Long-term stability of laser-induced defects in (fluoride-)phosphate glasses doped with W, Mo, Ta, Nb and Zr ions

Laser-induced defects in glasses are of considerable interest for many applications from optics to photonics. The importance of low-level impurities of polyvalent ions in aiding defect formation has been identified early on. (Fluoride-)phosphate glasses are used today as laser materials, waveguides, amplifiers and luminescing materials, all sensitive to a change of the materials transmission by the interaction with light during application. To better understand the processes of defect generation and recovery, a systematic comparison of defect formation in various glasses and for various radiation sources and dopants has been conducted over the last decades. Here we will focus on (fluoride-) phosphate glasses doped with 50 to 5000 ppm of the 4d and 5d ions Zr, Nb, Ta, Mo and W. Glasses were melted under air or under reducing conditions in order to shift the redox equilibrium of the dopants before irradiation with either the 193 nm or 248 nm excimer laser. Only for W, Mo and Nb reduced ion species were confirmed by optical and/or ESR spectroscopy in the pre-irradiated glasses. However, irradiation showed for all metaphosphates the presence of reduced dopant species (W5+, Mo5+, Ta4+, Nb4+, Zr3+), acting as extrinsic hole centers (HC) after being photo-oxidized by laser irradiation to the fully oxidized d0 ions (Mn+)+-HC. Only for Ta5+ with its (Ta5+)− -electron center (EC), photo-reduction to the tetravalent ion was observed. Defect recovery was followed up to 16 years after the irradiation experiments, showing that most (Mn+)+-HC were very stable, while intrinsic HC either recombined with EC or converted into extrinsic (Mn+)+-HC. Due to ubiquitous iron impurities, even these high purity glasses with iron levels of 5-10 ppm or less, showed the formation of (Fe2+)+-HC.