Michael Ruggenthaler
Max-Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany

” Towards a detailed understanding of strong light-matter coupling effects “

“An artistic depiction of how strong coupling to the photons of a cavity can modify the equilibrium bond length of a molecule”

In the last decade a host of seminal experimental results have demonstrated that properties and dynamics of molecules and solids can be modified and controlled by coupling strongly to the electromagentic field of a photonic environment, e.g. an optical cavity. However, many of the observed effects are not well understood and the common models of strong light-matter coupling lead to contradictory conclusions. It therefore becomes desirable to have first-principles approaches to strong light-matter coupling in order to obtain a so far elusive detailed understanding of photon-modified matter properties.

In this talk I will discuss the fundamental setting for such ab-initio methods, the Pauli-Fierz quantum field theory in Coulomb gauge [1], highlight subtle yet important issues, e.g., the significance of the bare masses of the particles, and introduce a hierachy of approximations in first-principles approaches to light-matter coupled systems [2,3]. I will then show how the recently developed quantum-electrodynamical density-functional theory [4,5] is able to treat all these levels of approximations, and demonstrate how a detailed understanding of strong light-matter coupling becomes accessible by ab-initio simulations. Among others I will highlight how decoherence and dissipation are naturally included in such simulations [1,5,6], that common models of light-matter interactions become less accurate if applied naively to vibrational excitations [7] and that collective coupling effects imply local strong coupling between light and matter [8]. This last finding is especially important, since it potentially resolves one of the main discrepancies between predictions of quantum-optical models and experimental results.

[1] Spohn, Herbert. Dynamics of charged particles and their radiation field. Cambridge university press, 2004.

[2] Ruggenthaler, Michael, et al. “From a quantum-electrodynamical light–matter description to novel spectroscopies.” Nature Reviews Chemistry 2.3 (2018): 1-16.

[3] Ruggenthaler, Michael, et al. “Quantum-electrodynamical density-functional theory: Bridging quantum optics and electronic-structure theory.” Physical Review A 90.1 (2014): 012508.

[4] Ruggenthaler, Michael. “Ground-state quantum-electrodynamical density-functional theory.” arXiv preprint arXiv:1509.01417 (2015).

[5] Jestädt, René, et al. “Light-matter interactions within the Ehrenfest–Maxwell–Pauli–Kohn–Sham framework: fundamentals, implementation, and nano-optical applications.” Advances in Physics 68.4 (2019): 225-333.

[6] Flick, Johannes, et al. “Light–matter response in nonrelativistic quantum electrodynamics.” ACS photonics 6.11 (2019): 2757-2778.

[7] Sidler, Dominik, et al. “Chemistry in Quantum Cavities: Exact Results, the Impact of Thermal Velocities, and Modified Dissociation” J. Phys. Chem. Lett. 11 (2020): 7525-7530

[8] Sidler, Dominik, et al. “Polaritonic Chemistry: Collective Strong Coupling Implies Strong Local Modification of
Chemical Properties” J. Phys. Chem. Lett. 12 (2020): 508-516