In our most recent work led by Bailey we report on solid-state high-harmonic generation spectroscopy (sHHG) inside a diamond anvil cell (DAC), opening an all-optical route to probe electronic structure under extreme pressure. Using MoS₂ as a model system, we tracked how pressure reshapes the material’s electronic structure and uncovered an isostructural electronic transition near 15 GPa, marked by a sharp suppression of harmonic emission and a pronounced rotation of the polarization anisotropy. Supported by first-principles calculations performed by the Qiu group at Yale, the work shows that these signatures arise from a crossover of the lowest direct bandgap from the K-point to the Γ-point and from interference between competing excitation pathways in momentum space. These results establish solid-state high-harmonic generation as a sensitive new spectroscopic tool for studying pressure-tuned quantum materials in regimes that have been difficult to access experimentally.

Original publication available open access at Science Advances:
https://www.science.org/doi/10.1126/sciadv.adz5621