Activation of high-lying dormant states by cascaded transition control in core@shell upconversion nanoparticles

Lanthanide-doped upconversion nanoparticles exhibit multicolor emission through discrete, narrowband transitions, enabling applications in bioimaging, lasers, and displays. However, the trade-off between leveraging abundant energy levels for multicolor output and mitigating rapid deactivation of high-lying states remains a fundamental challenge for achieving balanced white-light emission under moderate excitation power. This is particularly true for emitters such as Ho3+, which are highly susceptible to severe multiphonon relaxation and back energy transfer. Here, we address this by introducing a core@shell cascaded transition strategy that selectively populates Ho3+ high-lying states via engineered energy relays at a key intermediate level, Ce3+-mediated cross-relaxation control, and precise spatial ion arrangement. Our design achieves a ∼1500-fold boost in blue emission versus conventional architectures, enabling balanced red, green and blue output at 10 W/cm2 (10 mg/mL suspension). This white-light system surpasses mixed-nanoparticle media in stability, producing white lasers for photonics. Our approach provides a versatile route to high-lying states activation in lanthanide systems, with potential implications for multicolor imaging, displays, and anti-counterfeiting. The authors develop triply doped upconversion nanoparticles with cascaded energy transition pathways via precise spatial ion arrangement, which enables balanced RGB multicolor output and white lasing.