飞秒激光对 PMMA 隐形加工实验研究 (内封面文章·特邀)

Objective Conventional mechanical machining methods are unable to fabricate fine structures within transparent polymeric materials such as polymethyl methacrylate (PMMA), which limits their applications in optical microdevices and microfluidic chips. To overcome this limitation, this study systematically investigates the invisible machining behavior of PMMA using infrared femtosecond laser irradiation. Based on the law of refraction, a precise focusing model was established to describe laser propagation from air into the transparent medium, quantitatively revealing the spatial displacement between the theoretical and actual focal positions and analyzing the influence of refractive index mismatch on focusing characteristics. Furthermore, by systematically adjusting laser energy, scanning passes, and scanning speed, the formation mechanism, morphological evolution, and thermo-mechanical characteristics of internal microchannels under different processing conditions were examined. Experimental results show that, after focal correction and parameter optimization, high-quality and controllable subsurface cutting and microchannel fabrication can be achieved within the material, with channels exhibiting excellent morphological integrity and optical transparency. Finally, through precise control of energy deposition and scanning path, separation of 2 mm-thick PMMA samples was successfully realized. These findings provide a novel technical approach and theoretical support for high-precision internal machining of transparent materials, three-dimensional microstructure fabrication, and integrated photonic device development. Methods In this study, a low-repetition-rate infrared femtosecond laser processing system (Fig.1) was used to perform experiments on PMMA to investigate the effects of laser parameters on the morphology of microcavities. A precise focusing model was established to describe the laser propagation from air into the transparent medium, and the spatial displacement between theoretical and actual focal positions was analyzed (Figs.2-3). Microfabrication was carried out following the procedure shown in Fig.4, and the resulting microcavities were characterized using a laser confocal microscope and an optical microscope (Fig.5-Fig.7, Fig.9, Fig.10, Fig.12). The internal channels were further measured using a white-light interferometer (Fig.11), and, using the methods described above, PMMA samples were ultimately separated (Fig.12). Results and Discussions The experimental results indicate that there is a certain range of single-pulse energy within which internal modification of the material can be achieved. Increasing the number of scanning passes can enlarge the groove width. When the single-pulse energy is relatively low, increasing the number of scans can achieve the desired effect. Conversely, increasing the scanning speed can reduce the groove width. When the single-pulse energy is relatively high, increasing the number of scanning passes can also achieve the desired result. Therefore, strict control of these laser parameters is necessary to achieve subsurface processing of PMMA and realize invisible machining. Conclusions In this study, the invisible machining behavior of infrared femtosecond laser in transparent polymer PMMA was systematically investigated, revealing the intrinsic relationship between laser focusing, energy deposition, and internal structure evolution. Firstly, a focusing model for laser propagation from air into a transparent medium was established based on the law of refraction, enabling quantitative analysis of the spatial displacement between theoretical and actual focal positions. The influence of refractive index mismatch on focusing accuracy and energy distribution was clarified, providing a theoretical foundation for high-precision subsurface processing. Secondly, by systematically adjusting key laser parameters-including pulse energy, pulse duration, repetition rate, and scanning speed-the formation mechanism and morphological evolution of internal microchannels induced by femtosecond laser irradiation in PMMA were thoroughly examined. The results demonstrated that appropriate focal correction combined with optimized parameters allows stable energy deposition, resulting in continuous, regular, and controllable microchannel structures. Furthermore, the experiments successfully achieved high-quality separation of 2 mm-thick PMMA samples, confirming the feasibility and stability of femtosecond laser invisible cutting for high-resolution, low-damage internal processing. The processed cross-sections were smooth, with minimal heat-affected zones, demonstrating the advantages of nonlinear absorption and ultrafast localized energy deposition in transparent materials. In summary, this work provides a feasible experimental approach and theoretical guidance for the internal microstructure fabrication of transparent polymers and offers a technical reference for the development of three-dimensional microfluidic chips, optical components, and precision micromachining applications. Future work could further incorporate adaptive optics and dynamic focusing techniques to achieve deeper structure control and functionalized processing.