The nature of a particle tracking solution is very much dependent on the context of the experiment. As a consequence, this software will undoubtedly continue to evolve as new research is undertaken. By making it open-source, it is hoped that other researchers around the world will be able to use it as a basis for the development of their own tracking software. By using more advanced pattern-matching algorithms, it should be possible to improve the accuracy of the tracking procedure so that better spatial resolution is possible. Any asso-
ciated increases in computation time will hopefully be offset by improvements in processor performance. As it stands, my software requires the National Instruments IMAQ Vision add-on to be installed. This package can be expensive. To make my software truly free, the dependence on proprietary NI software would have to be removed. This would entail a considerable amount of work, but it is certainly a tractable problem.
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Particle Dynamics and Optical
Sorting in a Bessel Beam
4.1
Introduction
In the last section of Chapter 3, we considered briefly the dynamics of colloidal particles moving under the influence of a Mathieu beam. In this chapter, I investigate the behaviour or particles moving in another class of non-diffracting beam: the Bessel beam. Bessel beams were first proposed by Durnin in 1987 [1] as a propagation-invariant solution to the Helmholtz wave equation. In the transverse plane, zeroth-order Bessel beams are charac- terised by a bright core surrounded by a series of concentric rings, as shown in figure 4.1, a computer-generated representation of a zeroth order Bessel beam.
The bright core of the beam has been of particular interest to experimentalists over the last few years. It is this part of the beam that is often described as being propagation invariant, or non-diffracting, because overzmax, thepropagation distanceof the Bessel beam,
the diameter of the core does not increase appreciably. As an example Durninet al.[2, 3] produced a Bessel beam that had a central spot size of r0 = 60µm and a propagation
distancezmax≈85cm. By comparison, for 633nm light a Gaussian beam with a beam waist
w0= 60µm has a Raleigh range of justzR= 1.7cm. The Raleigh range is the distance over