Unique excitation power density and pulse width-dependent multicolor upconversion emissions of Y2Mo4O15:Yb3+,Ho3+ for anti-counterfeiting and information encryption applications

Developing smart upconversion materials with novel optical properties is of great significance for applications such as anti-counterfeiting and information encryption, and is always a research frontier and hotspot in this field. Here, a unique smart upconversion material that can respond to the power density and pulse width of the excitation light is reported. The upconversion photoluminescence (PL) color of the developed upconversion material can be changed from red to yellow and then to green when the excitation power density increases from 0.5 to 100 W cm−2. Besides, its upconversion PL color can also be tuned from red to yellow and then to green when the pulse width of the excitation light reduces from 10 to 0.1 ms. Importantly, mechanistic studies show that the transition path of the excited electron of the activator is altered due to the high phonon energy of the host material, and thus the special upconversion luminescence is generated. Furthermore, advanced anti-counterfeiting and information encryption applications are demonstrated by taking advantage of the novel optical properties of the developed upconversion material.