It is proposed to apply the direct measurement of spontaneous polarization using triangular waves to ferroelectric liquid crystals. The use of the triangular waves allows us to easily subtract the background contribution due to the conductive and the capacitive current and to accurately determine the spontaneous polarization, since a bump due to the polarization realignment appears on a straight base line. Moreover, the experiment with pulsed triangular waves clearly reveals no existence of the threshold voltage for deforming the helix and some characteristic properties of the dynamic reaction of the helix to the field applied.
Many optical properties are presented here in the reflection spectra for monodomain cholesterics of various cell thicknesses at various angles of incidence. It is reported for the first time that left-circularly polarized light incident on a left-handed helix gives rise to a beat structure in the subsidiary oscillation, while right-circularly polarized light causes a swell to be superimposed on the subsidiary oscillation. The beat structure is ascribed to the excitation of two sets of optical eigen modes, each set consisting of two optical eigen modes of slightly different wavelength. The polarization characteristics of the total reflection band as well as the selective reflection band are also discussed.
The reflection spectra from a monodomain cholesteric liquid crystal are measured for obliquely incident light. The first order reflection region is found to be divided into three regions at a large angle of incidence. The central region shows high reflectivity irrespective of incident polarization and is attributed to the total reflection. The total reflection peak is also observed between the second order reflection peaks even at a small angle of incidence. The reflection spectra are numerically computed by the 4×4 matrix method and are in good agreement with the experiments.
We have observed the growing process of high quality homogeneous cells directly in a high magnetic field by using a microscope with an elongated tube of 846 mm in length. For improving the cell quality, the application of the magnetic field is the most effective in comparison with other methods, such as the application of an electric field, SiO oblique evaporation, rubbing, etc. Factors affecting the cell quality were also studied by counting the number of characteristic defects that are always produced during the growing process. As expected, the cell quality is improved when the magnetic field is high, the cooling rate is low, the temperature control is accurate, the cell is thick, the sample is passed through the membrane filter and when the annealing time elapses. For the zone-refined sample, however, the cell quality is lowered unexpectedly; the reason for this is discussed.
Light propagation in helical structures is formulated as an eigenvalue problem based on Berreman's 4×4 matrix. Diagonalization of the secular equation is carried out to obtain the optical eigenmodes as a function of wavenumber and propagation direction relative to the helical axis, and sets of the optical eigenmodes are found to be classified into four types, whose appearance strongly depends on the wavenumber and the propagation direction. The selective and total reflections in cholesteric liquid crystals are characterized by the types of the eigenmodes for various angles of incidence.
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