Light–matter interaction in liquid crystal cells

In this thesis we study the interactions between light and matter in photorefractive liquid crystal<br/>cells. To model the liquid crystal alignment we develop a fast and accurate approximation of<br/>the normally stiff equations which minimise the Landau-deGennes free energy of a nematic<br/>liquid crystal. The resulting equations are suitable for all configurations in which defects are<br/>not present, making them ideal for device simulation. Specifically, they offer an increase in<br/>computational efficiency by a factor of 100 while maintaining an error of order (10?4) when<br/>compared to the full stiff equations. As this approximation is based on aQ–tensor formalism, the<br/>sign reversal symmetry of the liquid crystal is respected. We consider both the simplified case,<br/>where the director is restricted to a plane, and the full three-dimensional case. An approximation<br/>of the error is also given. We use the liquid crystal model to understand two different optical<br/>effects. The first of these is optical coupling. This effect is observed in liquid crystals in both the<br/>Bragg and Raman–Nath regimes. To account for this behaviour we develop an extension to the<br/>coupled wave theory which is suitable for all regimes of coupling. The model assumes that the<br/>refractive index grating, generated by the liquid crystal, has an arbitrary profile in one direction<br/>and is periodic (but not necessarily sinusoidal) in the other. Higher order diffracted terms are<br/>considered and appropriate mismatch terms dealt with. It is shown that this model is analytically<br/>equivalent to both the Bragg and Raman–Nath regime coupling models under an appropriate set<br/>of assumptions. This model is also verified through comparison to finite element simulations of<br/>Maxwell’s equations. The second effect we model is the coupling of surface plasmon polaritons<br/>at the interface between a metal layer and a photorefractive liquid crystal cell. We implement<br/>existing numerical models to gain a thorough understanding of the system. These models are<br/>qualitatively compared with experimental observations. Analytic approximations to describe<br/>the coupling of surface plasmon polaritons at the surface of the liquid crystal cell are developed.<br/>These expressions provide a great deal of insight into the coupling mechanisms and will be of<br/>fundamental importance in optimising these systems.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2011

Author dc:creator

• Daly, Keith Richard

Advisor dc:contributor.advisor

• D'Alessandro, Giampaolo