Novel optical fibres for on-chip optical tweezing and bio-applications

Abstract

Optical trapping of a single cell is a technique widely used in many scientific sectors due to the benefits of isolating and examining a single cell in isolation. Studies that use a conventional microscope-based tweezing system demonstrate optical trapping based on strongly focused laser light delivered through a high Numerical Aperture objective. However, this approach poses restrictions to the range of the applications of the tweezing system due to the use of the imaging optics for tweezing beam delivery. To overcome these restrictions, optical fibre-based systems to optically trap a single cell have been studied. In this thesis, an optical tweezing system based on machined four core optical fibres has been developed and applied to a variety of cells. Mirrors, with an angle slightly higher than the critical angle for the fibre to medium interface, have been fabricated on the end of a four-core fibre, to alter the propagation of the laser beam exiting the four cores. The four beams are directed to overlap, and the optical fibre trap acts in a manner similar to a conventional optical tweezer. The multicore fibre (MCF) trap is composed of four diverging beams which overlap to form a trapping volume, as opposed to the trapping volume of an optical tweezer which is formed by tightly focusing a single beam via a high numerical aperture objective lens. It is shown that micron-scale particles can be optically trapped in this overlap region of the MCF trap. Optical trapping of yeast cells, and also a wider range of cells such as red blood cells, U87 cells and mouse embryonic stem cells is reported in this thesis. The optical trapping system has been used also below a Raman microscope. This demonstrates the ability to trap cells under an analytical microscope without modifying the microscope optics, and to capture the Raman spectra from single trapped cells. The work presented in the thesis demonstrates a flexible system of small dimension, to trap cells for use under a wide range of microscopes, circumventing the need to focus a trapping beam through a high numerical aperture objective lens. The trap has been characterised by measuring the maximum trapping velocities and trap strengths, which are comparable to conventional optical tweezers. The trapping system has also been used to investigate alternative optical manipulation of 'special' particles, such as hollow glass spheres, that cannot be optically trapped with a conventional optical tweezers system due to their low refractive index. The machined multicore fibre uses the region between the beam overlap area and the fibre end face, to hold these low refractive index particles in place. The primary objectives of the work described in this thesis were to optimise the FIB fabrication of the mirrors on the MCF trap, build a robust, portable system capable of cell trapping and manipulation under different analytical microscopes, characterise the beam propagation characteristics, demonstrate stable trapping of some exemplar cell types, and compare the trap strength with conventional optical tweezers.