The shape of the 1.55 μm emission band of the Er³⁺-dopant in oxyfluoride nano-scaled glass-ceramics

We propose a diagram showing the sequence of Stark-split energy levels for the ⁴I_15/2 and ⁴I_13/2 states of Er³⁺ doped into oxyfluoride, nano-scaled glass-ceramics based on low- and room temperature measurements of respective emission and absorption bands. The room temperature broadest and flattest 1.55 μm telecommunications emission band ⁴I_13/2 → ⁴I_15/2 of Er³⁺ to date reported earlier for such glass-ceramics is interpreted using this framework. By investigating the low-temperature behaviour, we show that the shape of the 1.55 μm band at room temperature is mostly due to thermal population of higher Stark sublevels in the excited ⁴I_13/2 state. Two sites for Er³⁺ are proposed which relate, respectively, to the glassy and nano-crystalline phases of the glass-ceramics and their relative contributions to the emission spectrum are identified. The Er³⁺-site in the nano-crystalline phase is shown to be cubic with nearby charge compensation by one interstitial F⁻, without appreciable clustering of Er³⁺ ions.