High-harmonic generation in solids: imperfect recollisions, 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 theoretical findings on the microscopic mechanisms behind HHG, as well as potential spectroscopy applications of HHG.

The interband HHG mechanism can be pictured by the three-step model in terms of electron-hole-pair creation, propagation and recollision. I will show that such a picture should be expanded to encompass general cases: often the electron-hole pairs created away from the minimal band gap can dominate the HHG emission, and the electron-hole pair can undergo imperfect recollisions, i.e. recollisions where the electron and hole do not exactly overlap spatially.

Next, I will show that HHG can probe higher conduction-band properties of monolayer MoS2. 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.

Finally, I will characterize the anomalous HHG mechanism in solids, i.e. nonperturbative harmonics that are polarized perpendicularly to the laser polarization originating in the nonlinearities of the Berry curvatures. The anomalous harmonics will in general be competing with the interband harmonics, and I will show that the anomalous harmonics exhibit unique characteristics that can be using for identification and control. This also resolves a recent debate on the origin of the perpendicular-polarized harmonics.