Probing topology and correlations of quantum materials in strong laser fields

The interaction of intense laser pulses with quantum materials gives rise to a variety of intriguing physical phenomena. One of them is high harmonic generation (HHG), microscopically originating from the interplay of tunneling currents, dynamical Bloch oscillations, and electron-hole recollisions on a sub-cycle time scale. Focusing on the paradigmatic 3D topological insulator Bi₂Se₃, I will first discuss the potential of HHG driven by circularly-polarized fields as a probe of topological band structures. I will then examine how light-induced changes to the band structure dynamically modify the HHG emission spectrum. HHG in solids has traditionally been interpreted within the single-particle picture, where the electronic bands are assumed to remain “frozen” during the light-matter interaction. However, recent theoretical work [1] has challenged this notion and revealed that the onsite Coulomb repulsion in strongly correlated systems (Hubbard U) can be substantially modified by strong non-resonant laser fields and lead to a dramatic reshaping of the HHG spectrum. By combining time-resolved x-ray absorption experiments and exact diagonalization calculations, I will show that intense femtosecond pulses selectively induce a transient renormalization of the Hubbard-U in the prototypical cuprate superconductor La_2-xBa_xCuO₄. This result has far-reaching implications for HHG, attosecond spectroscopy, and ultrafast magnetism.

[1] N. Tancogne-Dejean, M. A. Sentef, and A. Rubio, Phys. Rev. Lett. 121, 097402 (2018)