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Title: Programmable diffractive optics for laser scanning confocal microscopy
Author: Boruah, Bosanta Ranjan
ISNI:       0000 0004 2671 6859
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2007
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Coherent light beams find applications in several exciting research areas where it is important to have absolute control over the phase and amplitude properties of the light beams. To name a few, in applications such as microscopy, optical trapping, interferometry etc. it is imperative to have dynamic control over the properties of the beam. One way of achieving this is with the help of a computer controlled diffractive optical element such as a liquid crystal spatial light modulator (LCSLM). Based on the principle of holography, such a technique can provide dynamic control over amplitude, phase and polarisation state across a coherent beam. This work describes about a ferroelectric liquid crystal spatial light modulator (FLC-SLM) system acting as a programmable diffractive optical element. The efficiency of the system to detect and correct aberrations in laser beams were demonstrated. The ultra sensitivity of singular light beams to the presence of azimuthal aberrations were observed and a novel aberration sensor based on these findings were implemented. The synchronisation of the display holograms on the FLCSLM panel with the acquisition of a CCD camera is demonstrated in a programmable reference beam phase stepping interferometry experiment. Laser scanning confocal microscopy is a widely used technique for the electronic visualization of optically sectioned microscopic structures. The information available from such a system is limited by the intensity and polarisation properties of the point spread function (PSF) at the sample plane, which are in turn dependent on the optical system and the sample being imaged. Aberrations due to incorrect optics or the sample itself may significantly degrade the PSF there by reducing the resolution and the signal available for image formation. Also the information obtained from a laser scanning confocal system working in the epifluorescence mode will also depend on the polarisation properties of the PSF, as molecular absorption and emission cross-sections are polarisation dependant. These polarisation properties can be particularly important for high resolution microscopy such as STED and molecular alignment studies when high numerical aperture lenses are used that can result in complex polarisation structure within the PSF. The thesis develops the theory necessary to understand the vectorial behaviour of the 3D PSF for an arbitrarily polarised beam. The design and implementation of a beam scanning optical microscope system, that uses an FLCSLM to control the optical field in the illumination pupil of the microscope objective, are described. The performance of the system in engineering and reconfiguring the 3D PSF is demonstrated. Use of programmable diffractive optics to generate different depletion beams for STED microscopy is also demonstrated.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available