“…The results revealed the importance of the sensitizer Yb 3+ ions in increasing the upconversion intensity and a higher concentration of this specie favours the erbium red and IR emissions. The observed Er 3+ emission around 1540 nm ( 4 I 13/2 → 4 I 15/2 electronic transition) has a FWHM of about 100 nm in the temperature range between 70 and 300 K. This type of rare earth doped materials have been receiving close attention due to their potential applications, such as optical amplification, new light-emitting-diode (LED) converted phosphors, laser technologies, etc [14,15,16]. Specifically, the TeO 2 based glasses, are considered as promising host materials, due to their favourable properties, which we mentioned in the first paragraph.…”