PhD Thesis Defense | Iker Gómez Viloria

Optical trapping with vortex beams: modeling of experimental settings through multipolar decomposition

Candidate: Iker Gómez Viloria

Supervisor: Gabriel Molina Terriza

Summary

This thesis presents a comprehensive study of on-axis optical trapping of spherical particles with vortex beams, combining experimental, theoretical, and numerical results. To this end, a custom optical tweezers setup was constructed, enabling the generation of vortex beam optical traps with well-defined helicity and the measurement of the trap stiffness constant. On the theoretical side, a novel method for calculating optical forces and torques is introduced, based on the integration of the Maxwell Stress Tensor and the well-defined helicity multipolar expansion of the light fields. This theoretical model yields analytical expressions that provide valuable insight into the role of beam angular momentum, helicity and optical system symmetries in vortex beam trapping. Building on this framework, we developed MOFT – Multipolar Optical Forces Toolbox, an open-source software package for simulating LG optical traps with spherical particles, available on GitHub. Altogether, this thesis provides practical tools and physical insight into the mechanisms of optical trapping with structured light, such as vortex beams, laying the groundwork for future developments in the field.

Sketch of on- and off-axis optical trapping trapping with a focused vortex beam and the optical forces and torques exerted on the trapped particle.