Abstract
The exact mechanism responsible for reversible photo-darkening in As-Ch glasses has been an open question. Many mechanisms have been suggested. Here I assume that self trapped excitons (STE) cause a local structural relaxation and end up in mid band states. I show that such an excitonic state produces a local frustration field which distorts the lattice. These states most probably decay non-radiatively through excitation of localized modes satisfying quantum statistics. I posit that these localized modes are the result of continuously broken ergodicity. These modes can channel to form local oscillations, which would act to raise the temperature, or couple to the valence electronic energy, which results in a local structural modification. I show the observed temperature dependence of both the band edge shift and the volume increase are predicted by the latter mechanism.
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