Optical absorption and emission properties of Gd3+ in silica host

We present a study of the optical properties of Gd-doped sol-gel silica glasses densified at different temperatures (from 450 up to 1050 °C) by means of optical absorption (OA) and radio luminescence (RL). The effect of a post-densification rapid thermal treatment (RTT) at approximately 1800 °C is also considered. Room temperature OA and RL measurements have revealed a slight low-energy shift of Gd3+ absorption/emission lines by densification temperature increasing accompanied by a parallel increase of Gd3+ RL intensities, especially strong after RTT. These effects are interpreted on the basis of structural modifications of the silica matrix and of the removal of non-radiative channels competing with Gd3+ emission. Moreover, RL spectra of fully densified samples display high-energy shoulders on the 6P7/2-8S Gd3+ emission possibly related to crystal field splitting of the 6P7/2 state. This interpretation is supported by the temperature dependence of RL spectra, investigated in the 10-320 K temperature interval: an increase of the intensity of high-energy components vs. temperature has been observed, which can be interpreted as due to thermally assisted excitation of electrons belonging to the lower 6P7/2 state to higher crystal field states and their subsequent radiative recombination.