Recently, solid-state high harmonic generation spectroscopy (sHHG) has shown promise as a new characterization tool capable of probing electronic band structure and associated properties in 2D semiconductors such as transition metal dichalcogenides (TMDs). Because sHHG is an all-optical technique, it can probe electronic properties of 2D materials in complex environments such as in high pressure cells or liquids. In addition, sHHG employs extreme nonlinearities to access momentum information that other optical spectroscopies cannot. sHHG retrieves electronic band structure information using a strong-field mid-IR pulse to first tunnel-ionize electrons from the valence to conduction band and then accelerate them through the conduction band. Harmonics are then either emitted due to nonlinear intraband currents or due to interband recombination. The resulting emission is highly anisotropic based on crystal symmetry, 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 harmonic anisotropy. Crystal symmetries impose constraints on each harmonic based on the angle between the driving field and crystal lattice leading to angular nodes. Band dispersion causes additional anisotropy and emission from different conduction bands can be identified by changes to the anisotropy. Specifically, lower harmonics are emitted from the first conduction band and higher harmonics are emitted from the second conduction band above its bandgap of 3.5 eV, made clear by a 30° angular shift in anisotropy between the two bands. This demonstrates the sensitivity of sHHG to electronic structure and its ability to probe specific bands independently. To build on this work, we measured MoS₂ at high pressures in a diamond anvil cell. Driving in the mid-IR allows selective probing of the lower bandgap MoS₂ without contributions from the surrounding diamond and He. Our preliminary sHHG results up to 30 GPa show symmetry and electronic structure changes through the structural phase transition at 25 GPa. This transition is associated with bandgap closing and MoS₂ is proposed to have metallic and superconducting phases at even higher pressures. With our demonstration that sHHG can probe electronic and structural properties of MoS₂ at high pressure, the door opens to use sHHG to study emergent phenomena of 2D materials in complex environments.

Bailey Nebgen

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