Liquid crystal switchable optical devices

This thesis will investigate liquid crystal materials for application in switchable optical devices, with a focus on dichroic dyes and the photoisomerisation of azobenzene-based materials. An electrically switchable and tuneable liquid crystal Lyot filter is presented which demonstrates excellen switching from transmission to blocking modes with a response time between 40 and 110ms depending on the wavelength rejection required. The effect of doping nematic liquid crystal 5CB with a new highsolubility anthraquinone dye is investigated. The solubility limit is found to be 7%(w/w), greater than any previously reported anthraquinone dye, allowing for better colour saturation in guest-host devices. The inclusion of this dye is found to improve the nematic temperature range of the system, with 5%(w/w) dye generating an increase in the nematic temperature range by 8.3◦C. The optical, elastic, and dielectric properties dependence on dye concentration are fully investigated and the observed dependencies discussed in terms of device implications. An anthraquinone dichroic dye doped liquid crystal elastomer is developed and the strain dependence of order parameter is measured via the polarisation angle dependent absorption of the anthraquinone dye. A temperature independent linear relationship between order parameter and strain is demonstrated, with excellent agreement with separate Raman spectroscopy data. A maximum order parameter of S = (0.28±0.01) is measured in the initially isotropic system. The transmission dependence on strain means that this material could be implemented as a simple optical strain sensor, with a continuous variation on transmission observed with applied strain. An azobenzene material BAAB2 is utilised in the investigation of the impact of photoisomerisation on nematic host 5CB. The nematic temperature range of BAAB2 un-irradiated is between 48 and 83◦C and the absorption peaks corresponding to the trans and cis excitations are found at 350nm and 435nm respectively. Doping of 5CB with BAAB2 at varying concentration between 0 and 10%(w/w) is undertaken and the change in nematic to isotropic phase transition temperature is investigated for various illumination conditions. In the un-irradiated state the nematic to isotropic phase transition temperature is found to increase from 36.3 to 41.4◦C, a direct result of BAAB2’s higher nematic temperature range when compared with 5CB. In the yellow-filtered state a reduction in transition temperature is observed with increasing cis concentration, with the relative change in intermolecular interaction energy measured as (−3.5±0.3)·10−3. Under continuous unpolarised UV irradiation the relative change in intermolecular interaction energy is 2.5 times larger, at (−8.7±0.2)·10−3. This is associated with the light induced torque generated out of the plane by the selective excitation of the BAAB2 material, confirmed via dielectrics measurements of the system under yellow filtered and continuous UV irradiation conditions. It is demonstrated that the BAAB2 doped 5CB system can be utilised as an all-optical automatic switchable laser protection device which switches from transmission to protection mode when the laser ”threat” irradiates the system. The response time dependence on temperature and laser power is fully characterised and the ability for this all-optical device to provide protection for CCD cameras is evaluated.