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We analyse the in¯uence of charged impurities and¯exoelectric polarization on the optical transmission of a hybrid aligned nematic liquid crystal cell. The theoretical results obtained within the framework of the Poisson± Boltzmann equation and Frank elastic theory are compared with the observed optical response [N. V. Madhusudana and G. Durand, J. Phys. Lett. 46, L-195 (1985)]. We show that impurities can be very important for the behaviour of the system in the low ® eld regime where the¯exoelectric e ect is relevant, and we determine the¯exoelectric coe cient, the anchoring strength, and the concentration of impurities in the sample previously studied by Madhusudana and Durand.

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A theoretical analysis is given of the acoustic wave propagation in periodically nonhomogeneous media made of a solid material whose stiffness tensor is uniformly rotating along a given axis. In the last years, such media have been studied theoretically as well as experimentally, in particular for what concerns sample preparation and possible applications. A detailed analysis of their acoustical properties is given here, based on fully analytic and simple propagation equations. For axial propagation: (i) the dispersion curves of media where the transversal field components and the longitudinal ones are not coupled show only one forbidden band, that gives selective Bragg diffraction; in the opposite case they show at least a second forbidden band, that involves the quasilongitudinal and one of the quasitransversal eigenmodes; (ii) in the first case (absence of coupling), the medium gives pure acoustical rotation for p<>lambda; (iii) in the presence of the coupling, regions of mode exchange between the longitudinal component and a transversal one are generally present. The cases of lossy media and of quasiaxial propagation are also considered, and the analogies between optical and acoustical properties discussed.

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