Diffraction in smectic C and nematic with short range smectic C order for oblique incidence of coherent laser light

We describe the application of a laser diffraction technique to the study of electroconvection (EC) in smectics C and nematics with short range smectic C order for oblique illumination with coherent laser light. We have used laser diffraction as a method for the direct quantitative determination of the amplitudes of the director field. We found that oblique incidence in general favours the reflexes of odd numbered order, which indicate the amplitude grating, produced by the director distortion in EC regime. The modification of the standard Carr-Helfrich mechanism, provoked by the diffraction study of the smectic C and nematic with short range smectic C order, was discussed.