Principle and Method of Ultrafast Laser Beam Shaping and Its Application in Functional Microstructure Fabrication

Significance As manufacturing quality requirements for functional microstructures increase, ultrafast laser micro/ nanomanufacturing has brought new challenges related to high processing efficiency, cross-scale processing, and selective or controllable processing, etc. The limitations of the spatial and temporal energy distribution of traditional ultrafast laser Gaussian beams and processing methods based on single-point focus scanning make it difficult to meet the latest manufacturing accuracy, efficiency, and cross-scale processing requirements. Therefore, researchers focus their attention on manufacturing methods based on ultrafast laser beam shaping. Progress Laser beam shaping can be divided into two types: spatial beam shaping and temporal beam shaping. Spatial beam shaping refers to tailoring the distribution of laser energy in the space domain, whereas, temporal beam shaping refers to changing the distribution of laser energy in the time domain. Compared with a traditional Gaussian beam, a shaped beam has new spatial and temporal energy distribution, which can meet the manufacturing requirements of specific structures or applications. By shaping the spatial profile of an ultrafast laser beam, the fabrication of microstructures with various shapes can be directly realized on exposure to single or multiple laser pulses. Common laser shaping methods include the spatial light modulator method (Figs. 1-3), lens array method (Fig. 4), and beam superposition method. Based on spatial beam shaping, the processing methods such as ultrafast laser direct writing, induction, and deposition can be used for the one-step fabrication of special spatial profile microstructures ( Figs. 5 and 6), high aspect ratio microstructures, and optimized processing of microchannels, microstructure arrays (Fig. 7), and laser-induced or deposited microstructures. By spatial beam shaping, the application range of an ultrafast laser in the manufacturing of functional microstructures can be expanded, the efficiency and precision of which can be improved. Temporal beam shaping transforms a conventional ultrafast pulse into a pulse sequence (Figs. 8 and 9). Each pulse sequence contains several subpulses with a time interval from a femtosecond to a picosecond range. The energy ratio between each subpulse can be derived. Temporal beam shaping can control electronic dynamics during lasermaterial interactions, which has a wide range of applications in the manufacturing of microchannels (Fig. 10), laserinduced periodic surface structures, nanoparticles (Fig. 11), nanostructures (Fig. 12), and thin films. To further improve the quality and efficiency of ultrafast laser processing, it is necessary to perform the coordinated shaping of ultrafast lasers in the time and space domains. On the one hand, spatial and temporal beam shaping can be performed separately in one optical path by combining double pulses and a Bessel beam (Fig. 13). On the other hand, it is possible to tailor an ultrafast laser in the spatiotemporal domain for coupling shaping by the simultaneous spatial and temporal focusing technology (Fig. 14). Cooperative shaping can considerably improve laser energy deposition efficiency and the three-dimensional symmetry of the intensity distribution of a laser beam focus (Fig. 15). Conclusion and Prospect The ultrafast laser beam shaping technology has the potential to greatly improve the variety, precision, and efficiency of functional microstructure manufacturing. A combination of the ultrafast laser beam shaping technology and microfabrication promotes the efficient and controllable manufacturing of large-area, high-quality functional microstructures, which accelerates the development of commercial scale-forming devices based on the microstructures. However, there are still some challenges with the ultrafast laser beam shaping technology. For example, the laser damage resistance of a shaping device weakens its processing ability, error of the complex shaping system affects its processing accuracy, and interaction mechanism between the shaped ultrafast laser beam and material to be processed is not fully known. These problems and challenges need to be overcome in the future. Facing the need for the miniaturization, integrated design, and large-scale manufacturing of functional microdevices, the ultrafast laser beam shaping manufacturing technology can be highly suitable for high-resolution, cross-scale, three-dimensional, and high-efficiency processing.