Upconversion luminescence through dynamically regulating the depletion of Ho³⁺: ⁵I₆ level for speed sensing

In this work, a strategy for dynamically adjusting the upconversion luminescence (UCL) color of NaGdF₄:Yb³⁺/Ho³⁺/Ce³⁺/Sc³⁺ is reported based on a phosphor wheel. It has been demonstrated that the rotation-dependent UCL mainly originated from the regulation of depletion mode for the Ho³⁺: ⁵I₆ level. Due to the dominant linear decay, a high-pure red UCL is observed under the steady-state excitation. However, as the proportion of the steady-state excitation decreases, the green-red emission intensity ratio gradually increases, followed by the color conversion from red to green. An approximate physical model is proposed to understand the dependence of I_G/I_R on rotation speed. We not only report a UCL material that shows potential application in velocity sensing but also provide new insights into wheel-based dynamic UCL regulation.