Low-frequency inelastic light scattering from chalcogenide glasses and alloys

Polarized low-frequency Raman and Brillouin spectra of the chalcogenide glasses ${\mathrm{As}}_2$ ${\mathrm{S}}_3$, Ge${\mathrm{S}}_2$, Ge${\mathrm{Se}}_2$, and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ and the glass alloy system ${\left({\mathrm{As}}_2{\mathrm{S}}_3\right)}_{1-x}{\left(\mathrm{Ge}{\mathrm{S}}_2\right)}_x$ have been measured using the near infrared 7525 or 7993 Å lines of a krypton laser. The low-temperature $T\sim 10$ K spectra of the four binary glasses indicate that the Raman coupling constant exhibits an ${\omega }^2$ frequency dependence in the limit $\omega \to 0$. In addition it is demonstrated that the spectral distribution of the low-frequency low-temperature Raman coupling constant can be described by a model proposed by Martin and Brenig. By fitting the predicted spectra of the model to the experimentally obtained spectra, the structural correlation ranges of the glasses have been determined. It is found that the structural correlation ranges of the ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ glasses are 6.5 ± 1 Å, while those of the Ge${\mathrm{S}}_2$ and Ge${\mathrm{Se}}_2$ glasses are 8.5 ± 1 Å. These results are compared with values of structural correlations obtained from x-ray diffraction spectra and optic-mode Raman spectra. The depolarization spectra predicted from the Martin and Brenig model are found to deviate significantly from the observed low-frequency depolarization spectra of the chalcogenide glasses. At higher temperatures the low-frequency Raman spectra of all samples exhibit a feature apparently centered at 0 frequency shift which cannot be described by the model of Martin and Brenig. A similar feature termed the light scattering excess has been reported for the low-frequency Raman spectrum of fused silica. A recent model by Theodorakopoulos and Jäckle describes the light scattering excess in terms of two-level defect states. The temperature dependence, depolarization ratio, and spectral distribution of the light-scattering excess are interpreted using this model and recent ultrasonic attenuation measurements. The composition dependence of both the structural correlation range and the light-scattering excess is presented for the alloy system of ${\left({\mathrm{As}}_2{\mathrm{S}}_3\right)}_{1-x}{\left(\mathrm{Ge}{\mathrm{S}}_2\right)}_x$.