High-harmonic generation under electronic strong coupling: A time-dependent combined quantum electrodynamics/quantum chemistry study

The creation of light-matter hybrid states, polaritons, in a cavity offers new intriguing opportunities to manipulate the electronic structure and electron dynamics of atoms and molecules. Here, we investigate the effect of strong electronic coupling between atoms or molecules and field modes of a Fabry-Pérot cavity on High-Harmonic Generation (HHG) spectra within a theoretical model study. We assume that the atom or molecule is driven by an intense classical laser field, giving rise to HHG, while being strongly coupled to quantized cavity modes as described by the Pauli-Fierz Hamiltonian in the framework of molecular quantum electrodynamics. Specifically, as a test case, we first consider a model Hamiltonian of a one-dimensional hydrogen atom coupled to a cavity mode, which can be treated "numerically exact" using grid methods. Furthermore, a hydrogen molecule coupled to a cavity mode is considered and treated within a recently suggested QED-TD-CI (Quantum Electrodynamics Time-Dependent Configuration Interaction) method [Weidman et al., J. Chem. Phys. 160, 094111 (2024)]. The resulting HHG spectra show (i) a suppression of the harmonic cutoff in line with the excitation of quantum light in the cavity and, in some cases, (ii) enhancement of some harmonics of the coupled light-matter system.