Highly sensitive optical ratiometric thermometry by exciting Eu³⁺/Tb³⁺'s unusual absorption lines

A guideline for designing sensitive thermometry is proposed. It reveals that using two emission bands that possess the opposite change tendencies against temperature makes it easier to achieve a larger relative sensitivity. Based on the guidelines, a highly sensitive strategy for optical thermal detection that depends on the Tb³⁺-to-Eu³⁺ emission ratio is designed by exciting Eu³⁺/Tb³⁺'s unusual absorption lines. This can be easily driven by a commonly used and cheap 405 nm laser diode. Moreover, its maximum relative sensitivity reaches up to 2.02% K^-1 at 610 K, one of the largest sensitivities reported so far.