The Flexoelectro-optic Effect for Photonics Applications
This thesis comprises an account of research carried out into the flexoelectro-optic effect, as observed in chiral nematic liquid crystals, and its potential for application in fibre optic communications components. The flexoelectro-optic effect provides a mechanism of fast, analogue rotation of the optic axis in chiral nematic materials via the application of an electric field to the sample. In particular, bimesogenic liquid crystal materials exhibit very large flexoelectro-optic tilt angles, and a large tilt angle per unit field in comparison to other mesogenic materials. In this work a new geometry for the flexoelectro-optic effect is developed in which the chiral nematic liquid crystal is aligned with its helical axis along the normal to the cell walls and the electric field is applied in the plane of the cell. It is shown that polymer stabilization of this device by the addition of a small percentage of reactive mesogen to mixture increases greatly the ability of the device to withstand high amplitude a.c. electric fields. Applied fields of up to 6.8 V/μm are shown to induce a maximum birefringence of ∆n=0.037, due to both flexoelectric and dielectric coupling, and ∆n=0.012 due to flexoelectric coupling only in a sample based on symmetric difluorinated bimesogens. This induced birefringence is shown to consistently respond to field application and removal on the sub millisecond timescale. Polymer stabilization of the same mixtures in the uniform lying helix texture is shown to affect the electro-optic response of the samples in a manner which is dependent on the concentration of reactive mesogen used, and the temperature at which the reactive mesogen is cured. A concentration of approximately 3% weight/weight, however, has little detrimental impact on the device characteristics, and curing of the sample at the lower end of the chiral nematic temperature range is shown to allow optimization of both tilt angle and response time of the samples. The effect is also employed to demonstrate a new method of fast electrical tuning of the output wavelength from chiral nematic photonic band edge lasers. An 8nm shift was induced in these devices by a 3.5 V/μm applied field.
flexoelectro-optic effect, analogue photonics, liquid crystal, chiral nematic
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