Optical trapping and liquid crystals

A detailed study of optical trapping and its application to liquid crystalline materials is presented. Colloids have been optically trapped within a thermotropic liquid crystal and this has led to the development of a new technique that is successful in determining the effective viscosities experienced by a colloid moving through a nematic liquid crystalline medium. In addition, the optical trapping of both nematic and, for the first time, chiral nematic droplets has been thoroughly studied through experiment. To begin, a study of the optical trapping of isotropic particles (polystyrene) in an isotropic medium (water) through theoretical modelling and experimental measurements is presented. This enables the behaviour of optical trapping forces to be understood for a wide range of parameters including particle size and birefringence. It is shown that force measurements carried out using the viscous drag technique are unreliable when the viscous drag force is applied over a short time period. Modelling of the forces exerted on a particle indicates that this phenomenon is caused by the optical potential such that the behaviour of a particle within a trap is dependent on the ratio between the trap stiffness and the applied viscous drag force. For 6 mum diameter particles, it is shown that reliable force measurements can be carried out when a viscous drag force of 8.5 pN is applied for more than 0.5 s. In addition, it is demonstrated that a second-order force function provides a close approximation of the transverse optical trapping force as a function of the particle displacement. It is reported that a colloid can be optically trapped within a thermotropic liquid crystalline medium. Viscous drag measurements allow the effective viscosities experienced by a colloid moving parallel and perpendicular to the director of the nematic material MLC 6648 to be deduced. They are nef[11] = 4.7 + 0.2 cP and = 23.7 +/- 2.0 cP, respectively. These values compare favourably with the known flow viscosity of MLC 6648, eta = 19 cP. A detailed study of the possible mechanisms responsible for the transfer of angular momentum to optically trapped nematic droplets is presented. The relative importance of wave plate behaviour, anisotropic light scattering, absorption and the light-induced Freedricksz transition is evaluated over a large range of droplet diameters (1 mum to 20 mum), material birefringence (0.15 to 0.26) and beam powers (50 mW to 400 mW). Measurements indicate that wave plate behaviour is responsible for at least 90 % of the total spin angular momentum transferred to a nematic droplet. In addition, it is suggested that the electric field vector of the trapping beam maintains the effective director of the droplet in parallel alignment. The optical trapping of chiral nematic droplets is presented for the first time. It is shown that the axial trapping force is severely reduced when the wavelength of the trapping beam corresponds to the region of selective reflection. The transfer of spin angular momentum from a linearly polarised beam to chiral nematic droplets is reported for the first time.