Fig. 2. Light-induced dimension crossover from a 3D to 2D CDW.

a Schematic of electron diffraction geometry for data presented in panels (d–g), where an ultrathin sample cleaved in the (0 0 1) plane is rotated by 8^∘ relative to the incident electron beam. b Illustration of the intersection between the reciprocal lattice and the Ewald’s sphere, which is locally approximated by a plane that corresponds to the diffraction pattern in (d) and (e). Green dots in the cross-sectional view mark the intersection points between the L–M–L line and the Ewald’s sphere. c Miller indices for four color-coded points in the diffraction images. d Static electron diffraction pattern taken at 88 K in the tilted geometry with zone axis equal to $\left(\overline{36}5164\right)$. The three hexagons correspond to the projection of the three Brillouin zones in (b) onto the Ewald’s sphere. e Differential diffraction pattern at 1 ps after photoexcitation by an 800-nm, 30-fs pulse. f Time evolution of the CDW peak at the L point (peak 4, blue) and the diffuse peak at the M point (peak 2, red). Intensity values are normalized by their respective averages before photoexcitation. See Supplementary Fig. 5b for traces of additional peaks. Inset illustrates the photoinduced change in the CDW dimensionality, sketched in the reciprocal space. g The same as (f) but plotted for extended pump-probe time delay, showing the partial recovery to a quasi-equilibrium plateau. In f and g, error bars of intensity change are smaller than the marker size; solid curves are fits to an error function multiplied by an exponential function. The incident fluence for data presented in (e–g) is 560 μJ/cm².