Power-tunable multicolor upconversion in nanocrystals under single-wavelength excitation

In this work, we demonstrate for the first time tunable upconversion luminescence in three primary colors using a single excitation wavelength of 980 nm, <i>via</i> altering the excitation intensity. A core/shell/shell nanocrystal of about 50 nm diameter was synthesized using a design strategy with 2% Er³⁺ and 98% Yb³⁺ in the core, and the outer shell is made of NaYF₄:Yb³⁺,Tm³⁺ (with 2% Tm³⁺ and 18% Yb³⁺), separated by an inert intermediate shell. This rationally designed architecture enables green, red, and blue light emissions by modulating the excitation power density, leveraging the photon-order-dependent upconversion process. As the power density of the 980 nm continuous-wave (CW) laser increases, the emission color shifts systematically from green to red and ultimately to blue, corresponding to the involvement of 2-photon, 3-photon, and 4-photon processes, respectively. Chromaticity coordinate shifts on the CIE diagram validated this dynamic color modulation, demonstrating precise control over emission pathways. The findings offer a simplified yet highly versatile excitation setup for full RGB tunability, paving the way for advancements in photonics and enabling possibilities in high-resolution color display and biomedical applications.