Origin of the refractive index modification of femtosecond laser processed doped phosphate glass

The origin of the local refractive index modification in femtosecond laser inscribed structures has been investigated with confocal microfluorescence imaging. We have identified the origin of both, positive and negative refractive index changes in a commercial Er-Yb codoped phosphate glass upon irradiation in the low repetition rate regime (1 kHz). Consistent relations among the photoluminescence behavior of the dopants (erbium and ytterbium ions), the local sign of the density change and the local modification of the refractive index by means of characteristic emission features such as the intensity and the spectral peak position have been established. Blue or redshift in the photoluminescence emission have been observed and related to a local perturbation in the crystal field caused by a modification of the mean distance among the dopant ions, and thus of the local matrix density. These conclusions are additionally supported by the spatial distribution of photoluminescence emission intensities, which have been interpreted in terms of energy transfer mechanisms underlying the overall erbium-ytterbium emission process.