Polarization-Resolved Particle Holography

Sammanfattning: In this thesis, measurement of particle fields using digital holography is the main subject. The questions investigated consider positioning, identification and sizing of nanometer and micrometer particles. The thesis explore these topics using both simulations and experimental measurements. Measuring particles is inherently a three-dimensional problem. Digital holography is, therefore, chosen as the measurement technique since it can record three-dimensional information in the interference pattern.Two main digital holographic setups are considered in this work, one single-view and one dual-view, both with off-axis configuration for the reference wave. Methods for positioning along the optical axis is the central question for the single-view system. This work presents a new method for axial positioning based on the wavefront curvature of the scattered light. In the reconstructed volume, along the optical axis, the scattered wave changes from converging to diverging around its location. This assumption is verified using simulations. An estimation of the position where this change occurs, hence, is an estimation of the actual axial position. Two different methods for quantification of the wavefront curvature is presented. The first uses the finite difference method of the reconstructed phase. The second uses a Chebyshev model for the phase-response. The difference between the two methods is that the one based on the Chebyshev model is more robust and less sensitive to noise.The dual-view system is an extension of the single-view setup where an identical system is placed perpendicular with the first system. The sample is illuminated from below, making the angle between the illumination and the two systems $90^\circ$. The concept of polarization-resolved registration is also incorporated in the detection. This detection is made possible by using two reference waves with linear and mutually orthogonal polarization at different off-axis tilts. One hologram can, therefore, be reconstructed into two complex amplitudes, one for each polarization component. The measurements using this system focus on how particle properties influence the polarization-response. The transition from single to dual-view polarization-resolved detection increases the complexity in the reconstruction. There is a need for accurate calibrations for this type of setup. The thesis contains details on calibrations of both the spatial mapping and polarization detection.The first application of the dual-view polarization-resolved setup is for identification and size estimation of nanometer-sized particles. The T-matrix method is used to establish a model for the sizing of spherical and spheroid particles. It is possible to estimate the size unambiguously up to approximately 200 nm for smooth particles. The sizing is limited to this lower size range.The second application investigates how the detected polarization varies for different kinds of microplastics. The measurements show that the microplastics have a complex polarization-response, indicating an irregular and non-spherical shape. In general, for both sizing and identification, the problem becomes more complex as the size increases and the particle shape is less smooth.This thesis shows that it is possible to estimate particle information from the polarization-response. However, the method is constricted both by the size and complexity of the particles.