Linear and non-linear optical properties of highly twisted chiral nematic liquid crystals

Willmott, Jonathan Raffe (2003) Linear and non-linear optical properties of highly twisted chiral nematic liquid crystals. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Chiral nematic liquid crystals exhibit a helical, periodic modulation in their physical properties and these may be arranged so that the periodicity is on length scales less than the wavelength of light. From the twisted nature of this periodic structure, interesting new photonic and electro-optic effects may be observed. If an electric field is applied perpendicular to the optic axis of an aligned sample, then the axis may rotate, allowing in-plane modulation of the intensity of light passing through crossed polarizers. We present measurements showing that we may rotate the optic axis by more than 45^o, allowing full contrast switching, in microseconds, for advanced displays. The key to these exceptionally high tilt angles is the use of liquid crystalline bimesogens, which inhibit dielectric coupling, to the applied field, whilst promoting flexoelectric coupling. As a consequence of flexoelectric polarisation, the centrosymmetry of the chiral nematic phase is removed, which allows us to double the frequency of intense optical electric fields, in a non-linear optical process known as second harmonic generation (SHG). We estimate the second order non-linear susceptibility tensor element, χ²₃₃ of some biemsogenic mixtures. We find that the intensity of harmonic radiation depends critically on the angle between optic axis, optical field and applied electric field. We describe our observation of the first ever super cooled blue phase, a self assembling 3D photonic crystal, with temperature stability tens of times greater than in any reported non-super-cooled blue phase. We present a method of generating laser light with a dye doped-mesophase matrix, in the novel photonic effect, of lasing in chiral nematic liquid crystals. We present results for a number of experiments designed to further our understanding of the lasing effect in order that we may design liquid crystal hosts for more efficient lasing. We find that liquid crystal hosts with different birefringence (Δn=n_e-n_o) exhibit dramatically different lasing efficiency. We attribute this to both the refractive properties of the host and also to the ordered conformation between host and dye. Finally we present our ideas for further work.