Rapid fabrication of microrings with complex cross section using annular vortex beams

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.

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.

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.

封面|利用结构化光场提升双光子聚合加工效率

图1 （a）涡旋光与环形菲涅尔光的波前与焦场能量分布信息。（b）不同拓扑荷的涡旋光与环形菲涅尔光的叠加。

飞秒激光双光子聚合加工技术具有应用材料广泛、无需掩膜、加工分辨率高的优点，成为了三维微纳功能结构制造的理想选择。但飞秒激光双光子聚合的区域仅局限在激光焦点附近，因此研究者们往往会使用高数值孔径（NA）的物镜以实现更小的体积元，从而提高加工分辨率。但体积元越小，相同结构所需要的体积元数量就越多，相应的加工耗时也就越长。因此，飞秒激光双光子聚合加工的高分辨率和高效率之间相互制约，这也使得双光子聚合往往仅适用于加工小体积高分辨的三维结构，难以应用于批量化制造和加工宏观尺度结构。因此，实现高精度、跨尺度三维微纳结构的高效加工有望突破其发展瓶颈。

为了提高飞秒激光微纳加工效率，研究者们利用激光的相干性和叠加性，将空间光调制器(SLM)运用到飞秒激光加工中。通过调制激光的相位、偏振等参数，在目标空间光场实现多种结构化焦场，并将其作为基本加工单元。将结构化焦场作为基本加工单元很好的解决了双光子聚合加工分辨率与效率之间的制约。一个结构化焦场可以被视为很多单点体积元的累加，同时又保持了超衍射分辨的特性。因此，采用结构化焦场可以在提高加工效率的同时也保持飞秒激光微纳加工高分辨的特性。

此外，研究者们通过对涡旋光参数进行调制，生成直径可调节的环状光场，继而实现管状结构的快速加工。目前，环状光场已经得到广泛研究，并应用于仿生血管加工中，但是简单的环形光场结构仅适用于加工单根微血管，当加工血管分叉等复杂部位时，依然需要采用逐点扫描的方式。如何生成更加复杂的焦场，实现仿生循环系统的快速加工依然是一个难点。

合肥工业大学张晨初副研究员与吴思竹教授课题组合作在Chinese Optics Letters 第20卷第2期上（Chenchu Zhang et al. Rapid fabrication of microrings with complex cross section using annular vortex beams）发表了基于涡旋光生成复杂结构光场的方法，并被选为当期封面。

该工作通过叠加涡旋相位和环形菲涅尔光相位，实现了涡旋光能流密度与环形光场直径的独立控制。环形光场的直径由环形菲涅尔光参数决定，同时结构光场的能流密度由涡旋相位控制。与传统的环形光场生成方法相比，该方法具有无零级光、高衍射效率、半径可精确线性调控的优点。此外，通过控制涡旋光参数，叠加非整数拓扑荷相位，环形光场可以变换为缺口环形光场，并且光场的直径、缺口位置以及缺口大小可以通过光场相位参数灵活调控，如图1所示。

该方法为图形化光场加工提供了一种新的解决方案，通过精确控制光场缺口的大小和方向，可以实现灵活加工血管中的分叉结构，该方法有望应用于仿血管结构一体化高效高分辨加工。