Non-linear Raman effects and photodarkening in chalcogenide glass

The intensities of Raman-scattering bands in the spectrum of vitreous \chem{As_{2}S_{3}} assigned to vibrations of intrinsic homopolar As-As ( \sim 234\un{cm^{-1}}) and S-S ( \sim 490\un{cm^{-1}}) bonds increase much more with the power of the exciting laser than does the principle vibrational band which is related to As-S-As glass-network vibrations ( \sim 343\un{cm^{-1}}). In addition, the frequency of this glass-network vibration softens with an increase of the laser power. We ascribe these non-linear Raman effects to a photo-stimulated increase in polarizability of the homopolar bonds accompanied by softening of glass-network vibrations. It is suggested that this non-linearity is related to photodarkening in \chem{As_{2}S_{3}}