Significance The widespread application of micro/nano processing technology has sparked interest in femtosecond laser processing, particularly for the fabrication of small- sized, high- precision, and three-dimensional structures. One of its key advantages lies in its ability to achieve ultra- high resolution beyond traditional diffraction limits, owing to nonlinear absorption effects such as two- photon absorption and threshold effects of focal excitation. Moreover, the integration of technologies like light field modulation greatly improves processing efficiency. Unlike traditional serial processing, which involves point- by- point scanning to create predetermined structures, light field modulation can transform a single focus into a multi- focus array, surface light field, or volume light field. This enables the rapid exposure of specific structures and provides flexible solutions for processing complex and time-consuming structures. Additionally, spatial modulation and spatiotemporal focusing technology can further improve axial processing resolution. Due to the significant advantages of femtosecond laser processing, it has found widespread applications in various fields such as micromachines, micro- optical devices, chiral research, and biomedicine. 1) Various micro- robots with different driving mechanisms can be fabricated using femtosecond laser processing, including ultrasonically driven robots, chemically driven robots, optically driven robots, electrically driven robots, and magnetically driven robots. These diverse driving mechanisms ensure that micro- robots can move precisely in different situations, enabling functions such as drug delivery and environmental monitoring. 2) Micro- optical devices have demonstrated great potential in fields such as imaging, sensing, communication, and signal processing due to their lightweight nature and high design flexibility. Utilizing techniques like two- photon polymerization, scientists can fabricate customized micro- optical devices with specific functions, such as achromatic lenses and gradient refractive index lenses. These micro- optical devices can also be integrated with fibers, photonic chips, CMOS cameras, and other components to form complete micro- optical systems. 3) Chiral structures created through femtosecond laser direct writing exhibit higher chiral optical responses compared to natural chiral materials, significantly advancing the field of chiral photonics. To expand the scope of chiral research, planar chiral structures, stacked planar structures, and three- dimensional spiral structures have been fabricated using femtosecond laser processing, providing a solid foundation for studying light responses in the orbital angular momentum dimension. 4) In the biomedical field, femtosecond lasers can be employed to fabricate cell culture scaffolds or construct three-dimensional capillary networks, thus promoting advancements in cytology and tissue engineering. Progress Researchers have conducted a series of studies to enhance the processing speed of femtosecond laser two- photon polymerization. The traditional point- by- point scanning method is a slow and inefficient serial processing approach. To achieve rapid exposure of specific structures, researchers have explored techniques such as adding diffractive optical elements or utilizing methods like light field modulation to convert a single focus into a multi- focal array, surface light field, or volumetric light field. Figure 2 shows schematic diagrams of different femtosecond laser processing systems, including systems with diffractive optical elements or spatial light modulators. However, the spatial light modulation method also has a significant drawback, which is the lack of axial energy distribution control. This results in the laser energy density at the defocus surface typically reaching the threshold of two- photon polymerization, making it challenging to control the thickness of the structure. To address this issue, a new temporal modulation dimension has been introduced, enabling the femtosecond laser to focus simultaneously in both temporal and spatial domains. This allows the laser energy density on the focal plane to meet the threshold for two- photon polymerization, while the energy remains insufficient on the defocus plane, thereby improving the axial processing resolution. The femtosecond laser's outstanding 3D processing capability has led to its widespread use in fields such as micromachinery, micro- optical devices, chirality research, and biomedical applications. Figure 4 showcases the magnetic- driven micro spiral structure and photothermal-driven micro robot created through femtosecond laser processing. By adjusting processing parameters, it becomes easy to control structural parameters such as height, radius, pitch, and cone angle. Following this, nickel and titanium metals are deposited onto the structure's surface using magnetron sputtering, providing excellent magnetic responsiveness and biocompatibility. For the photothermal-driven micro robot, a gradient distribution of cross- linking density within the hydrogel is initially formed to create a joint deformation unit, followed by achieving photothermal control through the deposition of silver nanoparticles. Figure 6 displays the applications of femtosecond laser technology in integrated optical devices, integrating micro/nano optical structures with fibers, photonic chips, and CMOS cameras. Figure 8 illustrates the helical dichroism of various chiral structures, encompassing planar chiral structures, rotational stacking of planar structures, and stereo structures. Additionally, it highlights the utilization of chiral spiral arrays in information display and encryption. Conclusions and Prospects Looking back over the past few decades, femtosecond laser processing has made significant strides in academic research. However, due to its high cost, two- photon polymerization technology has not yet achieved widespread applications in the industry. In the future, femtosecond laser processing technology will continue to advance towards higher processing resolution, shorter processing time, lower costs, and a wider variety of applicable materials.
