Observation of temporal reflection and broadband frequency translation at photonic time interfaces

Time reflection is a uniform inversion of the temporal evolution of a signal, which arises when an abrupt change in the properties of the host material occurs uniformly in space. At such a time interface, a portion of the input signal is time reversed, and its frequency spectrum is homogeneously translated as its momentum is conserved, forming the temporal counterpart of a spatial interface. Combinations of time interfaces, forming time metamaterials and Floquet matter, exploit the interference of multiple time reflections for extreme wave manipulation, leveraging time as an additional degree of freedom. Here we report the observation of photonic time reflection and associated broadband frequency translation in a switched transmission-line metamaterial whose effective capacitance is homogeneously and abruptly changed via a synchronized array of switches. A pair of temporal interfaces are combined to demonstrate time-reflection-induced wave interference, realizing the temporal counterpart of a Fabry-Perot cavity. Our results establish the foundational building blocks to realize time metamaterials and Floquet photonic crystals, with opportunities for extreme photon manipulation in space and time. Reflection cannot only occur at interfaces in space but also in time. Transmission-line metamaterials support time interfaces at which interference has been observed, forming a temporal version of a Fabry-Perot cavity.