C.Z. Van Doorn

Dynamic behavior of twisted nematic liquid‐crystal layers in switched fields

The Ericksen‐Leslie equations, describing the dynamic behavior of nematic liquid crystals, have been applied to a twisted nematic layer and have been numerically solved for a number of cases. The results account well for the observed dynamic behavior of displays using a twisted nematic layer. In particular, an explanation is given for the ’’bounce’’ in the optical transmission of a twisted nematic cell between polarizers after switching off the applied field. Materials having a frequency dependence of the sign of the dielectric anisotropy show a reversal of twist after the frequency of the applied field has been switched. This can also be explained. It is shown that fluid motion is essential to the occurrence of these phenomena.

Cathodrochromic Properties of Sodalite

The coloration and bleaching behavior of undoped bromide sodalite prepared by solid‐state reaction and by various low‐temperature hydrothermal reactions is studied. A high coloration sensitivity is obtained; for instance, a charge density of 0.5 μC/cm2 at 10 kV yields a contrast ratio of 2:1. The coloration and optical bleaching behavior is strongly dependent on the preparation method. A relation between bleaching rate and bromide deficit is found. A relation is suggested between residual coloration and the presence of oxygen on bromine sites. The effect of the doping with transition‐metal ions on the bleaching behavior is studied. The doping with Fe, Ni, and Cu causes a considerable increase in bleaching rate.

Transient effects in the electrically controlled light transmission of a twisted nematic layer

Abstract An oscillatory after-effect in the light transmission of a twisted nematic layer is often observed when the driving field is switched off. The effect is explained as being caused by fluid motion.

On the magnetic threshold for the alignment of a twisted nematic crystal

Abstract A computation of the capacitance and the optical transmission between parallel polarizers of a twisted nematic liquid crystal in a magnetic field shows that the “optical threshold” occurs at a higher value of the field strength than the “capacitive threshold”.

Angular dependent optical transmission of twisted nematic liquid-crystal layers

Abstract The angular-dependent light transmission of a twisted nematic layer between crossed polarizers in an electric or magnetic field depends mainly on the induced orientation of the director in the middle part of the layer. Measurements and computations compare favourably.

Two‐frequency 100‐line addressing of a reflective twisted‐nematic liquid‐crystal matrix display

Scanning of more than 100 lines of a reflective twisted‐nematic liquid‐crystal matrix display with full contrast can be achieved using a two‐frequency addressing scheme and a liquid crystal showing a dielectric anisotropy that is positive for low frequencies and negative for high frequencies. It is shown that the electric field inhomogeneity, caused by the large dielectric constants, changes the optical response to low and high frequencies as well as the angular dependence of the optical transmission.

Luminescence of O−2, Mn2+ and Fe3+ in sodalite

Abstract A luminescence emitted by sodalite at 77°K under 2537 A excitation and consisting of a number of peaks between 4500 and 7365 A at wavenumber intervals of 902 – 1032 cm -1 is ascribed to O - 2 . Emissions at λ max = 5420 A and λ max = 6830 A are ascribed to Mn 2+ and Fe 3+ , respectively.

Luminescence, transmission and width of the energy gap of CdTe single crystals

Synopsis Irradation of a CdTe single crystal or biasing a CdTe- p-n junction in the forward direction lead to a luminescence with a maximum at about 8880 A. From transmission measurements and measurements of the long wave limit of the photo emf. of a CdTe- p-n junction as a function of the temperature, the shift of the band edge was seen to vary from 2.34 × 10 −4 eV/°K at 77°K to 5.44 × 10 −4 eV/°K at 800°K.

Abnormal green “edge” emission in CdS due to an oxygen impurity

Abstract CdS powder, fired in an oxygen atmosphere, shows at low temperature an abnormal green “edge” emission, shifted 0.040 eV to higher energy with respect to the commonly observed “normal” edge emission. Other properties are very similar.

Abnormal green edge emission in CdS

Abstract As previously reported, heat treatment of CdS powder in oxygen causes the appearance of a green-photoluminescence, which shows close resemblance to the well-known green “edge” emission, but differs from it by being shifted by an amount of 0.040 eV towards higher energy. In the present note it is shown, that the oxygen treatment alone is not sufficient to invoke the abnormal emission and that the presence of chlorine is necessary also. It is not yet known, whether the chlorine forms part of the luminescence centre or merely aids the incorporation of oxygen.