Regensburg 2025 – wissenschaftliches Programm
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HL: Fachverband Halbleiterphysik
HL 2: 2D Semiconductors and van der Waals Heterostructures I
HL 2.7: Vortrag
Montag, 17. März 2025, 11:15–11:30, H15
Ultrafast All-Optical Probe of Broken Time-Reversal Symmetry in Monolayer WSe2 — •Paul Herrmann1, Sebastian Klimmer1, Thomas Lettau1, Till Weickhardt1, Anastasios Papavasileiou2, Kseniia Mosina2, Zdeněk Sofer2, Jan Wilhelm3, and Giancarlo Soavi1 — 1Friedrich Schiller University Jena, Germany — 2University of Chemistry and Technology, Prague, Czech Republic — 3University of Regensburg, Germany
The combination of broken space inversion and preserved time-reversal symmetry (TRS) underlies the spin-valley degree of freedom in monolayer transition metal dichalcogenides (TMDs). Introduction of an imbalance between the energy degenerate, but non-equivalent valleys (local extrema of the bandstructure) at the ±K points of the Brillouin zone breaks TRS. We probe broken TRS and a valley imbalance on ultrashort time scales by comparing the second harmonic (SH) intensity for a circularly vs. linearly polarized fundamental beam (FB). By numerically and analytically solving the semiconductor Bloch equations, we show that a two-photon resonant right/left circularly polarized FB interacts exclusively with the ±K valley. Thus, a circularly polarized FB probes the C3h wave vector group of the ±K valleys, in contrast to a linearly polarized FB probing the D3h group of the Γ point. This difference between wave vector groups at K and Γ fully captures and explains the experimentally measured deviation from the otherwise expected ratio of 2 in the circular vs. linear SH intensities.
Keywords: Time-reversal symmetry; Nonlinear optics; Two-dimensional material; Ultrafast