Desenvolvimento e caracterização de q-plates: geração de feixes helicoidais e vetoriais

Abstract

Q-plates are wave plates capable of transforming the polarization state and wavefront of light. The orientation of the fast axis of a q-plate varies along its cross section enabling the generation of light beams with orbital angular momentum and that have inhomogeneous polarization, the so-called vector beams. The goal of this dissertation is to present the characterization of q-plates, provided by collaborators from the Technical University of Denmark, composed of amorphous silicon nanopillars, periodically distributed, in order to generate the phenomenon of form birefringence. We theoretically show the functioning mechanisms of the q-plates that allow the generation of all vector beams present on the higher order Poincaré sphere, a geometric representation that encompasses the spatial profiles and the polarization state of these types of beams. To characterize the produced plates, we conducted experiments that could show vector beam generation and Laguerre-Gaussian modes known for its orbital angular momentum. For vector beams, we have analyzed the intensity profile of the beam after crossing a polarizer. It was possible to notice that the profile changes, showing the inhomogeneous character of the polarization state. In the case of beams with orbital angular momentum, their wavefronts have a helical shape, so we used a Mach-Zhender interferometer to analyze them. At the output of the interferometer, we obtained an interference pattern in the shape of petals, characteristic of helical beams. Hence, we conclude that the manufactured q-plates can generate the theoretically predicted transformations.