Solid-state high harmonic generation spectroscopy of MoS₂ in complex environments

Solid-state high harmonic generation (sHHG) has shown promise as a nonlinear optical characterization tool for electronic band structure in 2D transition metal dichalcogenides (TMDs) that has potential for measurements in complex environments such as in high pressure cells or liquids. sHHG retrieves electronic band structure information using a strong-field mid-IR pulse that induces tunnel-ionization, acceleration, and recombination of electrons and holes. Harmonic emissions result from intraband and interband currents during this process and are highly anisotropic, with each harmonic sampling different electron trajectories through the band dispersion of the material. We established the sensitivity of sHHG to symmetry and electronic structure in monolayer MoS₂ by exploring unique anisotropy features such as a 30° phase shift in anisotropy between harmonics emitted from the first conduction band (harmonics below 3.5 eV) and second conduction band (harmonics above 3.5 eV). Building on this work, we measured MoS₂ at high pressures in a diamond anvil cell, where the large bandgaps of diamond and He prevent their contribution to MoS₂ emissions. MoS₂ undergoes a phase transition associated with bandgap closing at 25 GPa and is proposed to have metallic and superconducting phases at higher pressures. Our preliminary sHHG results show symmetry and electronic structure changes through this phase transition, demonstrating its sensitivity to emergent phenomena of 2D materials in complex environments.