The initial holographic recording rate has been analytically studied for a photorefractive thin slab. The finite thickness is explicitly taken into account by using a two‐dimensional formulation (coordinatesxandzparallel and perpendicular to the slab faces, respectively). It is shown that for a thickness comparable to the grating period, the solution appreciably departs from the usual one‐dimensional result, i.e., edge effects are remarkable. In particular, significant space‐charge fields perpendicular to the slab faces are generated. These effects may substantially modify the diffraction properties of the recorded photorefractive gratings, as exemplified for a GaAs film. ©1995 American Institute of Physics.