Nonresonant interband Faraday rotation in amorphous semiconductors, as a function of photon energy, may be described by an equation derived for direct transitions in crystalline semiconductors. In this paper it is shown how this equation may be obtained for the former case also, assuming a parabolic density of states function S(E) and a correlation between valenceand conduction-band states. The analysis of experiments on chalcogenide glasses reveals a Faraday-rotation energy gap Eg"" that is significantly larger than the optical gap Eg '. The effect is attributed to transitions between extended states, so that it is meaningful to compare Eg" with the mobility gap Eg. For oxide glasses both gaps are comparable but for chalcogenide glasses Eg"" is too large by a few tenths of 1 eV.
Optical activity (OA) and Faraday rotation (FR) in SiO, are compared from both a theoretical and an experimental points of view. A simultaneous derivation of the one-oscillator formulas of F R and OA is presented. For OA this formula is extended t o a semi-classical expression, in analogy with earlier derivations of F R formulae. Starting from dispersion relations for crystals without inversion symmetry, i t is possible t o derive the OA formula, obtained earlier by Natori with the Kubo-Greenwood formalism. Within the limits of accuracy and in the wavelength range 300 to 700 nm the experimental curves of 09 and FR differ by a numerical factor only. Nevertheless the gap energies, obtained by fitting these curves with OA and F R formulas are different. The energy EER = 9 eV is believed to correspond to the real direct band gap.Ein Vergleich zwischen optischer Aktivitat (OA) und Faraday-Drehung (FR) wird anhand theoretischer und experimenteller Ergebnisse durchgefiihrt. Desgleichen wird eine Ableitung fur die Ein-Oszillator-Formeln fur FR und OA angegeben. I n Analogie zu einer friiheren Ableitung des FR-Ausdruckes wird eine halbklassische Gleichung fur die OA erhalten. Vom Dispersionszusammenhang fur einen Kristall ohne Inversions-Symmetrie ausgehend wird eine quantenmechanische Formel erhalten, die mit Hilfe des Kubo-Greenwood-Formalismus durrh Natori bereits friiher abgeleitet wurde. Innerhalb der Grenzen der MeDgenauigkeit und im WellenlLngenbereich von 300 bis 700 nm sind die experimentellen Kurven fur OA und F R proportional. Jedoch ergibt die Analyse mit Hilfe von theoretischen Spektralfunktionen einen Bandabstand, der fur beide Experimente nicht ubereinstimmt. Als SchluIlfolgerung wird angenommen, daD F R und OA nur an direkte Ubergange gekoppelt sind und daD der reale direkte Bandabstand in der Xahe von E:R = = 9 eV zu erwarten ist.
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