Improved Photothermal Heating of NaNdF4 Microcrystals via Low-Level Doping of Sm3+ for Thermal-Responsive Upconversion Luminescence Anti-Counterfeiting

This work reports the light-to-heat conversion (LHC) behavior of NaNdF4 doped with Sm3+. Due to the cross-relaxation between Nd3+ and Sm3+, the improved LHC is obtainable by introducing 5% Sm3+. When the laser power density is only 1.72 W/cm2, the spot temperature of NaNdF4:5%Sm3+ powder reaches as high as 138.7 ± 4.04 °C. More importantly, the photoheating response to the pump laser has favorable linear characteristics within a specific power range. A simple physical model is applied to analyze the relationship between photothermal heating and pump power. Finally, the temperature-responsive luminescence anti-counterfeiting is designed by combining the LHC material with the NaYF4:Yb3+/Ho3+/Ce3+ microcrystals. This novel strategy only requires two laser beams, and thus is more convenient to apply.