Optical properties of the small polaron in high electric fields

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Abstract
A simplified theory is given of the changes induced in the optical absorption of a small polaron by the presence of a static electric field bias ( epsilon ). Three cases are treated: a free carrier a carrier bound to an impurity center, and a carrier moving through a lattice in the presence of disorder potentials. Explicit formula are derived for weak and moderate fields ( epsilon 5 V cm-1), showing effects which are linear and quadratic in epsilon , with various temperature dependences. The symmetry properties of the optical high-field effects are discussed, and the problem of optical charge transfer in the Coulomb field of an impurity ion.