Optical characterization of Fe- and Cu-doped SiO 2 glasses prepared by the sol-gel method

The SiO 2 glasses obtained by the sol-gel process, doped with Fe or Cu and annealed either at oxidizing or at reducing (CO - CO 2 ) atmosphere, exhibit deep coloring caused by the absorption and scattering bands in the visible spectral region followed by a considerable change of the shape of the infra-red absorption bands in the region 1000 - 1250 cm -1 corresponding to the optical phonon's excitation. An analysis is made of the spectra obtained by the optical and photoacoustic as well as by the Raman techniques; results are compared with the theory taking into account the light absorption and scattering for the visible region and the influence of the phonons' scattering on the shape of the corresponding absorption bands for the infra- red region. We found that the coloration caused by the annealing of the doped glass at 100 degree(s)C is basically due to the individual Cu and Fe ions in interstitial sites. After annealing at 300 degree(s)C and higher temperatures in oxygen atmosphere the doping atoms enter substitutionally for Si; part of them form colloidal particles of the corresponding metal oxide. The latter exhibit both absorption and scattering which determine the glass color. Annealing under reduction conditions leads to the formation of Cu and Cu 2 O particles producing a deeper coloration; interaction of the former particles with carbon (graphite) coating causes a specific changes in the Raman spectra which could be attributed to the SERS - Surface Enhanced Raman Scattering.