Tailored optical vector fields for ultrashort-pulse laser micro-processing

Abstract

In this thesis, a Spatial Light Modulator (SLM) and a nano-structured S-waveplate were integrated with a picosecond laser system and employed to generate complex vector optical fields. Precise tailoring of optical vector beams is demonstrated, shaping their focal electric fields. Using novel optical setups, radial and azimuthal polarizations with and without vortex phase wavefront were created, producing complex laser micro patterning on a polished metal surface. Imprinting Laser Induced Periodic Surface Structures(LIPSS) elucidates the detailed vector fields around the focal region, which shows clearly how the Orbital Angular Momentum(OAM) associated with a helical wavefront induces rotation of vector fields along the optical axis of a focusing lens. In addition, unique, variable logarithmic spiral micro-structures were imprinted on the metal surface. These are the first experimental observations of such micro-structured spirals created by multi-pulse exposure with spiral vector fields which are shown to be due to superpositions of plane wave radial and azimuthal polarizations. A comparative analysis of micro-drilling with radially, azimuthally, circularly and linearly polarized beams with various wavefront was also carried out. The results are compared in terms of quality and efficiency, illustrating how the distinct machining properties associated with each beam make it best suited for specific processes or materials. A radially polarized beam was considered as the most efficient at drilling high-aspect-ratio features. By applying an axicon phase map on the SLM, annular shaped laser beams with adjustable dimension were created and employed in semiconductor thin film selective removal, which enhanced processing speed to a new level. Furthermore, annular beams with radial and azimuthal polarization were generated, imprinting complex concentric ring LIPSS pattern on a polished stainless steel surface. Side white light illumination tests demonstrated distinguishable bright strips on these rings due to diffractive properties of LIPSS, which offers a novel way in variable information encoding technologies. Finally, synchronization of vector field polarizations with micro-positioning at a material surface was demonstrated, leading to potential industrial applications of this work.