Valley-to-peak intensity ratio thermometry based on the red upconversion emission of Er³⁺

Upon 976 nm diode laser excitation, the temperature dependence of the red upconversion emission of Er³⁺ in CaWO₄:Yb³⁺/Er³⁺ phosphor was studied from 298 to 478 K. The spectrum was verified to consist of two Stark components originating from two Stark sublevels of ⁴F9/2 excited state to ⁴I15/2 ground state of Er³⁺. The valley-to-peak intensity ratio (VPR) of this double-peak spectrum was found to increase linearly with the rise of temperature. The maximum relative sensitivity of this VPR method was obtained to be about 0.20% K^-1 at 298 K. Moreover, a study on the power dependence was also performed, suggesting that VPR method is immune to the pump power and is thus suitable for monitoring the temperature.