  Understanding microscopic processes in materials and devices that can be
switched by light requires experimental access to dynamics on nanometer length
and femtosecond time scales. Here, we introduce ultrafast dark-field electron
microscopy, tailored to map the order parameter across a structural phase
transition. We track the evolution of charge-density wave domains in 1T-TaS2
after ultrashort laser excitation, elucidating relaxation pathways and domain
wall dynamics. The unique benefits of selective contrast enhancement will
inspire future beam shaping technology in ultrafast transmission electron
microscopy.
