Figure 1.

(a) The energy level diagram for Er$_{}^{3+}$ and Yb$_{}^{3+}$, where Yb$_{}^{3+}$ acts as a sensitizer in the upconversion energy transfer process to the Er$_{}^{3+}$ $_{}^4$I$_{11/2}^{}$ level. The upconversion process upon excitation at 980 nm yields green and red fluorescence of Er$_{}^{3+}$. The two green fluorescence bands at 524 and 547 nm are thermally coupled and hence their intensity ratio is temperature dependent. (b) A schematic of the confocal scanning microscopy setup used to measure the green fluorescence ratio with high temporal and spatial resolution. Pump light from the 980 nm laser is filtered via a 900 nm dichroic mirror (DC) where fluorescence from the EYT glass is sent through a series of filters. The total output is first filtered with a 650 nm shortpass filter, and then split, whereby each beam is filtered via a 540 nm shortpass (SP) or 540 nm Longpass (LP) filter, respectively, to measure separately the intensity for each green band with an avalanche photodiode detector (APD).