High-harmonic spectroscopy in solids: multiband effects and anomalous currents*

High-harmonic generation (HHG) has emerged in recent years as an all-optical probe of ultrafast dynamics and static properties of materials. Here, I report on our recent work on spectroscopic applications of HHG.

I will first present a combined theoretical and experimental study on HHG in monolayer MoS₂. I will show that the polarization properties of the high harmonics encode material-specific characteristics such as the vectorial character of the transition dipole moments from different valence-conduction-band pairs. This indicates that HHG, as an all-optical probe, can have several advantages over traditional spectroscopy methods such as angle-resolved photoemission spectroscopy: no requirement on ultrahigh-vacuum and the probing of vectorial dipole couplings.

Next, I will characterize the anomalous HHG mechanism in solids. The nonperturbative anomalous harmonics are polarized perpendicularly to the laser polarization direction, and originate in the nonlinearities of the Berry curvatures. I will show that the anomalous harmonics in general compete with the interband harmonics in terms of their yield, and that they exhibit unique characteristics that can be used for their identification and control. This also resolves a recent debate on the origin of the perpendicular-polarized harmonics, and will have consequences for Berry-curvature spectroscopy using HHG.