Turning NIR into blue afterglow: UV upconversion in CaWO₄:Yb,Tm for blue afterglow from the CaAl2O4:Eu2+,Nd3+ persistent luminescent phosphor

In this work, we demonstrate, for the first time to the best of our knowledge, a strategy enabling blue persistent luminescence after NIR charging through UV upconversion (UC)–persistent luminescence (PersL) system. Tm³⁺, Yb³⁺ codoped CaWO₄ crystals were synthesized by a solid-state method and exhibit strong ultraviolet (UV) upconversion emission centered at ∼363 nm under 980 nm excitation. Optimized UC crystals (Yb >7.5 Tm < ₀.₂₅) reached an effective UV irradiance of 2600 mW m⁻² at a power density of 0.1 W mm⁻². While these crystals are of great interest for applications including UV-visible solid-state lasers or as an internal UV radiation source for many photochemical reactions, we show that upon simple dry mixing of the UC and PersL components, NIR excitation at 980 nm successfully generated a visible blue afterglow from the CaAl 2 O 4 :Eu 2+ ,Nd 3+ phosphor. The optimal 50:50 composite exhibited an initial luminance of 1.23 mcd m⁻², exceeding the visibility threshold for safety signage (0.32 mcd m⁻²). Demonstrating blue afterglow under NIR irradiation through upconversion-generated UV light greatly expands the potential applications of persistent luminescence phosphors. It opens new possibilities for uses that require remote excitation and deep light penetration, such as safety signs, anti-counterfeiting, and optical signaling.