Helical Surface\nRelief Formation by Two-Photon Polymerization\nReaction Using a Femtosecond Optical Vortex Beam

Optical vortices possess a helical phase wavefront with\ncentral\nphase dislocation and orbital angular momentum. We demonstrated three-dimensional\nmicrostructure formation using a femtosecond optical vortex beam.\nTwo-photon polymerization of photocurable resin was induced by long-term\nexposure, resulting in the fabrication of cylindrical structures.\nThe ring shape represents the intensity profile of optical vortex\nbeam, and the diameter and height of the structures are related to\nthe laser power. Periodic helical surface relief was observed on the\ninner surface. Significantly, the helical direction of the surface\nrelief is consistent with the direction in which the orbital angular\nmomentum acts and changes depending on the sign of the topological\ncharge. Our proposed method can form three-dimensional microstructures\nwith helical periodic surface relief, and the pitch is smaller than\nthe diffraction limit without laser scanning. This method paves the\nway for further applications in optical devices such as three-dimensional\nchiral metamaterials.