Ultrafast Spectroscopy of Topologically Non-trivial and Chiral Materials Via High-order Harmonic Generation

High-order harmonic generation (HHG) is a non-linear optical process that has been well-studied in atomic and molecular systems using dilute gas targets. These studies included advanced understandings of the HHG mechanism. Subsequently, a spectrosocpic approach emerged, which can for example probe the orbital structure of the target molecule. In this talk, we will discuss the opportunities and challenges of extending the spectroscopic capabilities of HHG from isolated molecules to condensed matter systems. Benefits of the HHG approach is its all-optical nature and therefore its unprecedented access to the ultrafast time scales, and its applicability to a wide range of sample enviornments, unlike for example the requirment of ultra-high vacuum conditions in conventional spectroscopic approaches such as in scanning tunneling microsocpy (STM) and angle-resolved photoelectron spectrosocpy (ARPES). The challenges included undstanding fundamentally different HHG mechanism that required the correct understanding of the roles of high-density and periodicity that are naturally present in solid materials. I will show our recent experimental results, where we have scrutinized the novel mechanism of the solid-state HHG process for a successful ultrafast probing of topologically non-trivial and chiral surface states, monolayer two-dimensional crystals, and twisted heterosctructures. See our publication here: https://www.nature.com/articles/s41567-024-02640-8