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Title: Variable focal length liquid crystal immersed microlenses
Author: Commander, Lawrence
ISNI:       0000 0001 3561 0624
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1998
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This thesis describes research to produce variable focal length microlenses by immersing refractive photoresist microlenses in a layer of nematic liquid crystal. The nematic liquid crystal has an electrically controllable refractive index in one polarisation, and therefore, the focal length of the lensing interface formed between the refractive microlenses and the liquid crystal can be voltage controlled. Various designs of microlens cells have been fabricated, the first design had a focal length of f = -910μm at 0V, ∞ at ~1.5V and 560μm at 10V for a lens diameter of 100 μm and f = -1660μm at 0V, ∞ at ~1.5V and 1100μm at 12V for a lens diameter of 150μm. This original design microlens had a usable tuning range limited to 4V and above (plus 0V) due to large aberrations caused by the liquid crystal structures. The lenses' phase aberrations have been studied in a Mach-Zehnder interferometer. The first design (150μm diameter) had aberrations ranging from an RMS value of 0.18A to 0.55A at 632.8nm, peaking at around 2V. The peak in aberration of the original design was partly due to voltage being dropped across the dielectric of the photoresist. Microlenses, with an electrode on top of the photoresist, were fabricated to improve on the aberration performance of the original design. This "electrode on top" design of microlens had a focal length of = -1600μm at 0V, ∞ at ~1.5V and 1450μm at 8V for a lens diameter of 150?m. The electrode on top microlenses did not have the large aberrations of the original microlenses and, hence, their usable focal length range was not limited in the same way. The electrode on top design had an RMS aberration ranging from 0.071λ to 0.19λ without the peak of aberrations of the original design. A 1-D Deuling model for the liquid crystal structure has been used to analyse the phase aberrations of the lenses. Deuling's analysis has been extended to deal with asymmetric pretilt at the surfaces and a computer program has been used to model columns of liquid crystal across the microlens. The resulting phase profiles are qualitatively the same as the aberrations measured for the two spherical lens designs. The liquid crystal structure has been investigated by inspection in a polarising microscope, modelling of the liquid crystal and fabrication of simpler cylindrical microlenses. Using the microscope, it was seen that liquid crystal walls, disclinations and twist occur on the original design of microlenses. The walls' structure has been described as a combination of bend and twist of the liquid crystal directors. A structure for the liquid crystal after the walls have disappeared has been suggested with a twist of the liquid crystal being due to the fields inclined to the normal in the cell. The occurrence of wall structures has been reproduced in a 2-D finite element model of the liquid crystal which was also used to predict a design with reduced walls. This design, with a strip electrode opposite to the microlens, was fabricated as cylindrical microlenses and was found to agree with the model's predictions. However, additional walls occurred for conditions outside those which could be modelled. The final design of microlens with the electrode on top of the photoresist produced no walls in the liquid crystal and a reduced degree of twist, as well as the reduced aberrations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Optics & masers & lasers