Yb²⁺-Doped Silicate Glasses as Optical Sensor Materials for Cryogenic Thermometry

Optical sensors constitute attractive alternatives to resistive probes for the sensing and monitoring of temperature (T). In this work, we investigated, in the range from 2 to 300 K, the thermal behavior of Yb²⁺ ion photoluminescence (PL) in glass hosts for cryogenic thermometry. To that end, two kinds of Yb²⁺-doped preforms, with aluminosilicate and aluminophosphosilicate core glasses, were made using the modified chemical vapor deposition (MCVD) technique. The obtained preforms were then elongated, at about 2000 °C, to canes with an Yb²⁺-doped core of about 500 µm. Under UV excitation and independently of the core composition, all samples of preforms and their corresponding canes presented a wide visible emission band attributed to Yb²⁺ ions. Furthermore, PL kinetics measurements, recorded at two emission wavelengths (502 and 582 nm) under 355 nm pulsed excitation, showed an increase, at very low T, followed by a decrease in lifetime until room temperature (RT). A modified two-level model was proposed to interpret such a decay time dependence versus T. Based on the fit of lifetime data with this model, the absolute (S) and relative (S) sensitivities were determined for each sample. For both the preform and its corresponding cane, the aluminophosphosilicate glass composition featured the highest performances in the cryogenic domain, with values exceeding 28.3 µsK^-1 and 94.4% K^-1 at 30 K for S and S, respectively. The aluminophosphosilicate preform also exhibited the wider T operating range of 10-300 K. Our results show that Yb²⁺-doped silicate glasses are promising sensing materials for optical thermometry applications in the cryogenic domain.