Towards a detailed understanding of strong light-matter coupling effects

In the last decade a host of seminal experimental results have demonstrated that molecules and solids can be modified and controlled by coupling strongly to the electromagentic field of an optical cavity. 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 (bare masses of the particles) and introduce a hierachy of approximations in first-principles approaches [2,3]. I will then show how quantum-electrodynamical density-functional theory [4,5] is able to treat all these levels of approximations. 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. This last finding is especially important, since it potentially resolves one of the main discrepancies between predictions of quantum-optical models and experimental results.

References

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

[2] Ruggenthaler, Michael, et al., Nature Reviews Chemistry 2.3 (2018): 1-16.

[3] Ruggenthaler, Michael, et al., Physical Review A 90.1 (2014): 012508.

[4] Ruggenthaler, Michael., arXiv preprint arXiv:1509.01417 (2015).

[5] Jestädt, René, et al., Advances in Physics 68.4 (2019): 225-333.

[6] Flick, Johannes, et al., ACS photonics 6.11 (2019): 2757-2778.

[7] Sidler, Dominik, et al., J. Phys. Chem. 11 (2020): 7525-7530