Chromium-activated phosphors: from theory to applications

The development of chromium-activated phosphors is essential for applications such as near-infrared (NIR) lighting and <i>in vivo</i> bioimaging. The intra-configurational transition within the 3d orbital of chromium ions, which is susceptible to the surrounding crystal environment, leads to remarkable luminescence tunability from the NIR-I to NIR-II regions. The striking NIR luminescence property makes chromium-activated phosphors a versatile material platform for fundamental optical studies and practical device applications. This review comprehensively summarizes the recent developments in chromium-activated phosphors, encompassing the phenomenological crystal field theories, design principles, materials preparation methodologies, and emerging applications. We delve into the correlation between the local coordination environment (<i>e.g.</i>, the geometrical structure of polyhedra, nephelauxetic effect, and interaction coupling) and optical properties (<i>e.g.</i>, spectral profile, quantum efficiency, luminescence thermal stability, as well as persistent luminescence and mechanoluminescence), aiming to provide a theoretical basis and experimental guidance for further refinement of chromium-activated phosphors. We also discuss future opportunities and challenges of chromium-activated phosphors, such as extended optical tuning by controlling crystal size and energy level coupling.