UC San Diego

Molecular polaritons offer a promising avenue for manipulating light and matter properties through both single-molecule and collective strong light-matter coupling within optical cavities. Over the past decade, numerous theoretical and experimental studies have reported changes in optical and chemical properties as a result of this strong interaction. However, the field is fraught with inconsistent findings. Some experimental results cannot be reproduced or are later given non-polaritonic explanations, while theoretical models often fail to account for observed changes and make correct predictions. This disconnect between theory and experiment arises from the use of overly simplistic models to explain the highly complex nature of polaritonic systems in general, and organic molecules in particular.

Specifically, in the field of polariton chemistry, which aims to exploit collective strong coupling to modify chemical reactivity, there has been a tendency to interpret experiments conducted in the collective regime using single-molecule strong coupling models. In the case of single-molecule strong coupling, the excited states of individual molecules hybridize with cavity modes to create vibronic-polariton states, altering the energy levels of the molecules and hence their reactivity. In contrast, in the collective regime, polaritons are excitations delocalized over the entire ensemble of molecules, and it is unclear how they influence the local vibronic dynamics of individual molecules.

This thesis presents our efforts to unveil the novel photochemical and photophysical phenomena in organic exciton polaritons. Our findings can be summarized as follows: while collective strong light-matter coupling can significantly alter optical properties, such as the photonic density of states, it has negligible direct effects on the internal degrees of freedom of individual molecules, which are involved in chemical reactivity. Nevertheless, we conclude that polaritonic modifications to optical properties can influence molecular processes in a weak coupling manner, leading to long-range resonance energy transfer, and changes in absorption, emission, and Raman scattering rates. Further advancements require identifying the missing elements in our theories. Effects such as temperature and the multimode nature of optical microcavities may be crucial for understanding the experimental observations that remain unexplained to this day, and for definitively determining novel applications of collective strong light-matter coupling with organic molecules.