Session B05: The Chemical Physics of Molecular Polaritons II. Photophysics II

First principles approaches to excited states chemistry under strong light-matter coupling

In recent years, research at the interface of chemistry, material science, and quantum optics has surged and now opens new possibilities to study strong light-matter interactions at different limits [1]. In these limits, electrons, nuclei and photons have to be treated on the same quantized footing. Towards this goal, we have introduced a general time-dependent density-functional theory [2].

In this talk, we use a novel linear-response formulation [3] within a density-functional framework to study excited states of strongly light-matter coupled systems. We study how the potential-energy surfaces (PES) of a CO bond stretching in a formaldehyde molecule is modified and show the influence of strong light-matter coupling on avoided crossings. These results demonstrate the novel abilities to alter and open new chemical reaction pathways as well as to create new hybrid states of light and matter [4].

Our work opens an important new avenue in introducing ab initio methods to the nascent field of collective strong light-matter interactions.

[1] J. Flick, N. Rivera, P. Narang, Nanophotonics 7(9), 1479 (2018).
[2] J. Flick, P. Narang, PRL 121, 113002 (2018).
[3] J. Flick, D. Welakuh, M. Ruggenthaler, H. Appel, A. Rubio, ACS Photonics (2019).
[4] J. Flick, P. Narang arXiv:1907.04646 (2019).   

Communication between polaritonic and excitonic manifolds in an electronically strong coupled system

Optically dark states, which arise from intermolecular exciton coupling, are a crucial component in understanding the optical properties of natural and artificial light harvesters. Dark states also constitute a major density of states in the energy manifold of a strong-coupled system, which is generated from the interaction between a cavity photon and molecular excitons. Unlike optically bright upper and lower polariton states, dark state wavefunctions contain no photonic excitation and therefore have been considered to retain molecular character. In this framework, dark states not only serve as a deactivation pathway but also can participate in excitation energy exchange to states that are not coupled to cavity photons. We test this view by following the excited-state dynamics of bright to dark state interconversion in an electronically strong-coupled cavity composed of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). We show that dark states have a lifetime longer than that of uncoupled molecules indicating a significant change in the state character. Discussion on the communication between the dark states and the exciton manifold of uncoupled 4CzIPN triplet states will be presented.   

Mixed Quantum-Classical Methods for Molecules in Cavities

Describing chemical processes that are strongly correlated with quantum light requires an accurate, flexible, and computationally efficient treatment of light-matter interactions. Thus, in order to develop ab-initio theoretical descriptions of cavity modified chemical systems, extensions to the traditional theoretical tool-kits for quantum optics and quantum chemistry are required. Here, we investigate the extension of mixed quantum-classical trajectory methods as well as the concept of time-dependent potential energy surfaces, both traditionally introduced for electron-nuclear problems, to the photonic degrees of freedom. Within our work we find that classical Wigner dynamics for photons can be used to describe quantum effects such as correlation functions, bound photon states and cavity induced suppression by properly accounting for the quantum statistics of the vacuum field while using classical/semi-classical trajectories to describe the time-evolution. Investigating the time-dependent potential energy surfaces for photons we find significant differences with the potential used in conventional approaches. Furthermore, we analyze the exact time-dependent potential energy surface driving the proton motion of a cavity-induced chemical suppression.   

Mixed quantum-classical simulation of molecules in a cavity

Photons in an optical cavity can strongly interact with electronic and vibronic state of molecules, which provides great promise to manipulate chemical reaction in cavity. Mixed quantum-classical methods, such as mean-field Ehrenfest, have been successfully applied to traditional photochemical reaction and provide many valuable insights into the reaction mechanisms. In this talk, we will present our recent results on how to generalize the mean-field Ehrenfest dynamics to the molecules in cavity by treating nuclei classically and other degrees of freedom quantum mechanically. The derived quantum force is general and can be used to include any number of electronic and photon states. As an example, we use our method to study the dissociation of LiF in cavity and compare the results with exact quantum simulation, which shows the reaction outcomes can be controlled by the coupling strength and the dipole self-energy term. Our approximate approach produces accurate results compared with exact quantum simulation, and can be broadly applied to investigate molecular reactions in the cavity.