Photo darkening of rare earth doped silica

Abstract

The photo darkening (PD) absorption spectra from unseeded amplifier operation (by 915 nm pumping) of ytterbium / aluminum and co-doped silica fibers is after prolonged operation observed to develop a characteristic line at 2.6 eV (477 nm). This line is proposed to be due to inter center excitation transfer from type II oxygen deficiency centers ODC(II) to Tm3+ trace impurities. The ODC(II) is proposed to be the result of a displacive transition of a 4-fold silica ring hosting two 3-fold silicon units that presents two non-bridging oxygen to Yb3+ (as part of its 6-fold coordination by oxygen). The displacive transition is initiated by a charge disproportionation process which leads to NBO transfer in forming dioxasilirane (2-fold coordinated silicon with two NBO attached) next to silylene (2-fold coordinated silicon with a lone electron pair). In collaboration with a valence electron of Yb3+ a new ½ / 1½ chemical bond is formed on dioxasilirane which comprises the PD color center for the visible and near-infrared. Difference in solid acidity of the silica material co-doped with Yb/Al and Yb/P may explain the observed difference in spectral shapes by change of bond order to the formed chemical bond.

The three-electron bond =Si<O_2:∙Yb absorption center of pre-darkened ytterbium-doped silica

“…The direct ODC(II) to Yb emission excitation in these fibers is in [1] seen as either direct ODC(II) to Yb excitation transfer or that ODC(II) and Yb emission hold a common origin for UV excitation. The growing Tm 3+ /Er 3+ trace impurity emission found to follow increase of PD in [2] collaborate this.…”

“…In a previous work [2] the CC PD absorption is proposed to involve dissociation of superoxide bonds comprising a σ-bond ("1") in parallel to a polarized TEB↑ to Yb as shown in Fig. 1(a) state II↑ ("1+TEB↑").…”

“…For pre-darkened fibers the active CCs are taken to be in either state II or iii. The difference in spectral appearance of Yb/Al and Yb/P co-doped material upon PD is in [2] seen as result of change in the type of bonds involved in the CCs. This is here proposed to be due to different TEB ionic character (degree of polarization) in CCs of different sites.…”

“…The annealing of pre-darkened material is in this section described by a Markov chain for the different bonds of section 2 and 3 with phonon activated state transitions by displacive transitions of 4-member SiO 4 rings carrying the CC precursor as proposed in [2]. The CCs (state II) hold two three-fold coordinated oxygen (≡O) in = Si<O 2 as shown in Fig.…”

“…Several mechanisms and species potentially responsible for PD and the herewith associated PD color center (CC) have been proposed as discussed in [1,2] but a definitive explanation for the process remains a subject of debate.…”