Light-induced dark decays of photorefractive gratings and their observation in Bi12SiO20

Abstract

We find that the temporal shape and speed of dark decays of photorefractive gratings depend on the light intensity used to write the grating. We attribute this to the fact that the population distribution of impurity levels inside the crystal under illumination is light intensity dependent and differs from the thermal equilibrium distribution. We use a simple extension of the band conduction model of Kukhtarev which considers an additional shallow trap level to analyze dark decays observed in a Bi12SiO20 crystal. We find the excitation rate for thermally excited charge carriers and their diffusion length when this crystal is under illumination and obtain estimates for the shallow trap density (∼1.5×1015 cm−3) and the shallow trap occupation lifetime (∼0.2 s). We identify the shallow trap as a shallow hole trap by comparing the results from our dark decay analysis to results obtained from beam-coupling and light-induced erasure measurements.