Femtosecond laser induced two-photon polymerization has become an ideal choice for the fabrication of three-dimensional micro-nano functional structures due to its wide range of processing materials, maskless, and high processing resolution. However, since two-photon polymerization voxel is only limited to the vicinity of the laser focus, high-NA objective lenses are commonly used to achieve smaller volume elements in order to achieve higher processing resolution. But at the same time, the smaller voxel means the total number of voxels are greater to fabricate the same structure, making the corresponding processing time longer. Therefore, the high resolution and high processing efficiency of femtosecond laser two-photon polymerization are contradictory, which also leads to the fact that two-photon polymerization is only suitable for processing small-volume and high-resolution 3D structures. It is difficult to apply to batch manufacturing and processing macroscale structure. Therefore, how to achieve high-precision, high-efficiency processing of cross-scale 3D micro-nano structures is the bottleneck restricting its further development.
In order to improve the fabrication efficiency of femtosecond laser induced two-photon processing, researchers have used the coherence and superposition of laser to apply spatial light modulator (SLM) to femtosecond laser processing. By modulating the phase, and polarization of laser, a variety of structured focus can be realized in the target space. The structured focus can be used as a basic fabrication element. Using the structured focus as the basic processing unit solves the contradiction between the processing resolution and processing efficiency of two-photon polymerization: a structured focus can be regarded as the accumulation of many single voxels. Meanwhile these single-voxels maintain the super-diffraction-resolved property. Therefore, the use of structured focus can improve the processing efficiency while maintaining the high-resolution characteristics of femtosecond laser induced two-photon polymerization. By modulating the parameters of vortex beam, researchers have generated ring-shaped focus with adjustable diameter, and then have realized the rapid processing of tubular structures. The annular focus has been widely studied and applied in the processing of bionic blood vessels. But the simple annular focus is only suitable for processing a single microvessel. When processing complex parts such as blood vessel bifurcations, a point-by-point scanning method is still required. How to generate a more complex focal field and realize the rapid processing of the bionic circulatory system is still a difficult problem.
The research group led by associate researcher Chenchu Zhang and Prof. Wu Sizhu from Hefei University researched a novel structured beam generation method. The research results are published in Chinese Optics Letters 2022, Vol. 20, No. 2 (Chenchu Zhang et al. Rapid fabrication of microrings with complex cross section using annular vortex beams).
This work realizes the independent control of the vortex optical energy flow and the annular optical field diameter by superimposing the vortex phase and the annular Fresnel optical phase. The diameter of the annular light field is determined by the annular Fresnel light parameter, while the energy flow of the structured beam is controlled by the vortex phase. Compared with the traditional annular light field generation method, this method has the advantages of no zero-order beam, high diffraction efficiency, and precise linear adjustment of the radius. In addition, by controlling the vortex light parameters and superimposing the non-integer topological charge phase, the ring-shaped focus can be transformed into a gap-ring. The diameter of the gap-ring, the position of the gap, and the size of the gap can be flexibly adjusted by parameters of the light field, as shown in Figure 1.
Fig. 1 (a) The wavefront and intensity distribution of vortex beam and annular Fresnel beam. (b) The superposition of vortex beam with different topological charges and annular Fresnel beam.
This method provides a new solution for patterned light processing. By precisely controlling the size and direction of the gap-ring, the bifurcated structures in blood vessels can be flexibly processed. This method is expected to be highly efficient in the integration of vascular-like structures. There are further applications in high-resolution machining.