Creation and analysis of structured light fields for application in optical tweezers

Abstract

The work presented in this thesis focuses on the creation of customised structured light beams, their analysis, characterisation and application in optical trapping and tweezers. In the rst Chapter, we start by presenting an over-view of the thesis as well as a review of literature on laser beam shaping and its applications in optical tweezers. A theoretical description and de nitions of customised laser modes utilised in this thesis such as LG and HG beams are presented in Chapter 2. Concepts on digital laser beam shaping techniques to realise customised structured light elds are described in Chapter 3. Two methods of generating customized laser modes namely complex amplitude modulation and phase only modulation are considered. Based on the ability of the SLM to create multiple beams simultaneously, the multiplexing concept is also discussed. Following the same line, a new approach to obtain multiple vector beams on a single hologram is presented. In addition, an approach to create shape invariant vector at-top beams is discussed. Since we are also interested in the application of these custom light elds in optical tweezers, we discuss the fundamentals of optical trapping in Chapter 4. Experimental realisation of LG and HG beams is then presented in Chapter 5. With the use of the beam shaping methods in Chapter 3, we demonstrate light beam shaping and multiplexing. In addition, a quantitative analysis to determine the multiplexing properties of SLMs as well as an investigation on the maximum number of beams that can be multiplexed is presented. A novel experimental method that enables the simultaneous generation of many vector beams using a single digital hologram is described in Chapter 6. This method is interferometric in nature and relies on the multiplexing concept. We demonstrate the simultaneous generation of multiple vector vortex beams each with various polarization distributions. The exibility of our approach is further con rmed through the creation of multiple vector Bessel beams. Finally in Chapter 7, we present the application of our structured light elds in an optical trapping and tweezers system. The vector at-top beam is rstly considered where a new holistic classical and quantum toolkit to analyse this beam during propagation is presented. The experimental realisation of such beams which exploits the polarisation dependent e ciency of spatial light modulators is described. We then demonstrate the versatility of our vector at-top beam in an optical trapping and tweezing application. Following the experimental generation of multiple vector beams in Chapter 6, a novel vector holographic optical trap with arrays of digitally controlled Higher-Order Poincar e Sphere (HOPS) beams is also presented. We employ a simple set-up using a spatial light modulator and show that each beam in the array can be manipulated independently and set to an arbitrary HOPS state, including replicating traditional scalar beam HOTs. We demonstrate trapping and tweezing with customized arrays of HOPS beams comprising scalar orbital angular momentum and cylindrical vector beams, including radially and azimuthally polarized beams simultaneously in the same trap. Our approach is general enough to be easily extended to arbitrary vector beams.