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Title: Liquid crystal lasers
Author: Ford, A. D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This thesis examines the emission properties of liquid crystal (LC) lasers. The aim is to investigate correlations between the emission properties of the laser, in particular the threshold energy and the slope efficiency, and the macroscopic physical properties of the liquid crystal host. Using the threshold gain term obtained for a distributed feedback laser in the context of the coupled mode theory, an expression for the threshold energy (Eth) is obtained, in the form Eth d + 1/Δn²d² where d is the cell thickness and Δn is the birefringence. The slope efficiency is considered to be inversely proportional to the threshold energy and thus the laser emission properties are evaluated in the context of the host physical parameters. These relationships provide fits that are in good agreement with experimental data for the threshold energy and slope efficiency dependence on cell thickness. It is shown theoretically that a threshold-less laser can be achieved for large cell thicknesses if the absorption losses are neglected. For a given cell thickness, the emission properties from a range of monomesogens, nematogen mixtures and bimesogens provide evidence that LCs with high birefringence give rise to a low threshold energy. This is in accordance with the above expression. However, examining the emission properties of a high birefringence LC laser, suggest that a high birefringence does not necessarily give rise to high slope efficiency. The slope efficiency is shown to follow the relation, ηs = P₃/Eth where P3 depends on parameters such as saturation intensity and addition loss mechanisms. One possible loss mechanism highlighted in this thesis is associated to the elastic moduli of the host LC. This parameter provides an indication of the structural integrity of the helical structure of the LC host. The bimesogen with the largest elastic moduli gives rise to a slope efficiency of 20%. In addition to the chiral nematic phase providing the host structure for the band edge laser, the emission properties from three addition LC lasers, using alternative LC phase is also shown; the one dimensional chiral smectic C phase, the three dimensional blue phase I and the random laser that utilises the high scattering texture of the smectic A phase.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available