Theory of Multiphonon Absorption in the Transparent Regime of Amorphous Solids
We predict the multiphonon absorption α in amorphous solids, utilizing the statistical theory of Mitra et al, which relates optical properties of amorphous solids to variations in their local density. We find that if the ratio η of the mean density of an amorphous solid to that of its crystalline counterpart is greater than unity, then α decreases more slowly in the amorphous solid; if η<l, then either an increase or decrease is possible, depending on the width of the density distribution function. As an application of the method, we perform calculations for III–V semiconductors, utilizing an exactly soluble single-particle model which accounts for both anharmonicity and nonlinear moments. Analysis of their fundamental lattice resonance demonstrates that η<1 for these solids. Broadening of α and suppression of the temperature dependence is predicted to occur to varying extents for the III-V’s investigated. This suggests that various amorphous solids may not be as attractive as their crystalline counterparts for infrared applications requiring high transparency.
KeywordsLocal Density Amorphous Solid Density Distribution Function Amorphous Semiconductor Gruneisen Parameter
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