Nowadays, the well-known cylindrical vector beams (CVBs) – the axially symmetric beam solution to the full-vector electromagnetic wave equation – are widely used for advanced laser material processing, optical manipulation and communication and have a great interest for data storage. Higher-order CVBs with polarisation order greater than one and superpositions of CVBs of various orders (hybrid CVBs) are especially of interest because of their great potential in contemporary optics. We performed a theoretical analysis of the transformation of first-order CVBs (radially and azimuthally polarised beams) into hybrid higher-order ones using phase elements with complex transmission functions in the form of the cosine or sine functions of the azimuthal angle. Binary multi-sector phase plates approximating such transmission functions were fabricated and experimentally investigated. The influence of the number of sectors and a height difference between neighbouring sectors, as well as the energy contribution of the different components in the generated hybrid higher-order CVBs were discussed in the context of polarisation transformation and vector optical field transformation in the focal region. The possibility of polarisation transformation, even in the case of weak focusing, is also demonstrated. The simple structure of the profile of such plates, their high diffraction efficiency and high damage threshold, as well as the easy-to-implement polarisation transformation principle provide advanced opportunities for high-efficient, quickly-switchable dynamic control of the generation of structured laser beams.
Laser irradiation of various materials including metals, polymers, and semiconductors with vortex beams was previously shown to "twist" transiently molten matter providing the direct easy-to-implement way to obtain chiral surface relief. Specifically for metals, this effect was attributed to transfer of an orbital angular momentum (OAM) carried by a vortex beam. In this Letter, we report the formation of twisted metallic nanoneedles on surfaces of silver and gold films under their irradiation by a zero-OAM laser beam with a spiral-shaped intensity distribution. Our comparative experiments clearly demonstrate, for the first time to the best of our knowledge, that the formation of the chiral nanoneedles on the noble-metal films is mainly governed by the temperature-gradient-induced chiral thermocapillary mass transfer, rather than by OAM-driven rotation of the molten matter.
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