  Time crystals are an eagerly sought phase of matter with broken
time-translation symmetry. Quantum time crystals with discretely broken
time-translation symmetry have been demonstrated in trapped ions, atoms and
spins while continuously broken time-translation symmetry has been observed in
an atomic condensate inside an optical cavity. Here we report that a classical
metamaterial nanostructure, a two-dimensional array of plasmonic metamolecules
supported on flexible nanowires, can be driven to a state possessing all of the
key features of a continuous time crystal: continuous coherent illumination by
light resonant with the metamolecules' plasmonic mode triggers a spontaneous
phase transition to a superradiant-like state of transmissivity oscillations,
resulting from many-body interactions among the metamolecules, characterized by
long-range order in space and time. The phenomenon is of interest to the study
of dynamic classical many-body states in the strongly correlated regime and
applications in all-optical modulation, frequency conversion and timing.
