Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: Evidence for excitation transfer from oxygen deficiency centers to Yb³⁺

Emission spectra in the ̃240-1100 nm wavelength region as well as the temporally resolved decay of Yb³⁺ and point defect spontaneous emission have been recorded when aluminosilicate optical fibers doped with Yb are irradiated with ̃160 fs laser pulses having a central wavelength of ̃250 nm (hω=5 eV). Photoexcitation of the fibers in this region of the deep ultraviolet (UV) provides access simultaneously to the Type II Si oxygen deficiency center (ODC), the non-bridging oxygen hole center (NBOHC: an oxygen-excess defect), and the Ge ODC. Emission from all of these defects in the ultraviolet and/or visible is observed, as is intense fluorescence at 976 nm from Yb³⁺+. Absorption measurements conducted in the ̃230-265 nm region with a sequence of UV light-emitting diodes reveal a continuum peaking at ̃248 nm and having a spectral width of ̃18 nm (FWHM), confirming that the 250 nm laser pump is photoexciting predominantly the ODC. The temporal histories of the optically active defect and rare earth ion emission waveforms, in combination with time-integrated spectra, suggest that the Si ODC(II) triplet state directly excites Yb³⁺+ as well as at least one other intrinsic defect in the silica network. Prolonged exposure of the Yb-doped fibers to 250 nm radiation yields increased Yb³⁺+, NBOHC, and Si ODC(II) singlet emission which is accompanied by a decline in Si ODC(II) triplet fluorescence, thus reinforcing the conclusion-drawn on the basis of luminescence decay constants-that the triplet state of Si ODC(II) is the immediate precursor to the NBOHC and is partially responsible for Yb ion emission at 976 nm. This conclusion is consistent with the observation that exposure of fiber to 5 eV radiation slightly suppresses ODC absorption in the ̃240-255 nm region while simultaneously introducing an absorption continuum extending from 260 nm to below 235 nm (hω≈5.28 eV). These results suggest that ODC →E' center conversion assumes a role in excitation transfer to Yb³⁺+.