Optical temperature sensing with CaWO₄:Yb³⁺/Er³⁺ phosphors driven by a 405 nm laser diode

Traditionally, the 980 nm laser is used as the excitation source for Yb³⁺/Er³⁺ codoped phosphors. Different from this conventional way, it is shown here that the as-prepared CaWO₄:Yb³⁺/Er³⁺ (5/1 mol %) phosphors can be effectively triggered by the commonly used and cheap 405 nm laser diode. On such excitation, the Er³⁺ ion's characteristic green emission, which is ascribed to the ²H_11∕2∕⁴S_3∕2-⁴I_15∕2 transitions, is successfully detected with a good signal to noise ratio. The green emission's response to the temperature change is investigated in the 93–813 K range. It is confirmed that the ²H_11∕2-⁴I_15∕2 and ⁴S_3∕2-⁴I_15∕2 transitions obey the Boltzmann distribution well, which are thus suitable for optical ratiometric temperature measurement. Their maximum relative sensitivity is found to be 10.41% K⁻¹ at 93 K.