In conventional chemistry, molecules interact with light in a regime termed “weak coupling”, meaning that during their interaction, light does not significantly perturb the molecules, but simply acts as an irreversible energy source that brings the molecules to a higher energy state. In more visual terms, light interacting with a molecule can be understood as a ball “kicked“ up-hill, where light provides the energy “kick” to the molecule. Depending on the energy of light, different states of the molecule are accessed. For instance, in solar cells, sunlight absorbed by molecules slightly redistributes their electrons, eventually converting it to electrical current. In the opposite regime, known as “strong coupling”, the interaction between light and matter is no longer “irreversible”: after light “kicks” a molecule, the molecule is able to “kick” the light back. Such back and forth “kicking“ essentially represents a continuous energy flow between the light and the molecule, and is responsible for the formation of new states called polaritons that do not exists in the standard “weak coupling” regime. In recent years, the field of polaritonic chemistry has become increasingly popular as a new means to manipulate chemical processes with light by the formation of these new polariton states. Because these states are mixed consisting of both light and matter, they have hybrid properties and can potentially harness the best of both worlds.
To fully exploit the potential of both weak and strong light-matter interactions for targeted applications with molecular platforms, it is essential to develop a detailed microscopic understanding of the underlying physical and chemical phenomena. Towards this end, the overall goal of the present proposal is to explore the effect of the environment, such as the role played by disorder and vibrations, as well as the interplay between different molecular spin states, and how these impact the nature of weak and strong light-matter interactions. A particular focus on which aspects may be understood by classical means without invoking the quantum nature of neither light nor matter will be given, paving the way towards more realistic simulations of molecular polaritons where a full quantum treatment is prohibitive.
