Dielectric relaxation time measurement in absorbing photorefractive materials

We show that absorbing photorefractive materials characterized by a space-charge exponential relaxation time law exhibit an overall hologram optical erasure that is described by the so-called exponential integral function, as far as self-diffraction can be neglected. This fact is due to the bulk absorption producing an exponentially decreasing distribution of the erasure beam irradiance along the sample thickness that results in a correspondingly increasing dielectric relaxation time. The theoretical development in this paper is experimentally verified by the analysis of the holographic erasure in a nominally undoped Bi12TiO20 photorefractive crystal using the 514.5 nm laser wavelength where this material exhibits a relatively strong bulk absorption. Neglecting absorption in this experiment leads to a relaxation time that is about 4-fold larger than the actual value. (C) 2000 Published by Elsevier Science B.V. All rights reserved.