Highly nonlinear ultrafast dynamics in molecular liquids and quantum matter

High harmonic generation (HHG) is a nonlinear frequency up-conversion process that occurs when intense laser pulses irradiate matter. In gas-phase it has been extensively studied and is very well-understood (e.g. leading to the 2023 Physics Nobel Prize for attosecond science). In solids research is ongoing, but a consensus is forming for the dominant HHG mechanisms. In liquids however, no established theoretical model exists, and the underlying chemical and physical mechanisms remain unidentified. Advancement on this front may lead to novel light sources, and are especially appealing for ultrafast spectroscopy of chemistry in solutions. I will present our recent efforts in tackling this problem with a combination of ab-initio calculations[1], semi-analytical models, and experimental data obtained by collaborators at ETHZ. I will show that HHG in liquids is strongly affected by the electronic mean-free-path, thus allowing its extraction from all-optical measurements, and demonstrating the first application of high harmonic spectroscopy in liquids[2]. I will also show that higher-order scattering processes play a central role in the higher-energy part of liquid-HHG. Lastly, I’ll discuss my recent works on high harmonic spectroscopy of topological phases of matter (challenging the current conception in the field that topology strongly affects solid HHG)[3], and on strong-field driving of attosecond magnetization dynamics in quantum materials, constituting the prediction for the fastest magnetization dynamics to date[4].

[1] Neufeld et al., JCTC 18, 4117 (2022). https://doi.org/10.1021/acs.jctc.2c00235.

[2] Mondal*, Neufeld* et al., Nat. Phys. (2023). https://doi.org/10.1038/s41567-023-02214-0.

[3] Neufeld et al., PRX 13, 031011 (2023). https://doi.org/10.1103/PhysRevX.13.031011.

[4] Neufeld et al., npj Comp. Mat. 9, 39 (2023). https://doi.org/10.1038/s41524-023-00997-7.