Latent transition unlocks visible-to-ultraviolet upconversion for covert optical security

Upconversion luminescence (UCL) is transforming the landscape of optical anti-counterfeiting, particularly in the ultraviolet (UV) range, where invisible emissions enable highly advanced security features. Yet, scarce UV-emitting pathways have historically hindered significant progress in this area. Here, we present an intriguing finding: when LiLuGeO4 phosphors co-doped with Pr3+ and Bi3+ ions are excited with blue lasers, they unexpectedly emit UV radiation at 352 nm—a feature absent in singly doped counterparts. We attribute the previously unrecognized UV emission to a process we term “latent transition energy transfer” (LTET). In this mechanism, Pr3+ sensitizes Bi3+ without itself emitting UV light and without requiring spectral overlap, thereby bypassing conventional energy-transfer bottlenecks. Building on this insight, we have observed similar LTET-driven UCL behavior in other ion pairs (Pr3+–Tb3+ and Pr3+–Gd3+) within appropriate host materials. Demonstrated via UV imaging for security, this discovery unveils a new class of UV-emitting materials that enhance anti-counterfeiting and spectral control in luminescent systems.