# Temperature Sensing with Nd\({}^{3+}\) Doped YAS Laser Microresonators
## 1 Introduction  
Optically active ions present luminescent properties that provide information regarding the characteristics of the host material lattice. Particularly, fluorescent chemical elements, quantum dots, or proteins are commonly used to study the host material [1, 2]. Among them, trivalent lanthanide ions are widely used, because they present various fluorescent transitions throughout the UV to mid-IR electromagnetic range and they can be employed in different dielectric media such as inorganic glasses and crystals, or even in intracellular medium [3, 4].  
Yttria-alumina-silica (YAS) glass microspheres have an interesting application for in situ cancer radiotherapy, in which they are used as carriers of yttrium isotope [5]. This is possible due to the excellent chemical durability of YAS glasses. Other remarkable properties of these glasses include a high transformation temperature (about 900 \({}^{\circ}\)C) that is independent of the composition, high strength, optical transparency throughout the visible spectrum, and high electrical resistivity. Furthermore, the structure of YAS glasses make them favorable to be used as laser gain media for high concentrations of rare-earth dopant [6], and in all-fiber mode [7].  
In recent years, nanotechnological advances have led to the development of new methods for measuring temperature [8]. A new generation of optical sensors have been developed that take advantage of the variations in the emission spectrum of the material (intensity, wavelength, lifetime, band shape) due to changes in temperature [8].  
Some trivalent lanthanide ions show changes in the intensity of thermally coupled emission bands due to a temperature change [9; 10]. These changes are due to thermal population redistributions between two energy levels that are coupled to the same lower energy level [11]. This effect is studied using the fluorescence intensity ratio (FIR) technique, which compares the fluorescent intensities of two emission bands of a material at different temperatures. Therefore, several optical temperature sensors based on this technique have been developed using lanthanide ions with energy levels close to each other [11; 12; 13; 14; 15]. Although the FIR technique allows for simple and robust device designs, it cannot typically deliver temperature resolution at the sub-degree level.  
A straight method to enhance thermal resolution by at least an order of magnitude is embedding the active ions in a microcavity, such as a microsphere, featuring resonant modes called whispering gallery modes (WGM) [15]. Microspheres can host high-quality factor resonances, which means that they confine high-energy-density resonant modes [16]. Small nanoscale changes in the surface physical parameters of the microsphere (radius and refractive index) induce a shift in the resonant wavelength of the WGM. If these variations are the result of temperature changes, the WGM can be used to estimate the temperature with high precision. If the microsphere is thus doped with optically active ions emitting at the WGM wavelength, the resonant modes can be observed via the increased emission probability of the active ions due to the Purcell effect, resulting in sharp and narrow peaks [17].