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

Abstract

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 ²F_5/2 → ²F_7/2 transition and for visible luminescence related to the interconfigurational transition between the lowest energy state of the 4f¹²5d configuration and the ground ²F_7/2 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 ²F_5/2 and ²F_7/2 multiplets and the ²F_5/2 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.