Micro-optical devices have the characteristics of miniaturization and integration compared with ordinary optical devices owing to their extremely small size. Therefore, they have irreplaceable application value and importance in optical communication, optical display, optical processing, and optical information storage.
Femtosecond laser processing is flexible, efficient, and has several materials to use. As the laser is compressed for a short time, it produces a very high-power density. Furthermore, the interaction between the laser and material is nonlinear and nonequilibrium. Therefore, controlling the interaction process between laser and electrons, especially the local electron dynamics, is necessary for quality optimization of laser processing. Jiang et al. proposed a new electronic dynamic control (EDC) technique, whose core idea is to control the local transient electron dynamics by controlling the amplitude, phase, and polarization of the femtosecond laser in space and time. This will regulate the local transient electron dynamics of the material, and change the morphology and properties of the material. Based on EDC technology, laser processing quality and processing efficiency can be effectively optimized, which is of great significance in the processing of micro-optical devices.
The main methods of femtosecond laser processing of micro-nano-optical components include both laser-controlled material properties and morphology. Laser-controlled material properties alter the local refractive index of the material to fabricate microlenses, as shown in Figure 2(d). Notably, etching assistance can be further used. In Figure 1(a), Huang et al. obtain vibrantly colored gratings by using laser-induced nonablative periodic modification and etching of silicon. Figure 2(a) shows the method of laser-controlled material morphology in which two-photon polymerization is used to process a multilayer microlens group, or as shown in Figure 3(f), where direct writing subtraction is used.
The main methods used in EDC technology to improve processing efficiency and processing quality include temporal and spatial shaping of the laser. Time shaping controls the distribution of the laser field intensity in time so that the free electron density on the material surface can be controlled near the critical electron density, which not only increases the proportion of linear absorption, such as avalanche ionization, but also preventing high free electron density on material surface. It makes nonthermal phase transformation a crucial part of the main processing, considerably reducing the recast layer in material processing, and increasing the number of excited electrons and absorbed energy under the same energy. Spatial shaping achieves locally controllable selective removal of materials by controlling the spatial distribution of various laser parameters. For example, shaping the laser into two adjacent spots with a phase difference, processing on a gold film with only a wavelength of 1/14 width nanowires.
The devices for time-shaping laser mainly include the pulse sequence generating device, time-domain shaping system using 4f system, Michael interferometer for generating pulse sequence and its cascade, birefringent crystal for generating pulse sequence and its cascade (Figure 4). The devices for spatial shaping of the laser include dynamic shaping devices and static shaping devices based on SLM and axicons, cylindrical mirrors, and masks. By temporally shaping the laser, the etching efficiency improves the microlens processing and controls the size parameters of the obtained microlens, as shown in Fig. 5. Using a cylindrical mirror to perform static spatial shaping of the laser can efficiently process micro-optics, such as gratings. More flexible machining results can be obtained using dynamic spatial shaping, for example, simulating multibeam interference to improve machining efficiency or shaping the laser into multispot light, processing several two-dimensional graphics at a time, on-site material lattice processing or parallel processing can greatly improve the processing efficiency.
In this paper, we reviewed the methods for femtosecond laser processing of optical components. The technical methods for femtosecond laser processing of micro-optical components, including laser direct writing removal, laser three-dimensional printing, laser modification, and wet etching assisted laser processing methods were introduced for gratings, microlenses, and zone plates. We observed irreconcilable contradictions in the processing accuracy and efficiency of the unshaped laser. By shaping the femtosecond laser in temporal and spatial, the light field is not limited to the Gaussian distribution in time and space, which effectively controls the electronic dynamics of the processing and improves the processing accuracy and efficiency. The temporal shaping of the femtosecond laser effectively improves the energy deposition efficiency of the laser and enriches the application scenarios of the femtosecond laser. Furthermore, the spatial shaping of the femtosecond laser is an important way to improve the processing accuracy beyond the diffraction limit and improve the processing efficiency to achieve large-area processing. Therefore, the appropriate use of spatial-temporal shaping methods is an important method for improving the precision, efficiency, and application scope of femtosecond laser processing of micro-nano-optical components. Presently, the femtosecond laser electronic dynamic control and processing of optical components with spatial-temporal shaping is still faced by the relatively single-time shaping technology, which is yet to fully combine the spatial shaping method. The next step is to further develop the temporal and spatial shaping technologies and to combine spatial-temporal shaping in the fabrication of micro-nano-optical components through a deep understanding of the electronic dynamic regulation mechanism.
