Probing 2D Quantum Materials at High Pressure via Solid-State High Harmonics

Applying extreme pressure to solids can create new phases of matter that do not exist at ambient conditions. Investigating the chemical and electronic properties of these phases offers routes to design advanced materials that retain similar functionality under standard environments. Yet, direct methods to probe electronic structure under high pressure, in particularly within diamond anvil cells (DACs), remain limited. Here, we demonstrate that solid-state high harmonic generation (sHHG) spectroscopy provides an all-optical probe of the electronic structure of few-layer 2D semiconductor MoS₂ under extreme compression. sHHG, driven by a strong-field femtosecond mid-infrared (MIR) pulse, encodes information about band dispersion through electron–hole acceleration. Rotating the electric field polarization of the MIR w.r.t. the crystal allows recording the anisotropy that links to the spatial structure of electronic states. In the experiments, we observe odd-order harmonics from an exfoliated MoS₂ flake inside a DAC extending to the 13th order. Spectra recorded at ambient pressure and up to 30 GPa reveal distinct changes in harmonic yields and anisotropy, despite the absence of a structural phase transition. Stepwise pressure-dependent measurements uncover a minimum in sHHG emission intensity and a shift in angular anisotropy near 15 GPa. Theoretical analysis attributes this to pressure-induced band warping, where the direct gap shifts from the K to Γ point as the K gap closes and the Γ gap opens which has not been previously observed or predicted. This work constitutes the first realization of sHHG spectroscopy in a DAC, establishing a new all-optical route to probe electronic structure of quantum materials at extreme conditions.