We just published an article in Physical Review B on “Tensor network simulation of polaron-polaritons in organic microcavities”. In it, we discuss the consequences of strong light-matter coupling of electronic transitions on the vibrational (phononic) degrees of freedom of organic molecules. For these kinds of molecules in free space, excitation with visible light leads to the formation of an exciton, a bound electron-hole pair in the molecule. However, for many organic molecules, the exciton formation (i.e., electronic excitation) also leads to a significant reconfiguration of the nuclear positions, which can be understood as the formation of a so-called polaron, an electronic excitation dressed by a “cloud” of phonons (quantized molecular vibrations).
When such molecules are placed in an optical cavity with a strongly confined light mode that is resonant with the exciton, the system can enter into the so-called “strong coupling” regime, in which the fundamental excitations become hybrid light-matter states, so-called exciton-polaritons (or more precisely, polaron-polaritons, as they contain the traces of both strong exciton-photon and exciton-phonon coupling). Several works in the past few years have shown that under strong light-matter coupling, the lowest excited state (“lower polaron-polariton” or LPP) shows significantly reduced vibrational dressing compared to the bare exciton. However, these articles all used molecular models with just a single vibrational degree of freedom, whereas real molecules have a large number of them as all the atoms can move, and additionally interact with an almost continuous bath of phononic modes in the host medium in typical experiments. This motivated us to investigate a model where many vibrational degrees of freedom could be included explicitly.
In order to be able to find the eigenstates of a quantum model with many degrees of freedom, we exploited the numerical method of tensor networks. This method describes a high-dimensional quantum state as a connected network of many lower-dimensional tensors, and significantly reduces the necessary numerical effort as long as there is only limited entanglement within the system. Exploiting this method, we demonstrated that reduced vibrational dressing is still present in the LPP when all vibrational degrees of freedom are taken into account. Importantly, we were able to show that the influence of the phononic environment on the electronic and photonic properties of the LPP can be predicted from just two collective parameters of the vibrational modes, which can be reproduced by correctly choosing the frequency and coupling strength in a single-mode model. In addition, we explored vibrational properties of the LPP that can be addressed exclusively by our extended model and could be experimentally tested. For example, our findings indicate that vibronic coupling is more efficiently suppressed for environments dominated by low-frequency vibrational modes.