Modulations in the diffracted intensity in chiral smectic-C liquid crystals

Recently [K. A. Suresh, Yuvaraj Sah, P. B. Sunil Kumar and G. S. Ranganath, Phys. Rev. Lett. 72, 2863 (1994)] the intensity and polarization features associated with the optical diffraction in chiral smectic-C liquid crystal were studied in the phase grating mode. In the computations it was found that the diffracted intensity, as a function of sample thickness, has modulations of different length scales. In this paper, using a perturbative approach, we show that these modulations are a consequence of a coupling between different orders of scattering