Juan Gaitan: Electron-hole Dynamics under Circular Driving Fields

Friday, May 22, 2026

Elings 1601

Abstract

In atomic high-order harmonic generation (HHG), the electron-ion three step model has been verified to yield no harmonic emission under circular driving [1]. In high-order sideband generation (HSG), an analog of HHG, suppression of sidebands under a circular terahertz (THZ) driving field was also observed in early experiments, marking the success of the three-step model of electron-hole recollisions in solids [2]. However, under a circular THz field tuned from a linear polarization, the intensity of the 2nd-order sideband remained around 10% of the original value, indicating a breakdown of the three-step model. Recently, in monolayer MoS2, the low-order sidebands produced by circular mid-infrared (MIR) pulses were observed to be as intense as those produced by comparably-strong. linear MIR pulses [4]. Selection rules for HSG were proposed based on Floquet theory and common symmetries of the solid material and driving field, in spite of the experimental observation of sidebands that were not allowed by the selection rules. Here, we derive selection rules for HSG by considering the symmetries of the strongly-driven material as well as the effects of Berry curvature, which becomes essential in HSG under a circular driving field. To verify our predictions, we conduct sideband polarimetry [5] in GaAs-based quantum wells (QWs) with quasi-cw near-infrared (NIR) and THz lasers, whose ellipticities are continuously varied. We find that the intensities of the 2nd-order sidebands emitted from GaAs QWs can remain on the same order of magnitude as the THz driving field is tuned from a linear to a circular polarization. Unexpectedly, our experimental results show that the positive and negative 2nd-order sidebands need not have helicities opposite to that of the circular NIR laser. This suggests possible modulation of linear response of GaAs to sidebands by the circular THz field in HSG. Our work, along with further investigation of HSG under circular driving fields, provides insights on how basic material properties, such as crystal symmetry, can be modulated and studied through non-equilibrium dynamics.