Polaritons for Chemistry and Materials Science

Strong coupling of excitons with electromagnetic (EM) confined modes is achieved when the energy exchange rate between exciton and EM field modes becomes faster than the decay and decoherence rates of either constituent. As a result, New light-matter hybrid states called polaritons are formed. In this talk we will show how some material and chemical properties of molecules can be tuned and modified by taking advantage of this QED phenomenon.

Exciton transport plays a crucial rule in natural phenomena such as photosynthesis and in artificial devices such as organic solar cells but is inefficient in many organic materials. We will discuss how the formation of collective polaritonic modes can dramatically enhance exciton conductance when the molecules are strongly coupled to a EM mode, which can be exploited to “harvest” and direct excitations to specific positions by tuning the spatial distribution of the EM mode. We then show that in systems with a discrete EM mode spectrum, strong-coupling-enhanced exciton transport can proceed through “dark” modes that have no photonic component, but which nonetheless acquire a delocalized character in the strong-coupling regime.

In the second part, we discuss the influence of strong coupling on internal molecular structure and chemical reactions. While most models of strong coupling are based on simple two-level models, pioneering experiments have shown modifications of chemical reaction rates under strong coupling. In order to address this mismatch, we have developed a first-principles model that fully takes into account both electronic and nuclear degrees of freedom. We will first discuss the applicability of the Born-Oppenheimer approximation, which is challenged by the introduction of the new intermediate timescale of energy exchange between the molecule and the field. Based on these findings, we then show how photochemical reactions such as photoisomerization can be almost completely suppressed under strong coupling. We will also show how polaritons can lead to the formation of a polaritonic ``supermolecule'' involving the degrees of freedom of all molecules, opening a reaction path on which all involved molecules undergo a chemical transformation. Finally, we will discuss how the phenomenon of strong coupling could be utilized to modify ground-state chemical reactions.