Observation of Anomalous π Modes in Photonic Floquet Engineering.

Recent progress on Floquet topological phases has shed new light on time-dependant quantum systems, among which one-dimensional (1D) Floquet systems have been under extensive theoretical research. However, an unambiguous experimental observation of these 1D Floquet topological phases is still lacking. Here, by periodically bending an ultrathin metallic array of coupled corrugated waveguides, a photonic Floquet simulator was well designed and successfully fabricated to mimic the periodically driven Su-Schrieffer-Heeger model. Intriguingly, under moderate driven frequencies, we report the first observation of the anomalous Floquet topological π mode, propagating along the array's boundary. The different evolutionary behaviors between static and nonstatic topological end modes have been clearly demonstrated by the microwave near-field experiment. Furthermore, the experiment in the fast-driving regime also reveals the universal high-frequency behavior in driven systems. Our photonic simulator can serve as a versatile testing ground for various phenomena related to time-dependant 1D quantum phases, such as Thouless pumping and dynamical localization.