Abstract
In most theoretical descriptions of collective strong coupling of organic molecules to a cavity mode, the molecules are modeled as simple two-level systems. This picture fails to describe the rich structure provided by their internal rovibrational (nuclear) degrees of freedom. We investigate a first-principles model that fully takes into account both electronic and nuclear degrees of freedom, allowing an exploration of the phenomenon of strong coupling from an entirely new perspective. First, we demonstrate the limitations of applicability of the Born-Oppenheimer approximation in strongly coupled molecule-cavity structures. For the case of two molecules, we also show how dark states, which within the two-level picture are effectively decoupled from the cavity, are indeed affected by the formation of collective strong coupling. Finally, we discuss ground-state modifications in the ultrastrong-coupling regime and show that some molecular observables are affected by the collective coupling strength, while others depend only on the single-molecule coupling constant.
2 More- Received 10 June 2015
DOI:https://doi.org/10.1103/PhysRevX.5.041022
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Published by the American Physical Society
Popular Summary
When organic molecules interact with light modes confined in nanostructures, hybrid light-matter states (polaritons) can be created via so-called “strong coupling.” The participating molecules are affected by the interaction, and this effect can be used for controlling chemical reactions or modifying a material’s properties. Up until now, there has been no consensus on how these modifications occur, and accordingly, there has been no way to accurately predict these modifications or design novel structures to exploit them. Here, we demonstrate that it is possible to understand and predict these modifications by adapting well-known techniques from chemical physics. The extent of the modifications turns out to depend sensitively on which observable is interrogated.
We consider two model molecules, and we simplify our analyses by limiting the number of electrons to just one moving in one dimension. Progress up until now has been hindered by the fact that organic molecules possess a large number of internal (rovibrational) degrees of freedom and do not behave like simple two-level emitters. Here, we employ a first-principles approach that takes into account electronic, nuclear, and electromagnetic degrees of freedom on an equal footing. We demonstrate that the Born-Oppenheimer approximation, which underlies most of our understanding of chemical structure and dynamics and assumes that nuclear and electronic motion can be separated, can break down under strong coupling. However, we can predict exactly how this approximation breaks down; in addition, it also holds true in many important cases. By exploiting this approach, we show that molecular structure is modified even for so-called dark states, which are typically thought to be uncoupled from electromagnetic modes. By calculating absorption cross sections for different coupling strengths, we recover surprising effects such as correlated nuclear motion in separated molecules.
Our findings provide the basis for understanding how to manipulate chemical structure and reactions through strong coupling. We expect that our results will have wide-ranging implications for the design of novel nanostructures.