Surface Structuring with Polarization-Singular Femtosecond Laser Beams Generated by a q-plate

Abstract

In the last few years femtosecond optical vortex beams with different spatial distributions of the state of polarization (e.g. azimuthal, radial, spiral, etc.) have been used to generate complex, regular surface patterns on different materials. Here we present an experimental investigation on direct femtosecond laser surface structuring based on a larger class of vector beams generated by means of a q-plate with topological charge q = +1/2. In fact, voltage tuning of q-plate optical retardation allows generating a family of ultrashort laser beams with a continuous spatial evolution of polarization and fluence distribution in the focal plane. These beams can be thought of as a controlled coherent superposition of a Gaussian beam with uniform polarization and a vortex beam with a radial or azimuthal state of polarization. The use of this family of ultrashort laser beams in surface structuring leads to a further extension of the achievable surface patterns. The comparison of theoretical predictions of the vector beam characteristics at the focal plane and the generated surface patterns is used to rationalize the dependence of the surface structures on the local state of the laser beam, thus offering an effective way to either design unconventional surface structures or diagnose complex ultrashort laser beams.

Ultrafast Bessel beams: advanced tools for laser materials processing

“…Polarization features can be designed spatially and temporally [22,23]. All together, by engineering in space, time, and vectorial domains, [7,13,24,25], light confinement can harness key features of interaction with matter and build-up unique material transformation paths with highest precision. This is key in designing materials and functions, upscaling approaches for accurate and flexible processing.…”

Q-plate technology: a progress review [Invited]

“…Q-plates have been used quite extensively in the last four years to produce both pure OAM beams and vector-vortex beams to be used for laser ablation of metals and semiconductors. Amoruso and coworkers [88][89][90][91][92][93][94][95] have shown a variety of novel and interesting effects connected to the contemporary presence of structured wavefronts and ultra-short pulses. The central part of the ablated area (which corresponds to the hole of the doughnut, the singular point with vanishing intensity) is not affected by ablation, as one could expect, but only for low values of irradiation and number of shots.…”

Mapping complex polarization states of light on a solid

“…Periodic grating-like structures were induced by the laser within the ablation craters, as well as in the bulk of solids [1,2]. Primarily, patterns were observed to be oriented parallel [15] (perpendicular [16,17]) to the laser polarization on the surface (inside the bulk) of the material, although both orientations have also been reported on the surface [14,18]. However, SoPs of higher-order VV beams and complex polarization patterns produced by coherent superpositions of such vector beams have not been experimentally realized.…”

“…To visualize a polarization pattern, light has to interact with a medium that allows the pattern to be spatially resolved. In recent years, laser structuring of materials has been used to characterize the SoPs of Gaussian (linear) and first-order vortex beams (radial and azimuthal) [13,14]. Periodic grating-like structures were induced by the laser within the ablation craters, as well as in the bulk of solids [1,2].…”