Engineering the color output in a single upconversion microtube via intervention of the electronic population

The engineering of upconversion (UC) light color output is of great importance due to its promising application in biological imaging, data storage and solar cells. In this work, we showed that a tunable red-to-green emission ratio can be achieved by rationally designing the depopulation mechanism of intermediate state via increasing pump power density, pulse duration, pulse interval and Yb3+ concentration accompanied with the waveguide effect in a single Yb3+/Er3+ codoped NaYF4 microtube. We revealed that the red UC luminescence can be enhanced by three-photon process. As expected, three-photon and two-photon processes could be effectively switched by controlling pulse duration and power density. Compared with varying the power density, elevating the Yb concentration accompanied with the waveguide effect is more effective in modifying the color output. Our results provide insight into underlying physical mechanisms of intervening electronic population and present a general approach to constructing a new class of luminescent materials with tunable UC emissions.