Space charge field generation and thermal effect in a dye-doped twisted nematic liquid crystal cell

Abstract

Nematic liquid crystals exhibit long-range orientational order of the axes of the molecules, the average direction being described by the nematic director n. These materials are known for their large nonlinear optical properties, possess very broadband (0.4 μm-12 μm) birefringence and transparency, and have a large susceptibility to ac, dc and optical fields. These properties combine to make nematic liquid crystals ideal materials for image sensing, display and processing devices.

Optical reorientation in nematics is one nonlinear property that has been studied in detail. In analogy to reorientation by ac or static fields, an impinging optical field will realign the bireffingent director axis through dipolar interaction. In dye-doped nematic films, the dye dopant produces photocharges, which migrate and diffuse within the film and set up various dc space charge fields that exert a torque on the liquid crystal director axis, resulting in reorientation and changes in the refractive index. In particular, the most nonlinear optical effect observed to date has been reported in methyl red-doped nematic films, which show that these photoinduced space charge fields can be so large that no applied dc field is needed to create observable director axis reorientation effects.

The dissolution of dye molecules in the nematic liquid crystal within the solubility limit induces significant absorption in the visible range. A straightforward consequence of light absorption is heating, which leads to various thermo-optical effects. In this paper we discuss the combined effects of the induced space charge field and heating due to absorption in a twisted nematic cell, using the pump-probe configuration.