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Optical vortices, a type of structured beam with helical phase wavefronts and ‘doughnut’-shaped intensity distributions, have been used to fabricate chiral structures in metals and spiral patterns in anisotropic polarization-dependent azobenzene polymers. However, in isotropic polymers, the fabricated microstructures are typically confined to non-chiral cylindrical geometry due to the two-dimensional ‘doughnut’-shaped intensity profile of the optical vortices. Here we develop a powerful strategy to realize chiral microstructures in isotropic material by coaxial interference of a vortex beam and a plane wave, which produces three-dimensional (3D) spiral optical fields. These coaxial interference beams are generated by designing contrivable holograms consisting of an azimuthal phase and an equiphase loaded on a liquid-crystal spatial light modulator. In isotropic polymers, 3D chiral microstructures are achieved under illumination using coaxial interference femtosecond laser beams with their chirality controlled by the topological charge. Our further investigation reveals that the spiral lobes and chirality are caused by interfering patterns and helical phase wavefronts, respectively. This technique is simple, stable and easy to perform, and it offers broad applications in optical tweezers, optical communications and fast metamaterial fabrication. Helical microstructures can be directly polymerized into standard photoresists using beams derived from interfering holograms. Recent studies have shown that optical vortices can pattern polymer surfaces with the same left- or right-handed chirality of the spinning light beam, but only if the material’s structure has a built-in asymmetry. Dong Wu and co-workers from the University of Science and Technology of China report that optical vortices generated by liquid-crystal devices called spatial light modulators (SLMs) are stable enough to engrave chiral microstructures into more-common isotropic polymers. Directing femtosecond laser pulses onto an SLM produced holograms and plane waves that interfered and co-propagated into helices without the phase sensitivity of typical split-beam setups. This approach enabled controllable fabrication of spiral patterns with different lobes and orientations over large areas with a 100-nanometer-scale precision.
Ni et al. (Mon,) studied this question.