ON THE OPTICS OF TWIST GRAIN-BOUNDARY SMECTICS

We have studied theoretically the optical properties of twist grain bo undary smectics. We find many reflection bands even at normal incidenc e. In some of the reflection bands an incident light in any state of p olarization gets strongly reflected while in some others the strongly reflected state is of a circular polarization with the same or the opp osite handedness as that of the structure. At oblique incidence, depen ding upon the screw symmetry, a reflection band either has three sub-b ands of different polarizations or is a single band of a particular po larization. We find optical diffraction for light incident perpendicul ar to the twist axis. The diffraction pattern is completely different for TGB(A) and TGB(C). In addition in absorbing TGB(C) the pattern can even become asymmetric. From a Fourier inversion of the complex diffr acted amplitudes we can evaluate in some cases the sizes of the smecti c blocks and the grain boundaries.