# Temperature Sensing with Nd\({}^{3+}\) Doped YAS Laser Microresonators
## 5 Conclusions  
The FIR technique was employed on the 808 nm and 890 nm emissions of the YAS: Nd\({}^{3+}\) doped bulk glass to obtain a temperature calibration with the intensity ratios. An energy gap of 956 cm\({}^{-1}\) was obtained from the fit, in good agreement with the gap obtained from the absorption spectrum (\(E_{32}\) = 1053 cm\({}^{-1}\)). This technique provided a maximum value for the sensitivity of 1.60 \(\times\)10\({}^{-2}\) K\({}^{-1}\) and a temperature resolution limit of 3 K. On the other hand, the heating effects in a microsphere due to the laser source at 532 nm were studied by observing the displacement of the WGM peaks and determining the temperature using the FIR temperature calibration previously obtained. A maximum sensitivity value of 15 \(\times\) 10\({}^{-6}\) K\({}^{-1}\) and a temperature resolution limit of 0.2 K were obtained. This shows that the WGM displacement technique is an order of magnitude more accurate than the FIR technique. Moreover, with the microsphere acting as both a resonant cavity and a gain medium, laser emission was also observed under continuous pump in the emission band of \({}^{4}\)F\({}_{3/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{11/2}\) (1064 nm) of Nd\({}^{3+}\) ions, and a threshold power of 100 mW was determined. Furthermore, under 808 nm laser excitation, upconversion emissions were also observed on the microspheres. The emission bands were identified as \({}^{4}\)G\({}_{7/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{9/2}\) (530 nm), \({}^{4}\)G\({}_{7/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{11/2}\) (579 nm), \({}^{4}\)G\({}_{7/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{13/2}\) (675 nm), \({}^{4}\)F\({}_{9/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{9/2}\) (690 nm), and \({}^{4}\)F\({}_{7/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{9/2}\) (750 nm). It was found that the first three bands were upconversion transitions requiring two photons, while the other two were being populated by thermalization effects. Finally, the measurement of the temporal evolution of the upconversion emission showed that a mixture of ESA and ETU mechanisms are necessary to produce these upconversion processes.  
The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/3/1117/s1, Figure S1: Absorption spectrum of YAS:Nd\({}^{3+}\).  
Conceptualization, S.R. and I.R.M.; methodology, D.W.-M., F.P.-B., I.R.M., and L.L.M.; validation, D.W.-M., S.R., and I.R.M.; investigation, D.W.-M., F.P.-B., A.R., and I.R.M.; resources, I.R.M., V.N.S., V.I.S., and G.Y.S.; data curation, D.W.-M., F.P.-B., and I.R.M.; writing--original draft preparation, D.W.-M. and F.P.-B.; writing--review and editing, F.P.-B., I.R.M., L.M., and A.R.; project administration, I.R.M.; funding acquisition, I.R.M., L.L.M., A.R., and V.N.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Russian Science Foundation (Grant No. 19-19-00613), the Spanish MINECO (MAT2015-71070-REDC, MAT2016-75586-C4-4-P, FIS2016-77319-C2-1-R, PID2019-106383 GB-C44, and PID2019-107335 RA-I00), and EU-FEDER funds. L.L.M. and A.R. acknowledge support from the Spanish [PERSON] de la Cierva program (IJCI-2016-30498) and the Spanish Ramon y Cajal program (RYC-2017-21618), respectively.  
Figure 8: Temporal evolution of the upconversion emission of 530 nm of Nd\({}^{3+}\) ions.