Micro-helices are applied in microrobots and chiral metamaterials and require various features for structure fabrication in diverse applications. The femtosecond laser direct writing (FsLDW) technology can fabricate three dimensional (3D) microstructures based on two-photon polymerization (2PP) with sub-diffraction-limited resolution. This technology is used to fabricate micro-helix structures using the widely used point-by-point writing scheme with a single focus. However, this is relatively inefficient because of the repetitive scans along many helical trajectories in the fabrication process. Recently, one-step exposure with structured light has allowed the rapid fabrication of micro-helices, wherein helical beams are specially designed with vortex phases. However, state-of-the-art schemes can only produce microstructures with limited patterns owing to the complex and professional light manipulation techniques. To efficiently fabricate micro-helix structures with flexible features such as diameter, thread number, pitch, and chirality, we propose a scheme based on dynamic multi-focus patterns to fabricate multiple helical microstructures.

Micro-helix structures were fabricated by piling up the multi-focal voxels along helical trajectories. The helical motion of the voxels was divided into two components: a circular motion manipulated by dynamic holograms loaded on the spatial light modulator (SLM), and a linear motion controlled by a z-axis translation stage (Fig. 3). Based on our in-house fabrication system, we adapted the Gerchberg-Saxton (G-S) algorithm to compute dynamic multi-focus holograms on the SLM iteratively. Subsequently, the tightly focused femtosecond multi-focal beam patterns induced polymerization of the photoresist (SU-8). In this method, the diameter and thread number of the micro-helices were determined by the parameters of the hologram alone under fixed exposure conditions. The dynamic holograms with the z-axis translation stage allowed flexible control of the pitch and chirality.

The improved G-S algorithm adapted in our setup generates a well-defined multi-focus in the experiments, showing good consistency with the simulation (Fig. 2). These multi-focused beam patterns allow the flexible fabrication of various micro-helices by piling up the multi-focus voxels in a single helical motion (Fig. 3). Using a four-focus beam as an example, the geometric features of the corresponding voxels in 2PP are experimentally characterized in terms of their diameters and axial sizes (Fig. 4). In this experiment, the threshold power is determined as 0.01 mW for every single focus under 5 ms exposure time. Micro-helices are fabricated under the aforementioned exposure conditions. However, insufficient adhesion leads to the detachment of these microstructures with high aspect ratios from the silicon substrates (Fig. 5). To address this issue, another polymerized thin film is deposited on the substrate, which is cured using a UV lamp before proceeding with 2PP. This process significantly enhances the adhesion between the microstructures and substrate. Additionally, the capillary forces occurring during the developing step of post-processing distort the microstructures by pulling down the helical threads (Fig. 5). This issue is resolved by drying the micro-helices in supercritical carbon dioxide. Finally, five sets of dynamic multi-focus holograms are used to fabricate the micro-helices with different features (Fig. 6). The spiral diameter of these microhelices ranges from 2.4 to 19.2 μm as the number of threads increases from one to eight, with adjustable chirality and pitch. Introducing supporting micro-ribs also significantly enhances the stiffness of the micro-helices, which enables the height of the micro-helices to be up to 30 μm (Fig. 7). A possible reason for the different thread widths under different pitches in the voxel-stacking model is analyzed (Fig. 8).

In this paper, we propose a dynamic hologram scheme to fabricate micro-helices using 2PP. In our scheme, the helical motion of the focus in conventional direct laser writing can be divided into linear and circular motions. Linear axial motion is controlled by a mechanical translation stage, and circular motion is manipulated by a set of dynamically programmable holograms. We also prove that the improved G-S algorithm is effective in generating multi-focus femtosecond laser patterns with flexibly controlled parameters to achieve circular motion. Therefore, well-defined dynamic multi-focus patterns in combination with axial mechanical motion can allow the fabrication of micro-helices with flexible control over diameters, thread numbers, pitches, and chiralities. Compared with the FsLDW method based on the single focus, the multi-focus parallel writing scheme can increase the fabrication efficiency by N times for micro-helices with N threads. Additionally, this hologram-based scheme offers advantages in terms of system cost, as it eliminates the need for an expensive motion controller for sophisticated spiral mechanical movements. The proposed flexible and economical method of micro-helix fabrication holds great potential for various applications, such as microrobots, chiral metamaterials, and bioengineering.