Depleted upconversion luminescence in NaYF₄:Yb³⁺,Tm³⁺ nanoparticles: Via simultaneous two-wavelength excitation

We report optical depletion of upconversion luminescence (UCL) in NaYF₄:Yb³⁺,Tm³⁺ nanoparticles excited simultaneously by 980 nm and 1550 nm lasers. The UCL intensity is greatly depleted, while the downshifted emission at 1210 nm is obviously enhanced. The disturbances of 1550 nm photons on energy transfer between Yb³⁺ and Tm³⁺, cross relaxation (CR), the thermal effect and the stimulated emission depletion (STED) process are qualitatively evaluated. Our investigations verify that the unexpected depletion phenomena are governed by the STED process. The power densities of the 980 nm and 1550 nm lasers are both less than 100 W cm^-2, which will greatly reduce the thermal effect and damage and extend the applications of such nanomaterials. These results provide keen insights into controlling emission colors in optical processes, and offer potential applications in multicolor displays and STED nanoscopy.