Spectroscopic characterization of SrB4O7:Tm2+, a potential laser material and optical temperature sensor

Polycrystalline samples of SrB4O7:1% Tm2+ were prepared in air by a solid state reaction method. High resolution spectra and decay curves for infrared luminescence related to the intraconfigurational 2F5/2 → 2F7/2 transition and for visible luminescence related to the interconfigurational transition between the lowest energy state of the 4f125d configuration and the ground 2F7/2 state of the 4f13 configuration of Tm2+ were recorded as a function of temperature between 5 and 350 K. Energies of crystal field levels of the 2F5/2 and 2F7/2 multiplets and the 2F5/2 lifetime value were determined from low temperature measurements. With these data room temperature emission and absorption spectra were calibrated in the cross-section units. It was found that the infrared emission in SrB4O7:Tm2+ is long-lived with a lifetime value of 9.5 ms and its spectrum indicates that stimulated emission near 1.18 nm may be feasible. Examination of recorded decay curves for the visible luminescence revealed that the lifetime of the lowest energy state of the 4f125d configuration depends weakly on temperature in the region 5–280 K and then shows a steep decrease when the temperature grows from about 290 K to 350 K. The evaluated values of the thermal sensitivity parameter S for SrB4O7:Tm2+ in the region 300–330 K (27–57 °C) exceed 3.9% per K with a peak at 320 K (47 °C) equal to 6.30% per K. The S value is the highest reported thus far for luminescence thermometers operating in physiological temperatures.