  The tunability of materials properties by light promises a wealth of future
applications in energy conversion and information technology. Strongly
correlated materials such as transition-metal dichalcogenides (TMDCs) offer
optical control of electronic phases, charge ordering and interlayer
correlations by photodoping. Here, we find the emergence of a transient hexatic
state in a TMDC thin-film during the laser-induced transformation between two
charge-density wave (CDW) phases. Introducing tilt-series ultrafast nanobeam
electron diffraction, we reconstruct CDW rocking curves at high momentum
resolution. An intermittent suppression of three-dimensional structural
correlations promotes a loss of in-plane translational order characteristic of
a hexatic intermediate. Our results demonstrate the merit of tomographic
ultrafast structural probing in tracing coupled order parameters, heralding
universal nanoscale access to laser-induced dimensionality control in
functional heterostructures and devices.
