Waveguiding characteristics of cholesteric/nematic and smectic liquid crystal thin films

Previous studies of liquid crystal waveguiding characteristics have indicated severe drawbacks in the case of nematic thin film waveguides (1). A theoretical characterization of the nematic system was developed utilizing a Green´s function analysis of the depolarized scattering based on the thermal fluctuation theory of De Gennes (2). The calculations predict substantial losses for the zero-field case (~ 28 dB/cm) and these, together with the predicted structure in the power distribution, have been experimentally verified. A modification of the analysis to include the effect of electric field quenching of the thermal fluctuations predicts that significant damping of the scattering losses should be possible in the case of nematic molecules possessing large positive dielectric anisotropy. This prediction has been experimentally confirmed in the case of a number of room temperature nematogens. However, there remains a fundamental limitation of all pure nematic waveguide systems. This is that the dynamic response of nematic materials is governed, in the case of decay (i.e. turn-off) time, solely by weak elastic restoring torques and is thus generally slow (~ millisecond).