Strong-field physics and HHG in solids: topology, magnetism, Floquet*

In recent years strong-field laser-driven phenomena in condensed matter have been employed for novel ultrafast spectroscopies, e.g. all-optical band-structure reconstruction, probing exciton formation and recombination in real-time, and tracking ultrafast phase transitions. In this approach, intense laser pulses irradiate a sample, triggering non-perturbative responses such as high harmonic generation (HHG), photoionization, etc.; which can be analyzed to obtain information about the sample and ultrafast (potentially attosecond) out-of-equilibrium dynamics occurring within it. However, in order to faithfully extract information from spectra a deep physical understanding of the mechanisms generating the response, and a proper comparison between theory and experiment, are required. I will discuss this fundamental issue in the context of ab-initio calculations, and present our recent work attempting to probe material topology with HHG. I will show that topology is not necessarily imprinted onto spectra in a straightforward manner, and that its typical contribution to HHG can be rather weak, contrary to the common understanding in the field. I will also discuss our work on other emerging topics in strong-field physics in solids such as the role of Floquet light-driven phases of matter in the electronic response, and connections to ultrafast magnetism. Specifically, I will show that Floquet physics plays a major role in the tunneling process, and that strong-field laser irradiation allows inducing, and probing, attosecond magnetic responses, which are the fastest magnetic responses predicted to date.