Temperature and frequency dependences of the far-infrared and microwave optical absorption in amorphous materials

Abstract

Microwave and far-infrared absorption have been studied in several chalcogenide and oxide glasses (${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, ${\mathrm{Tl}}_2$Se${\mathrm{As}}_2$ ${\mathrm{Te}}_3$, 3Si${\mathrm{O}}_2$ · ${\mathrm{Na}}_2$O) at frequencies between 0.1 and 100 ${\mathrm{cm}}^{-1\mathrm{}}$ at 300 K and at 0.8 ${\mathrm{cm}}^{-1\mathrm{}}$ between 10 and 300 K. In addition, previously published data for Si${\mathrm{O}}_2$ and other amorphous materials are analyzed. The results exhibit comparable frequency- and temperature-dependent behavior for all amorphous materials. The magnitude of the absorption in the amorphous solids over the entire frequency range is strongly enhanced (by an order of magnitude) over the absorption in the corresponding crystalline solids. The enhanced absorption is interpreted in terms of disorder-induced optical coupling to Debye-like as well as to non-Debye-like modes. The non-Debye-like modes dominate the absorption at low frequencies $\frac{\omega }{2\pi c}\ll 10$ ${\mathrm{cm}}^{-1\mathrm{}}$ at temperatures $10<T<\mathrm{}300\mathrm{}$ K. It is shown that multiphonon difference processes as well as resonant optical excitations of two-level modes do not account for the experimental observations.