Dynamical optical response of nematic liquid crystal cells through electrically driven Fréedericksz transition:...

A dynamical optical characterization of planar nematic liquid-crystal cells electrically driven through the Fréedericksz transition is presented. Our method involves applying voltage steps with different starting voltage close to the Fréedericksz threshold. Measurements are performed on cells with various thickness, from a few microns up to 180µm, and highlight the transient molecular disorder occurring close to the Fréedericksz transition. We show that the transient disorder affects the molecular arrangement mainly in the reorientational plane of the splay motion induced by the planar cell geometry. Moreover, a disorder quantification in terms of optical transmission losses and temporal dynamics enables us to picture the Fréedericksz transition. This characterization provides the identification of the electrical driving conditions for which the effect of the reorientational disorder is minimized. When comparing cells with various thicknesses, it results that thick cells are characterized by a much smoother transition with respect to the conventional step-like Fréedericksz transition of the thin cells, hence, thick cells can be dynamically driven over a large range of voltages, even below the Fréedericksz threshold. The results are discussed in view of novel electro-optical applications of thick layers of nematics. As an example, the experimental conditions for realizing a rapid birefringence scan and the achievement of a large and tunable group delay for femtosecond pulses are presented.