Spatial photonic crystals (SPCs) are unique structures for light–matter interactions because they achieve a large and spatially periodic dielectric contrast on wavelength scales1–4. Their temporal analogues, photonic time crystals (PTCs), promise similar advances by periodically modulating optical properties in time5–11, but require strong, ultrafast modulation, which is challenging to obtain experimentally5,12–15. Driven metamaterials have been considered as a route to realize PTCs, yet all-optical implementations have remained unknown because of the challenge of achieving modulation on such short timescales. Here we demonstrate the all-optical realization of a photonic time crystal, achieved with a surface plasmon cavity metamaterial operating at terahertz frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the kinetic energy of the carriers and effective mass reaching up to 80% of their rest mass. Our spectroscopic measurements show a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% (refs. 16,17), and we predict plasmonic lasing to be within experimental reach. These results establish a robust platform for time-domain photonics in plasmonic systems.
