Our Comment in Physical Review Letters examines a recent report of helicity-dependent charge-density-wave dynamics in 1T-TiSe₂. Using MeV ultrafast electron diffraction with a near-collinear pump-probe geometry and carefully characterized circular pump polarization, we find no measurable circular dichroism in the transient CDW response for either 800 nm or 10.6 µm excitation. Our results suggest that the previously reported effect may be complicated by grazing-incidence excitation, which substantially alters the polarization state inside the sample. This work highlights the need for careful control of optical geometry and polarization when probing chiral order in quantum materials.

Measurements were done at the MeV-UED user facility at SLAC in collaboration with the groups of Alfred Zong (Stanford) and Giulio Cerullo (Politecnico Milan).

Link to the open access publication: https://journals.aps.org/prl/abstract/10.1103/p5d8-548h