UC San Diego

When optical transitions in materials couple strongly to photon modes, light and matter degrees of freedom can no longer be treated separately with the light-matter coupling taken as a perturbation. They form hybrid light-matter modes called polariton modes. Several experiments over the last decade have demonstrated a change in chemical reactivity under collective vibrational strong coupling, where vibrational modes in many molecules couple collectively to a photon mode, starting the field of polariton chemistry. However, from theoretical studies, there is no clear explanation for this phenomenon. Additionally, some recent experiments have shown no effect of collective strong light-matter coupling on reactivity.

In this dissertation, we propose strategies to enhance polaritonic effects and we also seek out avenues outside of chemical reactivity where the effect of strong light-matter coupling could be significant. We demonstrate that the effect of polaritons on chemical reactivity under vibrational strong coupling can be enhanced by vibrational polariton condensation. The macroscopic occupation of the polariton mode through Bose-Einstein condensation amplifies its contribution to the reaction rate.

To explain an experimentally observed reduction in the dehydration temperature in an open cavity system under vibrational strong coupling on resonance, we develop a theoretical model for heat flow based on kinetic equations. We hypothesized that the reduction in temperature is not due to an inherent change in the dehydration temperature of the material, but due to a reduction in the thermal contact resistance across the interface between the molecules and the plasmonic structure due to polaritons.

We also show that the topological properties of a system under electronic strong coupling can be modified through optical pumping with circularly polarized light. Our work relies on selectively saturating electronic transitions with circularly polarized light, and, therefore, creating a population imbalance in the system and breaking time-reversal symmetry.

In the last part of this dissertation, we propose a way to measure small differences in the equilibrium positions of the vibrational coordinate between different electronic states using higher-energy Fock states. This work does not require strong light-matter coupling. Higher-energy vibrational Fock states have a smaller length scale associated with them than the ground vibrational state which aids in inferring smaller displacements in the vibrational coordinate from the electronic absorption spectrum.