Effect of alkali halides on the persistent spectral hole burning in Ge-Ga-S glasses doped with Eu3+

Addition of CsBr and KBr into Eu3+-doped Ge-Ga-S glasses resulted in persistent spectral hole burning (PSHB) with high quantum efficiency, whereas the addition of alkali chloride did not show any evidence for a hole formation process. The characterization of the phonon side band (PSB) spectra revealed the site for Eu3+ ions located near the Ga-Br-. or Ga-Cl- units. The presence of Br ions near Eu3+ lowers the energy of the conduction band, thereby enabling the interaction between the 4f electrons and the resultant conduction band. The interaction made the photo-reduction of Eu3+ to Eu2+ possible. However, the higher coupling of phonon energy with Eu3+ together with the higher electronegativity of Cl- ion in CsCl-containing glass compared to its CsBr counterpart, hindered the photo-reduction of Eu3+ into Eu2+.