Fig. 3: Characterization of optical switching of UCL emission driven by topology-based ETNs in core-shell UCNPs under dual-beam irradiation for U-STED microscopy.

a Schematic energy level diagram of core-shell UCNPs illustrating the optical switching mechanism of 450-nm UCL emission mediated by topology-based ETNs under dual-beam irradiation. Key processes include EM within the Yb³⁺-Yb³⁺ network, ETU in the Yb³⁺-Tm³⁺ network, and CR in the Tm³⁺-Tm³⁺ network. The diagram compares two configurations: irradiation with a CW excitation laser at 980 nm (left) and dual-beam irradiation combining CW excitation and depletion lasers at 980 nm and 808 nm, respectively (right). b Depletion efficiency of 450-nm UCL emission from Yb,Tm@Y, Yb@Tm@Y, and Tm@Yb@Y UCNPs under dual-beam irradiation, using a CW excitation laser at 980 nm with varying power levels and a CW depletion laser at 808 nm at a fixed power of 10 mW. c Theoretical and experimental normalized intensity of 450-nm UCL emission from Yb,Tm@Y, Yb@Tm@Y, and Tm@Yb@Y UCNPs under dual-beam irradiation. Measurements were conducted with a CW excitation laser at 980 nm with power levels of 0.1 mW and 1 mW, combined with a CW depletion laser at 808 nm at varying power levels