AC Field Enhancement of Diffraction From Permanent Gratings in Dye-Doped Liquid Crystals

Permanent gratings can be written in doped liquid crystals with high dye concentration without any externally applied electric field, using low intensity, visible light. The gratings are adaptive as their diffraction efficiency can be easily modulated and controlled enhanced by an AC field. The diffracted intensity could also be modulated by a low frequency electric field with the magnitude of modulation decreasing for higher frequencies. The permanent gratings are durable, remaining in the cells for over a year even after application of high temperatures

Theory of Nematic-smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic-smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic-smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic-smectic interface which is perpendicular to the smectic layers