Evidence of formation of tightly bound rare-earth clusters in chalcogenide glasses and their evolution with glass composition

We present spectroscopic evidence for the formation of tightly bound Nd³⁺ clusters in chalcogenide glasses in which Nd³⁺ ions are so close that no appreciable fluorescence is observed. According to a theoretical calculation using the rate-equation modeling and comparison with experimental results, this type of cluster contributes as the main source of energy transfer among Nd³⁺ ions in Ga-free glasses, resulting in the strong fluorescence quenching. It is proposed that the total volume of such Nd³⁺ clusters sharply decreases with Ga doping, while the spatial distance among Nd³⁺ ions in clusters remains close enough for there to be no detectable fluorescence from these clustered Nd³⁺ ions. Such clusters are then completely dissolved in glasses once a critical Ga concentration is reached.