Observation of wave amplification and temporal topological state in a non-synthetic photonic time crystal

Photonic time crystals (PTCs) are materials whose dielectric permittivity is periodically modulated in time. Akin to conventional photonic crystals, PTCs give rise to momentum bandgap as well as to topological insulators in the time domain. Here, we experimentally demonstrate the properties of a k gap in a PTC with real dimensions, realized in a dynamically modulated microwave transmission-line metamaterial. Wave amplification is observed within the k gap, leading to a narrowing of the initial power spectrum and its shifting toward the gap. To probe the mid-gap topological state, we introduce a temporal interface separating two PTCs with distinct topological phases. The phase difference measured between time-reflected and time-refracted waves at the interface, together with the temporal confinement of the topological state, proves the realization of nontrivial temporal topology. By integrating k gap amplification with time-domain topological features, our work opens new avenues for light generation and manipulation in time-varying photonic materials.