A polaron approach to photorefractivity in Fe : LiNbO₃

The thermally activated, incoherent hopping of small electron polarons generated by continuous illumination in iron-doped lithium niobate is simulated by a Marcus-Holstein model for which all the input parameters are known from literature. The results of the calculations are compared with a comprehensive set of data obtained from photorefractive, photogalvanic and photoconductive measurements under green light excitation on samples with different doping levels and stoichiometries in the temperature range between $150K$ and room temperature. We show that the temperature and composition dependence of the photorefractive observables can be interpreted by a change in the abundance of the different hop types that a polaron performs before being captured by a deep Fe trap. Moreover, by a comparison between experimental and numerical data we obtain new insights on the initial photo-excitation part of the photorefractive process. In particular all results are consistent if a single value of the photogalvanic length $L_{PG}=(1.44\pm 0.05)\mathring{A}$ is assumed for all the samples and all the temperatures. The photo-generation efficiency ϕ under green light excitation (somewhere denoted as quantum efficiency) is also estimated. It appears to decrease from 10%–15% at room temperature to about 5% at 150 K. This behavior is qualitatively interpreted in terms of a temperature-dependent re-trapping probability of the light-emitted particles from the initial Fe donor center. (paper)