Photonic alloys: then, now, and next

Photonic alloys (PAs) are artificial photonic materials inspired by condensed-matter alloying, where mixed constituents tailor electromagnetic dispersion beyond conventional photonic crystals. Initially introduced as random mixtures of plasmonic and polaritonic microspheres, PAs evolved through digital-alloy implementations enabling deterministic band-structure control via short-period superlattices. Recently, topological photonic alloys (TPAs) have emerged, exploiting disorder and non-periodicity to realize robust chiral edge states and nontrivial bandgaps at vanishing topological constituent concentrations. This review unifies the development of PAs from early random and digital realizations to contemporary topological implementations. We discuss physical mechanisms, theoretical frameworks, and experimental realizations across dimensionalities and platforms, highlighting disorder, band alignment, and effective-medium concepts. Finally, we outline future directions including disorder-resilient topological photonics, programmable metamaterials, and semiconductor-compatible topological devices.