Whispering gallery modes in a holmium doped glass microsphere: Temperature sensor in the second biological window

The displacements on the whispering gallery modes (WGM) wavelengths of the three emission bands (660 nm, 760 nm and 1200 nm) of Ho3+ doped yttrium aluminosilicate microspheres were analysed as a function of the temperature. The displacements have been studied in order to evaluate the viability of these microspheres as thermal sensors. A 532 nm continuous wave laser was used to excite and heat the microspheres using a confocal microscope setup. Average displacement rates of the WGM peaks with the laser power of 5.3, 6.2 and 9.7 p.m./mW were found for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively. Temperature uncertainties of 0.6 K for the band centred at 660 nm, 0.5 K for the 760 nm band and 0.3 K for the 1200 nm band have been obtained. It is interesting to note that the maximum resolution corresponds to the band centred at 1200 nm that is especially interesting because it is inside the second biological window, which has medical applications.