White‐Light Sensitization Strategy for Upconverting Anticounterfeiting

Abstract Upconversion luminescence (UCL) presents a promising avenue for optical anticounterfeiting applications; however, its practical implementation is often limited by low visibility in bright environments. In this study, a white‐light sensitization strategy is introduced to significantly amplify UV UCL for improved security measures under well‐lit conditions. By integrating 808 nm infrared excitation with a white‐light flashlight exposure, a ten‐fold increase in UCL intensity at 354 nm is achieved from a NaYF 4 :Nd 3+ phosphor. This enhancement arises from a multi‐photon excitation process, wherein white light directly populates the high‐lying 4 G 7/2 and 4 G 5/2 intermediate levels of Nd 3+ , excited states otherwise only accessible via two‐photon infrared absorption. This white‐light sensitization approach enables robust UV UCL emission to be detected even in bright settings, overcoming a major limitation of traditional UCL‐based anticounterfeiting. Moreover, the feasibility of this method is demonstrated through UV imaging, highlighting its potential for advancing security and authentication technologies.