Resumen de Liquid crystal photonic devices based on conductive polymers = Dispositivos fotónicos de cristal líquido basados en polímeros conductores
David Pérez Medialdea
This thesis focuses primarily on two major lines: the effect of dissimilar alignment layers over the electro-optical and electrical responses in asymmetric liquid crystal devices, and manufacturing and characterization of liquid crystal lenses using conductive polymers as high resistivity electrodes. In the first part, several series of liquid crystal devices with dissimilar alignment layers (SiOx, slippery surface and PEDOT:PSS) are manufactured. Entire manufacturing protocols are fully exposed. The electro-optical responses and ion transport measurements are compared and analyzed to elucidate which phenomena are responsible of asymmetric responses. We found that different fluxes of charges injected or absorbed by the alignment layer are responsible of the asymmetry induced. In addition, the asymmetric electrical response (the impedance of devices are different for each field polarization) found below the threshold voltage involves the dominance of ionic effects over the spontaneous polarization as responsible of the asymmetry. One of the three alignment layers analyzed, PEDOT: PSS opens the way for the design and manufacture of LCDs using conductive polymers. This is the second major line developed in this thesis. We found that PEDOT:PSS can work simultaneously as an alignment layer and electrode (high resistivity electrode) paving the way for many applications such as flexible devices. Another important application of these LCDs based on organic compounds is the manufacture of liquid crystal lenses (MLCLs). An extensive study of the manufacture procedure and further characterization of these lenses are presented. Several types of conductive polymers are studied as potential candidates for high resistivity electrodes, concluding that PEDOT:PSS is the optimum. Due to the MLCLs configuration, the focal length can be tune varying both voltage and frequency of the applied signal. After a simple manufacturing procedure, MLCLs are obtained. Through a Hartmann- Shack wavefront sensor, the outcoming wavefront of the lenses are characterized. This characterization leads us to conclude that lenses with PEDOT:PSS as a high resistivity x ABSTRACT electrodes provide better lenses. Using identical electrodes, and therefore, the deposition of PEDOT:PSS in both substrates yield a better configuration. Due to the shrinkage diameter effect, the effective radius of the lens is reduced involving a greater reduction of the focal length. The drawback of this effect is the appearance of residual light in the image. The placement of a dynamic diaphragm may solve this problem. The shrinkage diameter effect is more pronounced in lenses with identical electrodes. A further analysis is performed to quantize the quality of the lenses. Such analysis consists in the calculus of the Modulation Transfer Function (MTF) as function of the spatial resolution at different voltages (at different focal lengths). MTF variation as function of aperture radius of the lens leads us information about the contrast (at a spatial resolution of 10 lp/mm) and resolution (30 lp/mm) for different voltages. In summary, this PhD work shows the study of asymmetric responses of asymmetric LCDs with dissimilar types of alignment layers. One of these alignment layers (PEDOT:PSS) is used as a high resistivity electrode in MLCLs. RESUMEN Este trabajo de tesis se centra básicamente en dos líneas: el estudio las capas de alineamiento en dispositivos de cristal líquido asimétricos y cómo afecta la naturaleza de las mismas a las respuestas electro-óptica y eléctrica, y la fabricación de lentes de cristal líquido usando polímeros conductores como electrodos de alta resistividad. En la primera parte, se diseñan y fabrican LCDs con distintas capas de alineamiento (SiOX, slippery surface y PEDOT:PSS). Las respuestas electro-ópticas y las medidas de transporte de iones son comparadas para dilucidar qué fenómenos son los que generan la asimetría presente en aquellas respuestas. De las tres capas de alineamiento presentadas, la última (PEDOT:PSS) abre el camino para el diseño y fabricación de LCDs usando polímeros conductores. Esta es la segunda línea desarrolla en esta Tesis. Una aplicación de estos LCDs basados en componentes orgánicos es la fabricación de lentes modales de cristal líquido (MLCLs). En este trabajo se expone ampliamente la fabricación de estas lentes y su caracterización. Para finalizar, se lleva a cabo un exhaustivo análisis de la calidad de las mismas.
