White-Light Sensitization Strategy for Upconverting Anticounterfeiting

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 NaYF₄:Nd³⁺ phosphor. This enhancement arises from a multi-photon excitation process, wherein white light directly populates the high-lying ⁴G_7/2 and ⁴G_5/2 intermediate levels of Nd³⁺, 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.