Kogelnik-like model for the diffraction efficiency of POLICRYPS gratings

We report on diffraction gratings consisting of continuous and well-aligned nematic films separated by polymer slices, which are originated in nematic-containing polymer composites when cured by a UV interference pattern. The diffraction efficiency of these gratings is experimentally investigated in detail; its temperature dependence reveals a rather complex, nonmonotonic shape, with a next-to-unit maximum at high temperatures and a next-to-zero minimum at lower ones. When the influence of an external electric field is investigated, the dependence of the diffraction efficiency on the applied voltage appears to be nonmonotonic too, the particular shape depending on the actual value of the sample temperature; in general, both switch-on and switch-off of the diffraction mechanism can be observed as an effect of the external field. For this kind of grating, a Kogelnik-like model has been implemented that makes use of only real values of some physical quantities, without the necessity of any fitting parameter. Numerical solutions account for the temperature dependence of the diffraction efficiency with good accuracy.