Microstructural and defect population change in electron beam irradiated Ge:SiO2 MCVD glasses in the conditions of refractive index change writing

Electron spin resonance (ESR) and ultra violet (UV) optical absorption spectra are measured in Ge:SiO2 glasses elaborated by modified chemical vapor deposition (MCVD) and irradiated by an electron beam of medium energy (10-50 keV). The doses (a few C/cm2) like the electron energy used, correspond to conditions for writing refractive index change for optical applications. These data yielded information on defect population changes. Raman spectroscopy was used for correlating these changes to microstructural changes. We point out a 5.1 eV absorption bleaching like surprisingly the one occurring under 5 eV laser irradiation correlated to an absorption increases around (4.4 and 5.8 eV). E′ centres are also produced. More specifically, we found in a previous work two domains of irradiation parameters for samples with large Ge content for which specific volume increases or decreases. We thus precise in this paper that (1) when compaction occurs, detectable by surface topography depression and D2 peak intensity increase, Si E′ centres are produced and a diamagnetic component absorbing strongly at 6.1 eV that may arise from sixfold coordinated Ge, (2) when expansion dominates, D2 peak does not decrease at all but long range order changes, detectable by modification of transverse optical (TO) and longitudinal optical (LO) mode intensity. In this case, Ge E′ and Ge(1) dominate and the previous absorption at 6.1 eV disappears. In addition, an absorption peak appears at 3.1 eV from an unknown defect.