Phase change materials in photonic devices

Recent developments in reconfigurable photonic devices highly rely on the effective refractive index change enabled by phase change materials (PCMs) as either dielectric surroundings or constituting materials. This universal characteristic, tunable refractive index, is shared among various kinds of PCMs and has been successfully utilized to achieve multilevel modulations for both free-space and integrated photonics. In this Perspective, we briefly recapitulate the fundamental mechanisms of phase transitions for three dominant PCMs. The progress in integrating different PCMs with on-chip silicon photonics and periodic antenna arrays are reviewed and analyzed in parallel. We also discuss the existing problems of PCM photonics, for example, the compatibility with commercial production line, the stability issue, and accessibility of the stimuli. In the end, we provide the outlook for the improving material engineering of PCM and multi-functional PCM-based photonics devices.