Wave Phenomena in Time-Dependent, Active and Topological Photonic Media: feature introduction

Time-dependent, active, and topological photonic platforms are redefining what light can do in materials engineered beyond the traditional passive and stationary regime. They unlock nonreciprocity without magnets, energy exchange between waves and modulations, new topological responses, and ultrafast control - all of which are increasingly transitioning from elegant theory to robust device applications. This Feature Issue (FI), curated in connection with the Metamaterials 2024 meeting in Crete, assembles contributions that advance this frontier across fundamentals, methods, and applications, spanning space-time metamaterials, non-Hermitian and topological photonics, and ultrafast spatiotemporal structuring. According to the structure of FI introductions, we briefly highlight the accepted contributions and situate them within the broader research landscape.