Fig. 1. Schematic of femtosecond laser two-photon polymerization fabrication system

Fig. 2. Generation of the double-focus beam. (a) Flow chart of improved G-S algorithm; (b) simulated double-focus beam; (c) experimental double-focus beam

Fig. 3. Design and SEM results of micro-helices. (a) Model of the double-helix; (b) double-focus holograms dynamically loaded on SLM; (c) right-hand single-helices and double-helices; (d) left-hand octuple-helices; (e) right-hand octuple-helices with ribs

Fig. 4. Voxels under various exposure conditions. (a) Voxel sizes under various exposure powers for 5 ms exposure time; (b) voxel sizes under 0.02 mW and 0.04 mW exposure power for various exposure time; (c) standing quadruple-focus voxel; (d) toppled quadruple-focus voxel

Fig. 5. Optical microscopic images of collapsed microstructures. (a) Detached from the substrate; (b) adhered to the substrate

Fig. 6. SEM images of micro-helices with different characteristics.(a) d_z=0.25 μm, exposure time of voxels is 20‒100 ms; (b) d_z=0.125 μm, exposure time of voxels is 20 ms; (c) d_z=0.5 μm, exposure time of voxels is 60 ms; (d) d_z=0.25 μm, exposure time of voxels is 60 ms; (e) d_z=0.125 μm, exposure time of voxels is 60 ms

Fig. 7. SEM images of micro-helices with supported ribs. (a) d_z=0.5 μm; (b) d_z=0.25 μm; (c) d_z=0.125 μm

Fig. 8. Analysis of the variations of the helical thread width with axial voxel spacing. (a) Schematic of piled-up voxels; (b) measured and calculated results of helical thread width under various axial voxel spacings