Particle tracking in 3D

Abstract

The evanescent field of an optical waveguide can be used to trap and propel microscopic particles. A 1µm particle is trapped on a strip waveguide and propelled forward until it reaches a 10µm gap at the centre of a loop, created by splitting the waveguide into two branches. According to simulations, the particle will be elevated 3µm in the gap and be stably trapped in the middle of the gap. The goal of this thesis is to implement an algorithm that can track a spherical particle in three dimensions and verify the simulated results. Information about all three coordinates is found by using off-focus images, in which diffraction rings will appear around the particle. The x and y coordinates are determined by calculating the centre of the particle in each image. A linear relationship between the radius of the diffraction ring and the displacement along the z-axis (vertical) is exploited to determine the z coordinate. Prior to tracking, it is necessary to make a calibration series where the radius is found for known displacements along the z-axis. First, the algorithm was applied on a video of a trapped particle made with darkfield microscopy. That did not work, because of too much noise from the waveguides near the gap. Instead, fluorescence microscopy was used. Unfortunately, trapping within the gap was not achieved with the latter technique, but a video of a particle moving along the waveguide was analysed. The precision of ±0.2µm with fluorescence will be sufficient to verify simulations where the particle is elevated 3µm.