror, thus having only reflection and absorption and no transmission, is an optimal design for maximizing the Purcell factor and spectral overlap between the cavity and molecule, while still sustaining an efficient measurability of the fluorescence.