White-light flashlight activated up-conversion luminescence for ultraviolet-B tagging

Optical tagging technology with emission in the ultraviolet region upon visible-light excitation is promising for objects identified in visually bright environments, while the relevant research is absent. Here we put forward a covert tagging concept, which is based on up-converting phosphors (e.g., Lu₃Al₅O₁₂:Pr^{3 +}) with emission peaking in the ultraviolet-B region (UV-B, 290−320 nm). A white-light flashlight serves as excitation source to make the up-converter emit, and an ultraviolet camera is applied to see such an emission wavelength range. This Letter expands the excitation source for an up-conversion process to a convenient flashlight for the first time, to the best of our knowledge. Moreover, such a flashlight-pumped UV-B tagging technology is generally applicable for many other phosphors, which can be utilized to mark and differentiate objects in commercial, civilian, or military applications.