Trapped between two beams – higher order laser mode manipulation for cell rotation

Abstract

Laser light is an exceptionally powerful tool which has been utilised across all natural sciences and engineering. The very high intensities of extremely controllable light have allowed for a diverse range of studies to be carried out. When the intensities are large enough, the act of redirecting the light can create a force which can be sufficient to move small transparent objects. In biology one application of this phenomenon forms a tool for trapping and handling microscopic cellular samples in a contactless way using two laser beams. Such a laser-based tool is the Optical Stretcher, it was invented for measuring the mechanical properties of single cellular biological samples. The work presented in this thesis built upon the Optical Stretcher and to gain expertise in the field, several different biological samples were tested using it, gaining insights into the impact of particular proteins to cell mechanics. The Optical Stretcher, along with the vast majority of cell trapping experiments utilises a rotationally symmetric laser beam, which allows the cells to be moved and held in place, but their orientation is random and subject to large fluctuations. Controlled orientation of cellular specimen can lead to improved 3D imaging of the sample and is an important field of study. Previous work has shown that it is possible to orient a cell using a specially shaped laser beam, however the experimental setups were not well suited to use in biological labs. Henceforth, this thesis investigated and engineered a Dual Beam Laser Trapping device called the Higher Order Mode Cell Rotator, in short HOMCR, in order to build a powerful all-in-fibre tool for tomographic cell rotation. The major component giving rise to the HOMCR was a polarisation controlling device that alters the state of light by squeezing on the laser fibre and inducing local changes in the polarisation profile of the laser light. By characterising this device, its capability has been shown for the first time to manipulate the two lobe higher order modes travelling in optical fibres, leading to an all-in-fibre dynamic cell rotator which was used successfully to trap and orient individual cells and larger biological samples.