Tricolor Upconversion Phosphors of LiYO₂:RE³⁺/Yb³⁺ (RE = Tm, Ho, Eu) for Metamerism Anti-Counterfeiting and 3D Volumetric Display

The development of blue/green/red tricolor upconversion (UC) phosphors under invisible light irradiation has attracted significant interest in emerging applications such as anti-counterfeiting, three-dimensional (3D) optical data storage, and solid-state 3D displays. In this study, a series of LiYO2:RE3+,Yb3+ (RE = Tm, Ho, Eu) phosphors are successfully prepared via first precipitation process followed by calcination, and completely optimized for efficient tricolor UC luminescence with 980 nm laser excitation. Mechanisms involving two- or three-photon absorption processes and low-phonon-energy-suppressed multi-phonon relaxation have been proposed for Ho3+(Eu3+)/Yb3+ and Tm3+/Yb3+ codoping, respectively. In addition, the RE3+ (Yb3+) concentration-induced phase transition of the LiYO2 host is discussed for interesting possibilities in spectral regulation. Based on the broad color gamut achieved by the tricolor UC phosphor mixture, homochromatic and polychromatic metamerisms are experimentally fabricated via screen printing for high-level anti-counterfeiting and information security. Moreover, for a proof-of-concept demonstration, prototypes of 3D optical data storage and volumetric displays are constructed by embedding tricolor UC mixtures and their 3D printed patterns in polydimethylsiloxane. It is believed that the continuous exploration of tricolor UC under invisible light excitation for metamerism anti-counterfeiting and 3D displays can promote the development of high-level information security and advanced volumetric display technologies.