Multicolor Tunable Upconversion Luminescence via Near-Infrared Manipulation of Population Pathways of Er³⁺ Ions Excited-State Levels for Volumetric Color Displays

Precise control over multicolor luminescence of upconversion nanoparticles (UCNPs) is of significant importance for their applications in widespread fields of research. However, realizing the tunable emissions in single UCNPs with a single lanthanide element remains a great challenge. Herein, without multiple lanthanide elements, a new strategy for the regulation of the excited-state level population pathways of the same lanthanide activator Er³⁺ ion to obtain multicolor-tunable upconversion luminescence is proposed through utilizing 980 nm coupled 1973 nm synergistic excitation. Unlike typical single wavelength excitation, the synergistic excitation may affect population pathways of the excited-state level of Er³⁺ ion by adjusting excitation wavelength and power density, resulting in a dynamically adjustable change in luminescence color output. Notably, multicolor luminescence involving green, chartreuse, yellow, orange, and red can be tuned dynamically in the NaYF₄:Er³⁺ single UCNPs by using the tunable 980/1973 nm synergistic excitation. This dynamic luminescence color variation from these UCNPs has demonstrated promising potential applications in volumetric color display. The results provide a new approach to achieve multicolor-tunable upconversion luminescence at single nanoparticles level and open up the possibility of developing true three-dimensional volumetric color display technologies with resolution at the nanometer range.