Novel laser beams for optical trapping and tweezing.

Abstract

Optical trapping and tweezing has been around for the last 30 years and since found its place in the fields of physics and biology. Over the years this technique has advanced exceedingly and is a unique tool to carry out research in the micrometre and nanometre scale regime. The aim of this dissertation was to illustrate that an optical trapping and tweezing system is an effective tool for the manipulation of micron sized particles and that using such a system allows one the ability to accurately and precisely measure optical forces in the piconewton scale. A custom built single gradient optical trapping system was built to illustrate the manipulation of micron sized particles. Here we will highlight some of the key components of such a system and give an explanation of how these components affect the optical trap. To enhance this system, we exploit the ability to shape light and in particular laser light to generate novel laser beams. This was achieved using a diffractive optical element known as a spatial light modulator (SLM). A spatial light modulator is an electronically addressed optical element which when incorporated into an optical system effectively manipulates the phase of light in order to generate various novel laser beams. In particular these novel laser beams include Laguerre-Gaussian, Bessel and recently proposed Bessel-like beams. Each of these beams contains interesting properties which can be beneficially exploited. Laguerre-Gaussian beams are particularly known as ‘donut’ shaped beams since they have a central dark hole. Increasing the order of these Laguerre-Gaussian beams leads to an increase in the central dark region. These beams are of particular interest since they carry orbital angular momentum. This is not easily observed; however, when incorporated into the optical trapping system, leads to the rotation of trapped particles due to the transfer of photons carrying orbital angular momentum. Bessel and Bessel-like beams on the other hand are classes of beam that possess interesting non-diffracting and self-reconstructive properties upon encountering an obstacle. Here the generation and properties of these novel laser beams will be discussed in detail. Furthermore it is well known that these novel laser beams prove highly useful when incorporated into an optical trapping system hence we will illustrate the effects on a trapped particle when incorporating a Laguerre-Gaussian beam carrying a topological charge of one. It is expected that the trapped particle should rotate due to the transfer of orbital angular momentum. The knowledge gained from beam shaping and the means to trap micron sized particles optically allows one the ability to incorporate this technique in a number of fields, including the promising field of microfluidics. This is an emerging field that deals with investigating fluid properties at the nano and microlitre regime. Optical tweezers integrated into a microfluidic device are beneficial since they are an adequate tool for measuring fluid flow using Stokes’ Law.