Optical signal and optical axes in uniaxial and biaxial nematic phases

ABSTRACTIn this work we will study the behavior of the optical signal and optical axes along the continuous transition of a lyotropic system from a nematic discotic (ND) phase to a nematic calamitic (NC) phase; we will study in detail the case in which there is a biaxial nematic (NB) phase at the interstice between these two uniaxial phases. It will be shown that the optical signal and optical axes change simultaneously in order to agree with the optical signals of the uniaxial phases at their borders: the optical signal is positive at the frontiers with the ND phase and it turns to be negative at the frontiers with the NC phase. Concomitantly, the optical axes rotates from the e→z direction to the e→x direction. We will illustrate these findings with experimental data that confirm our results.